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Article
Occupational Exposure and Environmental Release: The Case
Study of Pouring TiO
2
and Filler Materials for Paint Production
Ana Sofia Fonseca
1,
*, Anna-Kaisa Viitanen
2
, Tomi Kanerva
2
, Arto Säämänen
2
, Olivier Aguerre-Chariol
3
, Sebas-
tien Fable
3
, Adrien Dermigny
3
, Nicolas Karoski
3
, Isaline Fraboulet
3
, Ismo Kalevi Koponen
4
, Camilla Delpivo
5
,
Alejandro Vilchez Villalba
5
, Socorro Vázquez-Campos
5
, Alexander Christian Østerskov Jensen
1
, Signe Hjortkjær
Nielsen
1
, Nicklas Sahlgren
1
, Per Axel Clausen
1
, Bianca Xuan Nguyen Larsen
1
, Vivi Kofoed-Sørensen
1
, Keld Al-
strup Jensen
1
and Joonas Koivisto
1,6,7,8
National Research Centre for the Working Environment (NRCWE), DK-2100 Copenhagen, Denmark;
[email protected] (A.C.Ø.J.); [email protected] (S.H.N.); [email protected] (N.S.); [email protected] (P.A.C.); [email protected] (B.X.N.L.);
[email protected] (V.K.-S.); [email protected] (K.A.J.); [email protected] (J.K.)
2
Finnish Institute of Occupational Health, FI-00032 Työterveyslaitos, Finland; [email protected] (A.-
K.V.); [email protected] (T.K.); [email protected] (A.S.)
3
Caractérisation de l’Environnement (CARA), INERIS, 93310 Verneuil-en-Halatte, France; Oliv-
[email protected] (O.A.-C.); [email protected] (S.F.); Adrien.DERMIGNY@in-
eris.fr (A.D.); [email protected] (N.K.); [email protected] (I.F.)
4
Clean Air Technologies, FORCE Technology, DK-2605 Brøndby, Denmark; [email protected]
5
Human & Environmental Health & Safety, LEITAT Technological Center, 08005 Barcelona, Spain;
[email protected] (C.D.); [email protected] (A.V.V.); [email protected] (S.V.-C.)
6
ARCHE Consulting, B-9032 Ghent, Belgium
7
Air Pollution Management, DK-2100 Copenhagen, Denmark
8
Institute for Atmospheric and Earth System Research (INAR), University of Helsinki, FI-00014UHEL Hel-
sinki, Finland
*
Correspondence: [email protected]; Tel.: +45-3916-5492
1
Citation:
Fonseca, A.S.; Viitanen, A.-
K.; Kanerva, T.; Säämänen, A.;
Aguerre-Chariol, O.; Fable, S.;
Dermigny, A.; Karoski, N.;
Fraboulet, I.; Koponen, I. K.; et al.
Occupational Exposure and Envi-
ronmental Release: The Case Study
of Pouring TiO
2
and Filler Materials
for Paint Production.
Int. J. Environ.
Res. Public Health
2021,
18,
418.
https://doi.org/10.3390/
ijerph18020418
Received: 26 November 2020
Accepted: 23 December 2020
Published: 7 January 2021
Publisher’s Note:
MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and insti-
tutional affiliations.
Abstract:
Pulmonary exposure to micro- and nanoscaled particles has been widely linked to adverse
health effects and high concentrations of respirable particles are expected to occur within and
around many industrial settings. In this study, a field-measurement campaign was performed at an
industrial manufacturer, during the production of paints. Spatial and personal measurements were
conducted and results were used to estimate the mass flows in the facility and the airborne particle
release to the outdoor environment. Airborne particle number concentration (1 × 10
3
–1.0 × 10
4
cm
−3
),
respirable mass (0.06–0.6 mg m
−3
), and PM
10
(0.3–6.5 mg m
−3
) were measured during pouring activ-
ities. In overall; emissions from pouring activities were found to be dominated by coarser particles
>300 nm. Even though the raw materials were not identified as nanomaterials by the manufacturers,
handling of TiO
2
and clays resulted in release of nanometric particles to both workplace air and
outdoor environment, which was confirmed by TEM analysis of indoor and stack emission samples.
During the measurement period, none of the existing exposure limits in force were exceeded. Par-
ticle release to the outdoor environment varied from 6 to 20 g ton
−1
at concentrations between 0.6
and 9.7 mg m
−3
of total suspended dust depending on the powder. The estimated release of TiO
2
to
outdoors was 0.9 kg per year. Particle release to the environment is not expected to cause any major
impact due to atmospheric dilution
Keywords:
paint industry; particle emissions; occupational exposure; environmental release; expo-
sure determinants; powder handling
Copyright:
© 2021 by the author. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://cre-
ativecommons.org/licenses/by/4.0/).
1. Introduction
Recent studies have shown that the release of ultrafine particles to air (UFPs; defined
as the fraction of fine particles with an electrical mobility diameter
≤100
nm) originating
from handling of conventional non-nano classified materials may be substantial [1–3].
Int. J. Environ. Res. Public Health
2021,
18,
418. https://doi.org/10.3390/ijerph18020418
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They may even dominate the release of UFPs also in industries using manufactured na-
nomaterials (NMs; in the EU defined as materials in which 50% of the particles by number
have one or more external dimensions in the range of 1–100 nm). In most cases, workers
are exposed to a heterogeneous mixture of different particles, which can make the quan-
titative exposure characterization and risk assessment very complex [1,4].
The paint and coating manufacturers are well known as a highly important down-
stream user of large amounts of NMs and conventional non-nano materials [5,6]. By 2022,
the paint industry alone is expected to reach a global value of 209.4 billion U.S. dollars [7].
Paint is considered a suspension of organic and inorganic particulate materials (e.g., cel-
lulose, TiO
2
, ZrO
2
, ZnO, Ag, and CeO
2
) in a liquid composed of a binder (resin), a volatile
solvent or water, and additives to impart protective, durability, decorative, dirt repel-
lence, color, gloss coating, or other properties to a substrate [8–10]. In general, the paint
manufacturing process involves powder handling, pouring, mixing, dispersing, thinning
and adjusting, filling of containers, cleaning operations, and warehousing [11]. Vast vol-
umes of the materials added in conventional paints are handled as dry powders, which
may fall under the EU NM definition or contain a fraction of nanoparticles and release
UFP during their handling [6].
Previous studies have shown that paint and coating manufacturers emitted 6000, 600,
and 400 tons of VOCs (volatile organic compounds), PM
10
(mass of particulate matter col-
lected with a 50% cut-point of 10 µm in aerodynamic diameter,
D
ae
), and PM
2.5
(50% cut-
point of 2.5 µm
D
ae
), respectively to the outdoor environment [12]. Van Broekhuizen et al.
[5] also confirmed a significant UFP release and an associated worker exposure reaching
>1 × 10
5
cm
−3
during manufacturing of conventional non-nano waterborne paint, which
involved pouring conventional pigment grade TiO
2
and fillers such as calcium carbonate
(CaCO
3
) and talc (Mg
3
Si
4
O
10
(OH)
2
). Koponen et al. [13] found worker exposures to PM
1
(50% cut-point of 1 µm
D
ae
) in different paint producing facilities varying from 0.2 to 0.8
mg m
−3
. Hence, there is evidence that paint production can result in a significant worker
exposure and environmental release of UFP and fine pigment and filler dust particles.
There is also evidence that many of the pigments and fillers used in paints can cause
diverse negative health effects after lung exposure, e.g., [14–18]. The risk of pulmonary
hazard appears to be generally higher in connection with exposure to NM and UFP as
compared with coarser particles and similarities can be found between the effects of NM
and UFP in the ambient air pollution [19]. The European Agency for Safety and Health at
Work [20] consider UFP as one of the major risks in workplace microenvironments. Con-
sidering public health, exposure to the general ambient and indoor air pollution has also
been linked to adverse health effects including cancer, respiratory, cardiovascular, and
nervous system diseases [21–25]. PM
2.5
air pollution has been ranked as the 6th highest
risk factor for early death [26]. While fossil fuel combustion particles are considered to
play the most important role in the observed public health effect of ambient air pollution,
industrial sources can have important local influence, e.g., [27,28]. Hence, it is important
to understand the potential pollution impact of an industrial plant on its near-field sur-
roundings. This, both in regards to potential direct environmental exposure and accumu-
lation of persistent pollutants, which can affect both humans and ecosystems [29,30].
Occupational and environmental exposure assessments are therefore important steps
in a risk management program in order to maintain the workplace exposure below limit
values and assure the protection of the environment [31,32]. High quality industrial expo-
sure studies are also needed to further understand the exposure determinants, which are
critical for occupational and environmental exposure model development and testing [33–
39].
In this study, a workplace field measurement campaign was performed at a paint
factory producing three different paint batches (12.4–14.5 tonnes per batch). Work tasks,
such as handling solvents and pouring of conventional pigments and fillers, with poten-
tial impacts on human health and environment, were studied in different pouring lines.
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The main objective was to demonstrate a holistic approach in safety assessment by evalu-
ating: (i) workers personal exposures to PM in terms of total number, mass, and lung de-
posited surface area concentrations; (ii) mass flows of the airborne particle emissions in
the near-field/far-field (NF/FF) and via exhaust air ducts; and (iii) environmental expo-
sure.
2. Materials and Methods
2.1. Work Environment and Measurement Locations
Particle and gas measurements were conducted at a paint manufacturer located in
the vicinity of Copenhagen (Denmark) from 29 January to 2 February 2018 during the
production of three different paint batches. In two different pouring lines, named hereaf-
ter as mixing station (MS) and pouring station (PS), eight different powders (pigments
and fillers) were manually poured into the hopper by one worker. In addition to the use
of local exhaust ventilation systems at workstations, the facility was naturally ventilated
where the outdoor replacement air entered across the building shell and through open
doors. The layout of the working environment and placement of the measurement devices
and samplers are shown in Figure 1 and photos of the stations, stack emission, and envi-
ronmental measurements are shown in Supplementary Figure S1.
Figure 1.
Layout of the working environment and location of instrumentation and samples.
Particles in the breathing zone (BZ) were measured with the personal measurement
instruments attached in a carry bag to the worker’s shoulder. At the stationary NF meas-
urement points in PS and MS the inlets of the instruments and the samplers were approx-
imately at a height of 1.5 m and 0.5 m from the powder pouring activities. Therefore, the
NF was defined as the volume around the process activity where the worker stands dur-
ing the work process. At the stationary FF measurement points, the instruments and sam-
plers were placed from 7.5 to 10 m from the NF locations (Figure 1).
In the MS, small bags (25 kg) with TiO
2
pigment (Tioxide TR81, Huntsman P&A UK
Ltd, Hartlepool, United Kingdom), modified alumino-silicate (OpTiMat
®
2550, Imerys,
Barcelona, Spain), and microspheres (Expancel, Akzo Nobel, Bohus, Sweden) were
opened with a knife and manually emptied by pouring directly into a mixing tank from
the edge of an opening area of 0.3 m
2
and a resulting drop height of 1.4 m inside the tank
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(Supplementary Figure S1a). This mixing tank had local exhaust ventilation (LEV) located
under the pouring point attached to the funnel leading to the mixer. This LEV was con-
nected to the mixing stack (EXH1; Figure 1), which had an exhaust air velocity of 5 m s
−1
and a volumetric dry flow rate of 10,700 m
3
h
−1
(Supplementary Figure S1b).
In the PS, powders were poured either from small bags (SBs; 25 kg) or big bags (BBs;
500 kg) through a quadratic opening with an area of 1.27 m
2
(Supplementary Figure S1c)
and conducted via tubes into a mixing tank (Supplementary Figure S1a). The SBs were
opened with a knife and manually poured from the edge of the tank and an internal tank
drop height of 1 m whereas BBs were lifted with an electric forklift above the pouring
funnel. Empty bags were introduced into a disposal container (Supplementary Figure
S1c). The PS had local exhaust ventilation (LEV) at the rim along three sides of the pouring
inlet. The exhaust duct of this LEV (duct diameter = 125 mm; mean air velocity = 13 m s
−1
;
and volumetric flow rate = 576 m
3
h
−1
) was connected to the pouring stack (EXH2; Figure
1) with a registered exhaust air velocity of 5 m s
−1
and a volumetric dry flow rate of 1258
m
3
h
−1
(Supplementary Figure S1b). Pouring activities in the PS involved the handling of
the followed five materials (Table 1): (i) calcined clay (PoleStar™ 200P, Imerys, Cornwall,
United Kingdom); (ii) calcined kaolinite (Ultrex 96, BASF, Ludwigshafen, Germany); (iii)
Talc (Finntalc M15, Mondo Minerals B.V., Helsinki, Finland ); (iv) dolomite (Microdol 1,
Norwegian Talc AS, Fjell, Norway); and (v) calcite CaCO
3
(Socal P2, Solvay Chemicals
International SA, Brussels, Belgium).
Table 1.
Description of pouring activities and physicochemical characteristics of the materials under study.
σ:
standard
deviation;
d
50
: average particle size; SSA: specific surface area; VSSA: volume specific surface area;
DI:
dustiness index;
SRD: small rotating drum method; OEL: occupational exposure limit.
Pouring Activity Description
Material
Name
TiO
2
pig-
ment (93%
rutile), (Ti-
oxid TR81;
CAS-Nr.
13463-67-7)
Functional-
ized alu-
mino-sili-
cate clay
(Al
2
Si
2
O
5
,
OpTiMat®
2550; CAS
No. 93763-
70-3)
Thermo-
plastic mi-
crospheres
(Expancel
461 WE 20
d36; CAS-
No. 75-28-5)
Calcined
clay
(Al
2
Si
2
O
5
;
PoleStar™
Location
Paint
Batch
#1
MS
#2
#3
Material Characteristics
OEL
d
Bulk Den- BET-SSA VSSA
DI
SRD
±
σ
(mg
Measurement
d
50
Chemical Compo-
±
σ
(m
2
(mg kg
−1
)
Shape
a
sity ±
σ
¥
a
m
−3
)
(µm)
sition
Day
c
(g cm
−3
)
b
(m
2
g
−1
) cm
−3
)
30 January
2018
Major: Ti, O
31 January
Minor: Si, Al, Zr,
0.25
Sphere 0.94 ± 0.03 12.7 ± 1.3 53.2 3.0 * ± 1.3 6
e
2018
P, and organic
coating
1 February
2018
MS
#1
29 January
2018
25
Major: Si, O
Minor: Al, K, Na,
and organic coat-
ing
Plate
0.16 ± 0.001
5.2 ± 4.0
13.6
149.9 ±
10.8
2
#1
MS
#3
29 January
2018
1 February
2018
20–30
Organic (2%)
Sphere
N/A
N/A
N/A
155.6 ±
66.3
5
f
PS
#1
30 January
2018
2
Major: Si, O, Al
Minor: K, Fe
Plate
0.52 ± 0.01 10.4 ± 0.05
27.0
13.3 ± 0.2
2
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200P; CAS
No. 92704-
41-1)
Calcined
kaolinite
31 January
(Al
2
Si
2
O
5
(O
Major: Si, O, Al
PS
#2
0.8
Plate 0.35 ± 0.01 6.2 ± 0.01 16.0 7.1 ± 0.4
H)
4
; Ultrex
2018
Minor: Fe, Na, Ti
96; CAS No.
92704-41-1)
Dolomite
30 January
#1
(CaMg(CO
3
)
2018
Major: Ca, O, Mg
2
; Microdol
PS
7.5
Sphere 1.09 ± 0.04 3.2 ± 0.3
9.7 23.3 ± 1.2
Minor: Si, S
31 January
1; CAS Nr.
#2
2018
16389-88-1)
30 January
Talc
#1
>96% Talc (Mg-Sil-
2018
(Mg
3
Si
4
O
10
(
5 (Par- icate with residue
OH)
2
;
ticles < magnesite and
Plate 0.46 ± 0.01 5.6 ± 0.2
15.1 69.1 ± 4.9
Finntalc
PS
31 January 2 µm: chlorite); MgO;
M15; CAS
#2
20%) SiO
2
; Al
2
O
3
and
2018
No. 14807-
FeO)
96-6)
Calcite
(CaCO
3
,
Socal® P2,
Fine
Grades,
1 February
Major: Ca, O.
PS
#3
N/A
Rod
0.57 ± 0.01 6.7 ± 0.6
18.2 0.6 * ± 0.4
calcium
2018
Minor: Si, S, Mg
carbonate
>= 98%;
CAS No.
471-34-1)
a
Determined by SEM or TEM-EDS;
b
Determined according to the procedure given in EN17199-3:2019 [40];
c
Mass-based
respirable dustiness determined by small rotating drum (SRD; EN17199-4:2019 [41]);
d
Respirable OEL according to the
Danish Working Environment Authority [42];
e
Calculated as Ti 8-h time weighted average;
f
Respirable inert mineral dust;
¥
Information available in the material safety data sheets provided by the manufacturer; * Below the limit of quantification
(7 mg kg
−1
); N/A: Not available data.
2
5
f
5
f
5
f
2.2. Raw Materials Characterization
The following characterization was made for the pigment and filler powders:
•
Primary particle size, morphology, and elemental composition by scanning electron
microscopy (SEM; using a FEI Quanta 200 microscope), operating at an accelerating
voltage of 1–2 kV and at magnifications between 20,000× and 50,000×, and transmis-
sion electron microscopy (TEM; using a Jeol JEM 1400 Plus microscope), operating at
an accelerating voltage of 120 kV, and coupled to an energy dispersive X-ray spec-
troscopy (EDS; AZTEC from Oxford Instruments). The powders were dispersed in
aqueous media with a small amount of ethanol for deagglomeration and deposited
on a Ni TEM grids;
Specific surface area (SSA) analysis by using the Brunauer–Emmett–Teller (BET)
method with nitrogen absorption using an Autosorb-1-MP (Quantachrome, USA;
[43]);
Dustiness in terms of respirable mass fraction, by using the small rotating drum
(SRD; EN17199-4:2019 [41]), respirable dust sampling using GK2.69 cyclone, and size
distribution analysis using an electrical low-pressure impactor (ELPI; Dekati model
ELPI and ELPI+, Dekati Ltd., Kangasala, Finland);
•
•
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•
Bulk density according to EN17199-1:2019 [44] using a measuring cylinder of known
volume of 10 cm
3
and an analytical balance with a resolution of 0.01 g.
The physicochemical characteristics, and current occupational exposure limits (OEL)
are described in Table 1 for each material. According to the manufacturer, the particle
sizes
d
50
varies from 250 for TiO
2
to >5 µm for talc, dolomite, and clays. The main organic
solvents and additives added to the mixer before and after the powders were white spirit,
high boiling esters (coalescent agents), glycols, and glycol ethers. Other materials such as
different types of cellulose were also added to these paint batches. However, these events
were not monitored due to the small amounts used.
2.3. Measurement Strategy
The measurement strategy adopted in this study followed the Tier 3 approach for
particle exposure assessment published by the Organisation for Economic Co-operation
and Development [45] and EN 17058:2018 [46]. It included real-time particle monitoring
combined with collection of samples for gravimetric, morphological, and chemical analy-
sis during working and non-working periods.
For background (BG) discrimination (particles from sources other than the target pro-
cess), a combined approach of temporal and spatial analysis was adopted [32]. The non-
working periods were used to define the BG concentrations at all measurement points by
using the measurements obtained prior the target activity in the paint manufacturing fa-
cility.
2.4. Particle Monitoring and Sampling Techniques
The measurements included real-time monitoring of particle concentrations and size
distributions, collection of particles on filter samplers, and TEM samples collected simul-
taneously from NF, FF, BZ, and stacks (Figure 1). The following real-time particle moni-
tors were used:
•
Particle mobility size distributions were measured by NanoScan (NS; TSI NanoScan
model 3091, TSI Inc., Shoreview, MN, USA) for particles from 10 to 420 nm in 60 s
intervals [47,48].
Optical particle sizer (OPS; TSI model 3330, TSI Inc., Shoreview, MN, USA) was used
to measure the optical particle size distributions in 16 channels from 0.3 to 10 µm in
60 s intervals [49–51].
Aerodynamic particle size distributions were measured using an electrical low-pres-
sure impactor (ELPI; Dekati model ELPI and ELPI+, Dekati Ltd., Kangasala, Finland)
in 14 size channels between 6 nm or 7 nm and 10 µm with 1 s intervals [52].
Portable condensation particle counter (CPC; TSI model 3007, TSI Inc., Shoreview,
MN, USA) were used to measure the total particle number concentration from 10 nm
to > 1 µm in 1 s time resolution [53,54].
Miniature diffusion size classifiers (DiSCmini (DM); Testo SE and Co. KGaA,
Lenzkirch, Germany) were used to measure total particle number, mean particle di-
ameter, and the lung deposited surface area (LDSA) of particles with modal diameter
in the range of 10–300 nm with 1 s time resolution [55]. To avoid artifacts due to
coarse particles, the DM was equipped with an inlet separator with a cutoff diameter
of 700 nm.
Aerosol black carbon detector (BC; Berkeley; [56]) and aethalometer AE33 (Magee
Scientific, USA) were used to measure BC mass concentration with 60 s intervals.
Alphasense optical particle counter OPC-N2 was used to measure PM
1
, PM
2.5
, and
PM
10
, and particle size distributions in 60 s intervals [57,58].
SidePak
TM
(model AM520, TSI Inc., Minnesota, USA) were used to measure particles
in the size range 0.1–10 µm [59,60].
•
•
•
•
•
•
•
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For the personal DM and SidePak, the particles were sampled through transparent
conductive Tygon™ tubing (Saint Gobain Performance Plastics, Courbevoie, France) [61],
while electrically conductive silicone sampling lines were used for the rest of the instru-
ments. Diffusional losses for the NS and ELPI sampling lines were corrected according to
Cheng (2001) [62].
In the case of total particle number concentrations and particle size distributions, we
primarily present the results obtained with the ELPI, because it is the only instrument for
which measurements were made at the NF of both MS and PS. However, when ELPI data
was not available at the MS, particle emissions were characterized by NanoScan and OPS.
The mobility and optical particle number size distributions measured by the NS and OPS
were combined to form a wide size-range dN/dLog (D
p
) particle number size distribution
for both NF and FF measurements according to Mølgaard et al. [63]. To make this combi-
nation it was assumed that a particle mobility and optical diameter were equivalent even
though optical diameter may differ from mobility diameter depending on the particle
shape, refractive index, and size [64]. The combined particle size distributions were based
on the mobility size concentrations by NS from 10 to 300 nm (15th channel of NS was
removed) and optical size concentrations by OPS from 300 nm to 10 µm.
The offline methods utilized in this study comprised:
•
Collection of respirable dust (d
50
cut size of 4 µm) for gravimetric and inorganic chem-
ical analysis by using Fluoropore™ (Millipore, Billerica, MA, USA) membrane filters
37-mm polytetrafluorethylene (PTFE) with a 0.8-µm pore size mounted in cyclones
GK2.69 (BGI Inc., Waltham, MA, USA) or SCC1.062 (BGI Inc., Waltham, MA, USA),
connected to portable sampling pumps (Apex2, Casella Inc., Bedford, United King-
dom) operating at 4.2 L min
−1
or 1.05 L min
−1
, respectively [65].
Note: Particle mass concentrations were gravimetrically determined by pre- and
post-weighing the filters collected using an electronic microbalance (Mettler Toledo
Model XP6) with ± 1 µg sensitivity located in a climate-controlled weighing room
(relative humidity (RH) = 50% T = 22 °C). Three blind filters were stored to be used
as laboratory blanks to correct for handling and environmental factors.
Collection of size-fractioned fine dust for gravimetric analysis by using a 4-stage cas-
cade impactor, without pre separator mounted with 47-mm aluminum foils (Dek-
ati
®
Gravimetric Impactor-DGI, model DGI-1571, Dekati Ltd., Kangasala, Finland) at
a flow rate of 70 L min
–1
, which results in calculated
d
50
cut-off diameters of >2.5, 1.0–
2.5, 0.5–1, and 0.2–0.5 µm. An after-filter collected particles <0.2 µm. Weighing was
conducted as mentioned above.
Isokinetic sampling on the quartz filter according to EN 13284-1 (reference method
for characterization of TSP) for determination of TSP mass concentrations and com-
bined with adsorption solutions according to EN 14385:2004 [66] (reference method
for characterization of heavy metals in atmospheric emission of stationary sources).
This filter is then analyzed together with the adsorption solutions in order to deter-
mine the concentration of 11 metals (As, Cd, Cr, Co, Cu, Mn, Ni, Pb, Sb, Ti, and V) by
means of inductively coupled plasma - optical emission spectrometry (ICP-OES).
Collection of airborne particles on 400-mesh Cu grids precoated with holey carbon
film by using a mini-particle sampler (MPS; Ecomesure; [67]) connected to a pump
(Apex2, Casella Inc., Bedford, United Kingdom) operating at 0.3 L min
−1
during 2–5
min sampling time. Aerosol samples collected by MPS were analyzed by TEM (Jeol
JEM 1400 Plus microscope), operating at an accelerating voltage of 120 kV, and cou-
pled to an EDS system (AZTEC from Oxford Instruments, High Wycombe, United
Kingdom). In situ EDS chemical analysis of agglomerates and individual particles
were performed with an acquisition time of 100 s.
Airborne gas-phase organic compounds ((S)VOC) were sampled on Tenax TA with
GilAir5 pumps (Gilian, Florida, USA) for 79–81 min with a flow of 93–110 mL min
−1
resulting in sampled volumes of 7.9–9.6 L. The Tenax TA tubes were cleaned before
•
•
•
•
•
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•
sampling in a stream of pure nitrogen at 300 °C for 180 min and 340 °C for 30 min
using a sample tube conditioning apparatus (TC-20, Markes International,
Llantrisant, United Kingdom). The Tenax TA tubes were analyzed by thermal de-
sorption gas chromatography and mass spectrometry (TD-GC–MS) using a Perkin
Elmer Turbo Matrix 350 thermal desorber coupled to a Bruker SCION TQ GC-MS
system (Bruker Daltonics, Bremen, Germany). Desorption was carried out in a He
flow of 1 mL/min at 275 °C for 20 min and desorbed (S)VOCs collected in a cold trap
at
−20
°C, followed by
flash
desorption of the cold trap at 275 °C for 1.5 min transfer-
ring the (S)VOCs to the GC column. The column was a 5% phenyl polydimethylsilox-
ane of 30 m × 0.25 mm with 0.25µm film thickness (VF-5MS, Agilent Technologies,
California, USA). The GC oven program was 40 °C for 2 min, then 20 °C/min to 150
°C hold for 10 min, then 5 °C/min to 275° hold for 6 min, and finally 3 °C/min to 300
°C hold for 1 min. The transfer line and the source were kept at 280 °C. The MS was
operated with electron ionization (EI) in scan mode (mass range m/z 40–500). Tenta-
tive identification of the organic compounds was performed by MS Data Review,
Version 8.0.1 (Bruker, Billerica, Massachusetts, USA), and NIST/EPA/NIH Mass
Spectral Library Version 2.0g, May 19, 2011 (NIST, Gaithersburg, Maryland, USA).
The terpenes were identified using authentic standards as well. The air concentra-
tions of (S)VOCs were semi-quantified using n-decane as a calibration standard.
Velocity measurements of local exhaust ventilation stacks were measured by a pitot
tube.
Particularly for the environmental monitoring and sampling, an original methodol-
ogy not yet published was followed. Description can be found in the Supplementary In-
formation.
2.5. Data Processing
All online instruments were time-synchronized and intercompared overnight, the
day before the actual measurements. Worker area exposures in terms of total particle
number concentration (N) were considered statistically significant when the following ap-
proach, described by Asbach et al. and Kaminski et al. [68,69], was fulfilled:
�������� > BG + 3. σ
(1)
where
WA
is the mean particle number concentrations in the workplace (either NF or BZ)
during the pouring activity, BG is the mean spatial background registered concentrations
simultaneously at FF, and
σ
BG
is the standard deviation of the BG concentration. However,
this approach should be carefully used and interpreted in the presence of secondary
sources of particles and especially in small workplaces with poor ventilation systems. In
the present study, the combined approach of temporal and spatial analysis allowed us to
distinguish the target particles from the background.
The cumulative worker exposure was calculated as an 8-h time weighted average (8
h-TWA). In this study, we calculated the 8 h-TWA based on the workers exposure dura-
tion during pouring activities (total daily duration varying from 2.2 to 5.8 h) and the spa-
tial background concentrations (measured in FF) for the remaining hours.
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3. Results
3.1. Raw Materials Characterization
Table 1 summarizes the characteristics of the eight different pigment and filler mate-
rials studied in this exposure study. Representative electron micrographs of the eight ma-
terials are shown in Supplementary Figure S2 and used for validation or modification of
particle characteristics. The characterization data demonstrate a considerable range in
BET-SSA values (e.g., dolomite and TiO
2
with 3.2 and 13 m
2
g
−1
, respectively), particle
grain sizes (0.25 to 30 µm), bulk densities (0.16–1.09 g cm
−3
), and particle shapes (e.g., plate
talc, plate clays, rod calcite, and spherical TiO
2
and dolomite). Therefore, establishing a
general relationship between emission patterns and materials physicochemical character-
istics might be challenging [70–73].
Respirable dustiness mass-fractions obtained by the SRD method ranged from < limit
of quantification (7 mg kg
−1
) to 156 mg kg
−1
with TiO
2
and calcite having the lowest and
microspheres and OpTiMat clay having the highest values. The standard deviations were
<43%, and as low as 2% for calcined clay PoleStar 200P, which demonstrates a general
reproducibility and homogeneity among the three replicas considered in dustiness test
and the method itself. According to the EN 15,051 ranking scheme, these materials are in
the category of powders with a very low dustiness level (<10 mg kg
−1
), low (between 10
and 50 mg kg
−1
), and moderate dustiness index levels (between 50 and 250 mg kg
−1
).
Overall, the pigment/filler powders used at the paint manufacturer were not identi-
fied as NMs by the manufacturers considering current regulation [74]. However, the Eu-
ropean Commission also proposes to define a material as a NM when it has a volume
specific surface area (VSSA) greater than 60, 40, and 20 m
2
cm
−3
for spheres, rods/fibers,
and flakes, respectively (EU, 2018). Considering uncertainties, Wohlleben et al. [75] pro-
posed the VSSA trigger thresholds of only 24, 16, and 8 m
2
cm
−3
for spheres, rods, and
flakes, respectively. Hence, considering the shapes of the current study materials and their
VSSA values, only clay PoleStar 200P certainly classifies as NM while only microspheres
and dolomite can be excluded as not being NMs. The rest of the materials are potentially
NMs according to these parameters.
3.2. Emissions and Exposure to Chemicals and Particles
Emissions and worker exposure were here analyzed considering the PS or MS and
the material being poured (Table 2). Supplementary Table S1 shows the measured organic
compounds, which were mainly solvents and coalescent agents. The measured terpenes
(α-pinene, 3-carene, and limonene) were probably fragrances. Texanol and the following
compounds were coalescent agents used as additive in waterborne paints. The concentra-
tions were generally highest in the solvent room and lowest in outdoor air. The NF, FF,
and personal concentrations were generally of comparable magnitude, except during the
production of paint batch #2 where NF concentration was lower. All field blanks were
below limit of detection defined as 10 times the signal-to-noise ratio. None of the com-
pounds with Danish TLVs exceeded these values (Table S1). For the glycol ethers the high-
est measured concentrations were from <1 to 14% of the TLV. For white spirit this value
was 65%. The results may be regarded as inaccurate due to the calibration with decane
since response factors in GC–MS varied significantly. The air concentration may also be
somewhat underestimated due to some degree of breakthrough (loss of analyte during
sampling) caused by large sampling volumes. However, these results are still deemed
useful as an indication of the actual concentrations.
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Table 2.
Descriptive statistics for the measured particle number concentrations (N), and lung deposited surface area (LDSA) by using DiSCmini (DM) (size range 10–700
nm) for non-working hours and for each poured material during each batch production and respirable dust and PM
10
.
Variables
Amount Poured (kg)
Pouring Rate
(kg min
-1
)
-
39.1
61.5
62.5
BZ
Mean
N/A
8.3 × 10
3
1.9 × 10
4
9.6 × 10
3
2.6 × 10
4
1.9 × 10
4
1.1 × 10
4
±σ
N
(cm
-3
)
NF
Mean
±σ
4.8 × 10
3
1.1 × 10
4
1.5 × 10
4
9.2 × 10
3
FF
Mean
±σ
3.7 × 10
3
3.4 × 10
3
2.3 × 10
3
2.0 × 10
3
BZ
Mean
±σ
N/A
37.2
86.1
53.8
55.7
68.3
39.1
LDSA
(µm
2
cm
-3
)
NF
FF
Mean
±σ
Mean
±σ
35.7
57.6
78.5
65.3
21.8
53.9
52.1
45.3
34.1
22.9
40.7
27.6
22.8
14.3
15.5
11.5
Respirable Dust/PM
10
(µg m
-3
)
BZ
N/A
N/A
N/A
N/A/1624 *
NF
76/N/A
1450 **
467.9/1150 **
621.8/1060 **
FF
25/N/A
76.4/N/A
60.3/N/A
47.3 (<DL)
Non-working hours
(14h of meas-
-
urements)
Batch #1 (55 min) 2150 kg (86 SBs × 25 kg)
Batch #2 (24 min) 1475 kg (59 SBs × 25 kg)
TiO
2
pigment
2625 kg (105 SBs × 25
Batch #3 (42 min)
kg)
Functionalized
alumino-sili-
cate clay
Batch #1 (14 min)
N/A
(Al
2
Si
2
O
5
, OpTi-
Mat® 2550)
N/A
Microspheres
Batch #1 (17 min)
(Expancel)
Batch #3 (21 min)
N/A
Calcined clay
(PoleStar™
Batch #1 (75 min) 925 kg (37 SBs × 25 kg)
200P)
Calcined kao-
linite (Ultrex
Batch #2 (94 min) 500 kg (20 SBs × 25 kg)
96)
Dolomite (Mi-
Batch #1 (122 min) 1500 kg (3 BBs × 500 kg)
crodol 1)
Batch #2 (117 min) 1500 kg (3 BBs × 500 kg)
1200 (2.4 BBs × 500)
Talc (Finntalc
Batch #1 (64 min)
M15)
Batch #2 (87 min)
1600 (3.2 BBs × 500)
Calcite (Socal®
Batch #3 (43 min) 333 (13.3 SBs × 25 kg)
P2)
3.0 × 10
3
6.7 × 10
3
2.1 × 10
4
5.8 × 10
3
1.2 × 10
4
1.0 × 10
4
1.1 × 10
4
6.7 × 10
3
N/A
N/A
5.3 × 10
3
9.2 × 10
3
6.4 × 10
3
4.4 × 10
4
N/A
28.0
36.3
12.2
37.3
N/A
57.6/482 **
34.1 (<DL)
N/A
N/A
12.3
N/A
4.5 × 10
3
6.3 × 10
4
3.1 × 10
2
3.2 × 10
4
4.1 × 10
3
1.1 × 10
4
4.6 × 10
4
1.6 × 10
3
5.5 × 10
3
2.4 × 10
3
1.9 × 10
4
3.4 × 10
3
6.0 × 10
2
3.8 × 10
2
9.6 × 10
2
N/A
11.6
0.4
61.7
221.4
57.7
12.4
19.3
77.0
14.3
0.2
4.2
8.2
6.3
1.5
2.52
0.01
N/A
N/A
N/A/1120 *
457.7/1050 **
N/A
N/A/630 *
N/A
47.3 (<DL)
N/A
4.5 × 10
5
5.3
12.3
12.8
18.5
18.4
7.7
1.7 × 10
4
3.6 × 10
4
2.3 × 10
4
2.7 × 10
5
1.9 × 10
5
8.4 × 10
3
2.4 × 10
4
1.3 × 10
5
6.4 × 10
4
6.2 × 10
5
5.0 × 10
5
5.8 × 10
3
6.4 × 10
3
2.8 × 10
4
5.0 × 10
4
4.0 × 10
5
1.9 × 10
5
6.9 × 10
3
7.3 × 10
3
4.0 × 10
3
8.2 × 10
4
2.1 × 10
5
1.1 × 10
6
6.3 × 10
5
4.8 × 10
3
3.9 × 10
3
8.1 × 10
3
5.9 × 10
3
5.6 × 10
2
2.3 × 10
3
1.3 × 10
3
6.0 × 10
3
3.3 × 10
3
6.4 × 10
2
44.2
63.5
44.2
449.4
313.9
21.4
49.7
187.2
90.5
981.5
810.4
10.4
22.0
33.5
56.5
327.6
185.6
17.0
0.2
0.7
1.7
7.9
4.5
0.1
14.4
9.1
11.2
13.1
16.1
11.1
0.01
0.03
0.03
0.08
0.06
0.02
N/A/2140 *
N/A/1040 *
N/A/1750 *
N/A/5630 *
N/A/6510 *
N/A/300 *
N/A/650 *
N/A/610 *
N/A/1260 *
N/A/4880 *
N/A/2930 *
N/A/299 *
N/A
N/A
N/A
N/A
N/A
N/A
4.9 × 10
3
4.3 × 10
3
Particle number concentrations significantly different from BG (FF) according to Equation (1) are shown in underlined bold. Mean ±
σ:
arithmetic mean and corresponding
standard deviation; BZ: breathing zone; NF: Near field; FF: far field; DL: detection limit; SB: small bags; BB: big bags; N/A: Not available data. * Measured PM
10
mass
concentrations by using SidePak (model AM520) at the PS ** Measured PM
10
mass concentrations by using OPC-N2 at the mixing station (MS).
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3.2.1. Pouring Activity at the Mixing Station
Particle measurements performed at the MS during non-working hours and specific
pouring activities are summarized in Table 2.
During non-working periods (14 h measurements starting on Sunday 28 January at
16 h to Monday at 6 h), the mean particle number and LDSA concentrations measured in
the MS at NF were 4800 ± 3000 cm
−3
and 35.7 ± 21.8 µm
2
cm
−3
, respectively, and the particle
sizes were <200 nm (Supplementary Figure S3). The average NF and FF respirable mass
concentrations were 76 and 25 µg m
−3
, respectively (Table 2 and Supplementary Figure
S3). Results from electron microscopy analysis showed that the background particles con-
sisted mainly of soot and few pigment/filler particles with vapors condensed on them
(Supplementary Figure S4). The daily mean BC was 556 ± 109 and 366 ± 107 ng m
−3
meas-
ured by the BC AE33 and BC
ABCD
sensors, respectively and data from the two instruments
correlated well (R
2
= 0.82; time series shown in Supplementary Figure S5). The highest BC
concentrations were observed between 6:00 and 12:00 CET and similarly, but at lower lev-
els in the afternoon and evening hours reflecting daily variation in traffic. This indicates
that particles infiltrated from the outdoor environment. Indoor BC sources were not iden-
tified during this measurement campaign.
Measurements during work hours resulted in considerable episodic increased parti-
cle concentrations in all batch formulations. Figure 2 shows an example of the MS NF, FF,
and BZ total particle number concentrations and particle number size distributions meas-
ured while pouring microspheres (Expancel) and 105 SBs of TiO
2
during formulation of
paint batch #3. The color plot displayed in Figure 2b shows the particle diameter on the y
axis, and time of the day on the x axis, with the particle number concentration expressed
as dN/dlogD
p
in each size interval. The normalized concentration dN/dlogD
p
corresponds
to the differential number of particles per differential log diameter within an interval of
the size distribution where
N
is the particle number and
D
p
is the particle diameter.
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Figure 2.
Time series during pouring activities involved in the paint formulation of batch #3 at the MS of (a) total particle
number concentration, and (b) particle number size distributions measured by electrical low-pressure impactor (ELPI) at
NF and mean particle size diameter measured by DM at NF and FF. The blue horizontal lines show the filter sampling
periods.
Pouring of the same powders and clay OpTiMat during formulation of different paint
batches, resulted in similar average work day concentrations (Table 2 and Supplementary
Figures S6–S9) with higher episodically increased dust concentrations in the NF and BZ
as compared with the dust concentrations in the BG (Figure 2, and Table 2). However,
increased particle concentrations were also seen in the FF as a result of powder handling
activities.
The time-series data showed generally similar trends in BZ and NF measurements
with the DM, but NF measurements showed relatively high fluctuations during pouring
of microspheres while BZ measurements had higher concentrations and fluctuations dur-
ing pouring of TiO
2
. A general increase is observed in the NF and BZ particle number
concentrations between ca. 12:00 and ca. 13:20, which was probably attributed to other
sources (not identified) during the lunch break. The NF DM and ELPI data, showed a
remarkable difference in the concentrations and variations between ca. 11:00 and 12:00
including the period with pouring of microspheres (Figure 2). This difference may be ex-
plained by the presence of particles with modal diameters larger than 300 nm (particle
size distributions shown are shown in Supplementary Figure S10). Although the DM has
an inlet separator with a 0.7 µm
d
50
value, some larger particles can still penetrate and
dominate the electrometer signals and result in erroneous particle number concentrations
[55,76]. However, for some unknown reason, this difference was not registered in the BZ
DM measurements.
Even though the particle number concentrations were similar, the highest level of
respirable mass concentration was registered at NF while pouring 105 SBs of TiO
2
(PM
4
=
622 µg m
−3
; Table 2 and Figure 2). This suggests that dust mass concentrations were linked
to the amount of material used when the pouring rate was similar (60 kg min
−1
). BZ dust
concentrations were consistently higher than in the NF (Table 2). This type of observation
is not unusual and may be due to several factors, such as closer proximity to the source
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than the NF measurement during the actual pouring process, influence of other processes,
including folding and handling emptied bags, which was done 1 m further away from the
NF, and the unknown dominant air flow directions around the MS.
During pouring activities in the MS, the particle size diameter increased when com-
pared to non-activity levels and differences in the particle size distribution patterns
among types of materials poured can also be observed. On average, mean emissions were
moderately higher and coarser during TiO
2
pouring and microspheres than clay OpTiMat
pouring (mean particle size distributions shown in Figure S10). Microscopic analysis of
the samples collected during TiO
2
pouring activity confirmed the presence of crystalline
TiO
2
in higher number counts (Figure S11a,b). These particles were agglomerated (Figure
S11a) having primary particle sizes of 100–500 nm (Figure S11b), which is close to the
d
50
indicated by the manufacturer (see Table 1). In addition, other micrometer particles such
as platelets of talc (Mg
3
Si
4
O
10
(OH)
2
) and kaolinite (Al
2
Si
2
O
5
(OH)
4
) were detected (Figure
S11c,d).
3.2.2. Pouring Activity at the Pouring Station
Figure 3 shows an example of the total particle number concentration and size distri-
bution as a function of time during pouring calcined kaolinite (Ultrex 96), talc (Finntalc
M15), and dolomite (Microdol 1) during the paint formulation of batch #2. The measure-
ment results obtained during the formulation of batch #1 and pouring of talc, dolomite,
and calcined clay Polestar 200P (Supplementary Figure S12) were comparable to the meas-
urements during formulation of batch #2. The time series of the total particle number con-
centration and particle size distribution during pouring 332 kg of calcite in the batch #3
paint formulation are illustrated in the Supplementary Figure S13.
Figure 3.
Time series during pouring activities involved in the paint formulation of batch #2 at the PS of (a) total particle
number concentration, and (b) particle number size distributions measured by ELPI at NF and mean particle size diameter
measured by DM at NF and FF. The blue horizontal lines show the filter sampling periods.
The concentrations in terms of particle number, mass and LDSA increased up to 3
orders of magnitude in the NF, BZ, and FF from the preactivity concentration levels dur-
ing the day (Table 2; Figure 3 and Supplementary Figures S12 and S13). Figures 3, S12b,
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and S13b show that different pouring events coincide with peaks of particle number con-
centrations at NF in the size range of 300 nm–7 µm. Figure S14 confirms that the mean
particle size distributions obtained for each poured material were exclusively different for
particles above 300 nm. Hence, the general increase in the total number concentration,
mass, and LDSA could be assumed to be due to particles released from powder pouring
activities.
Pouring of 925 kg calcined clay and 500 kg kaolinite from SBs, during the formulation
of batch #1 and #2, respectively, increased the concentrations >1 × 10
4
cm
−3
and 1 × 10
2
µm
2
cm
−3
at NF and BZ (Figure 3 for batch #2 and Supplementary Figure S12 for batch #1) while
kept stable at FF location (<4 × 10
3
cm
−3
and 10 µm
2
cm
−3
). However, the NF ELPI concen-
trations were nearly constant <1 × 10
4
cm
−3
and similar to the concentration levels meas-
ured before the activity. These differences can probably be attributed to the aerosol meas-
ured, which is composed of particles with modal diameters larger than 300 nm (mean
particle size distributions shown in Supplementary Figure S14).
The PM
10
concentrations measured in the NF and in the BZ ranged from 0.6 to 2.1 mg
m
−3
with the highest concentration registered during pouring of kaolinite. Lower PM
10
mass concentration <0.1 mg m
−3
were previously measured by Koivisto et al. (2015) at NF
while pouring calcined kaolin. This difference between the current and previous concen-
tration measurements is probably due to the low respirable dustiness index of the kaolin
material used in the previous study, which was only 30% of the dustiness index of the
calcined kaolinite used in the present study or attributed to setup differences. The dust
collected during pouring of calcined clay (Figure 4a) and kaolinite (Figure 4b) were dom-
inated by compact and thick Al-Si platelets with >100 nm primary particle size. Some of
the collected fragments seemed to be fiber-like particles with a diameter smaller than 3
µm, a length larger than 5 µm and an aspect ratio (length:diameter; L:D) greater than or
equal to 3:1, thus meeting the criteria for a respirable fiber according to the World Health
Organization definition [77].
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Figure 4.
Example of microscope images of particles collected NF during pouring activities of (a) calcined clay (PoleStar
P200); (b) kaolinite (Ultrex 96); (c,d) talc (Finntalc M15); (e) dolomite (Microdol 1); and (f) calcite (Socal P2).
Pouring of talc from BBs resulted into the highest concentrations (>limit of detection
1 × 10
6
cm
−3
and 1 × 10
3
µm
2
cm
−3
) at both BZ and NF locations by using the DM whereas
kept relatively constant at 1 × 10
4
cm
−3
by using ELPI at NF (Figure 3 and Supplementary
Figure S12). Similar as for the clays and kaolinite pouring activities, these high concentra-
tions are probably due to the presence of coarse particles in the DM air sample as indicated
above (Figure S14). In this study, the PM
10
concentrations measured in the NF and in the
BZ were indeed the highest among all the materials considered. It ranged from 2.9 to 4.9
mg m
−3
at NF and 5.6 to 6.5 mg m
−3
at BZ. The dust collected contained micrometric plate-
lets (Figure 4c,d). Additionally, among these particles, some appeared to meet the criteria
for a respirable fibers according to the WHO definition (L:D > 3:1). A concentration peak
was detected at FF between 11:30 and 12:00 (Figure 3), which most likely originate from
other activities in the FF of the PS; e.g., driving a forklift, moving sacks, using solvents,
rather than from the pouring activities. Lower FF concentrations were expected due to the
dilution of the concentrations.
Identical to previous pouring events, pouring of dolomite from BBs (1500 kg in total)
also increased the concentrations (>1 × 10
5
cm
−3
and 1 × 10
2
µm
2
cm
−3
) measured in the NF
and BZ by using DM whereas ELPI NF kept concentrations stable at 1 × 10
4
cm
−3
(Figure 3
and Supplementary Figure S12). This is again possibly due to the presence of particles
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with modal diameters >300 nm (Supplementary Figure S14) leading to unreliable DM re-
sults.
The PM
10
concentrations measured in the NF ranged from 0.6 to 1.3 mg m
−3
, and in
the BZ ranged from 1.0 to 1.8 mg m
−3
. Koivisto et al. (2015) reported similar levels at NF
while pouring dolomite from BBs. Dust collected during this pouring event contained
mainly micron-size Mg-Ca particles consisting of >100 nm primary particles (Figure 4e).
Pouring 332 kg of calcite CaCO
3
during the paint formulation of batch #3 produced a
clear number concentration peak at both NF and BZ (elevated from 4 × 10
3
to 2 × 10
4
cm
−3
),
which was not observed by ELPI at NF or at FF by the DM (Figure S13).
This consistent pattern among most of the pouring activities led us to confirm that
pouring calcite is also characterized by high coarse particle concentrations (Supplemen-
tary Figure S14), which can lead to untrustworthy DM results. The corresponding PM
10
concentrations measured in both the NF and BZ were the lowest among all the materials
poured (0.3 mg m
−3
), which can be explained by the small particle diameters (mean parti-
cle size distributions shown in Supplementary Figure S14) and the low dustiness level.
Microscopic analysis confirmed the presence of aggregated particles with spheroidal and
elongated shape (Figure 4f).
Similarly to what has been registered in the MS, the exposure measured in the
worker’s BZ was consistently higher than the NF concentrations (Table 2).
3.3. Comparison of Worker Exposure Concentrations with Recommended Exposure Limits
For the comparison between the exposure concentrations and OEL, it was assumed
that workers exposure during pouring activities at the mixing and pouring stations had a
daily duration between 2.2 and 5.8 h, and the rest of 8 working hours was spent FF.
Based on chemical identity of the collected particles during each event, a nano refer-
ence value (NRV
8h-TWA
) of 4 × 10
4
cm
−3
set by Social and Economic Council of the Nether-
lands for biopersistent granular materials with density < 6 × 10
3
kg m
−3
was assigned [78].
For short-term exposures of maximum 15 min (NRV
15min-TWA
), a value of 2 × NRV
8h-TWA
was
used (van Broekhuizen et al. [79]). The NRVs are background-corrected concentrations,
which, however, were influenced by source emission events.
The calculated 8 h-TWA exposure
N
concentrations calculated from ELPI (or NS and
OPS) were in the range of 3.7 × 10
3
cm
−3
and 6.0 × 10
3
cm
−3
(Table 3) and hence not exceeding
10 percent of the NRV. Even considering the advised short-term value, none of the pour-
ing materials would have exceeded the NRV, because particle concentrations concentra-
tion levels were <1.4 × 10
4
cm
−3
(not BG corrected).
Table 3.
Background corrected 8 h-TWA particle number (N) and PM
10
exposure concentrations
obtained for each working day, and comparison with the nano-reference value (NRV) and the
permissible exposure limit (PEL).
N
8h-TWA
(cm
−3
)
Day 1 3.7 × 10
3
Day 2 6.0 × 10
3
Day 3 5.1 × 10
3
Day
NRV
8h-TWA
(cm
−3
)
[78]
4.0 × 10
4
N
8h-TWA
/ PM
10 8h-TWA
NRV
8h-TWA
(mg m
−3
)
0.1
1.4
0.2
2.1
0.1
0.3
PEL
8h-TWA
(mg m
−3
)
[42]
10
PM
10 8h-TWA
/
PEL
8h-TWA
0.1
0.2
0.03
In terms of mass concentration, the Danish Working Environment Authority [42] has
a PEL for 8-h TWA of 10 mg m
−3
for total suspended inert mineral dust and TiO
2
. For
respirable inert mineral dusts, and clays the 8-h TWA PEL is 5 and 2 mg m
−3
, respectively.
The National Institute for Occupational Safety and Health [80] has recommended an ex-
posure limit (REL) of 2.4 mg m
−3
for fine (respirable; EN 481:1993; ISO 7708:1995) TiO
2
dust. Considering the Danish 8 h-TWA, the 10 mg m
−3
PEL for TiO
2
in total dust, and the
2.4 mg m
−3
NIOSH REL for fine TiO
2
, it is considered that the TiO
2
exposure do not exceed
any of these legal or recommended values during any working day given that the PM
10
levels were in the range of 0.3–2.1 mg m
−3
. Even though respirable mass sample was not
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collected for kaolinite pouring, it is not expected that exposure to respirable mineral clays
would have exceeded the PEL of 2 mg m
−3
because PM
10
was 2.1 mg m
−3
. Koivisto et al.
[35] also showed that personal exposure levels to pigments and fillers were below the
current OELs during preparing comparable formulations.
3.4. Environmental Release
Outdoor measurements in the facility surroundings (Supplementary Figure S1d)
were performed during production activities of three different paint batches. Due to
strong winds (85% between 1.4 and 4.4 m s
−1
and 8% between 4.4 and 8 m s
−1
) during the
measurement period and (relatively) and low particulate emissions from the exhaust
stacks, no measurable impact from the production activities was found in the vicinity of
the plant: exposed samplings and non-exposed samplings show elemental concentrations
not significantly different.
Stack emissions were characterized on three days. The exhaust stack (PS or MS), and
sampling period was chosen based on the most representative activity in the facility.
Therefore, three sampling periods were completed for the pouring stack and one for the
mixing stack.
Results from the stack emission connected to the MS (mixing stack) during TiO
2
pour-
ing activity for the production of paint batch #2 (1475 kg from 59 SBs) revealed an emitted
mass rate of 20.3 g ton
−1
(grams per poured tonne of TiO
2
) in TSP at a concentration of 550
µg m
−3
(Table 4). Total particle concentrations collected by DGI (i.e PM
2.5
+ particles with a
diameter > 2.5 µm) and TSP reference method are not directly comparable due to the
known effect of particle losses on walls for DGI [81].
Table 4.
Emissions detected at the stack of both mixing and pouring stations in terms of the total particle number, total
suspended particles (TSP), and size fraction particle mass.
ELPI
Variables
DGI Sampler
TSP Reference Method
[66]
(mg m
−3
)
0.55
5.8/20.3
9.67
-
4.19
6.93
12.0/16.7
5.2/5.8
-
8.6/11.3
PM
1
PM
0,5
PM
0,2
N total × 10
3
PM
2.5
+ >2,5 µm PM
2.5
−3
)
−3
) (mg m
−3
) (mg m
−3
) (mg m
−3
)
−3
)
(mg m
(mg m
(cm
Concentration batch #2
2.69
0.38
0.34
0.28
0.18
0.16
(7:33–12:43; TiO
2
pouring)
Flow (g h
−1
/g ton
−1
)
-
4.0/14
3.7/13 3.0/10.5 2.0/7.0 1.7/6.0
Concentration batch #1
4.78
*
*
*
*
*
(7:30- 12:32)
Concentration batch #2 (13:00–14:40)
5.15
2.85
1.87
0.89
0.47
0.26
Concentration batch #3 (12:13–14:57)
9.13
3.49
2.29
1.07
0.44
0.30
Average
6.35
3.17
2.08
0.98
0.46
0.28
Flow batch #1
-
-
-
-
-
-
(g h
−1
/g ton
−1
)
Flow batch #2
-
3.6/4.0
2.3/2.6 1.1/1.2 0.6/0.7 0.3/0.3
(g h
−1
/g ton
−1
)
Flow batch #3
-
4.4/23
2.9/15.2 1.3/6.8 0.6/3.1 0.4/2.1
(g h
−1
/g ton
−1
)
Average
-
4.0/13.5
2.6/8.9 1.2/4.0 0.6/1.9 0.4/1.2
(g h
−1
/g ton
−1
)
* DGI spectrum distribution not estimated due to the saturated sample.
Pouring station
Mixing
Measured particle sizes by using ELPI in the mixing stack ranged from 50 nm to 1
µm (Supplementary Figure S15). The size distribution given by DGI results (Table 4)
shows the same trend with majority of particles (in mass) under 2.5 µm size and a signif-
icant proportion below 0.2 µm. Table S2 in the supplementary information contains the
elemental concentrations and emission factors measured in TSP and in PM
2.5
, PM
1
, PM
0.5
,
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and PM
0.2
fractions obtained by the DGI impactor for the mixing stack. The relevant chem-
ical elements found were Si at a concentration of 63 µg m
−3
, Ti at 28.7 µg m
−3
(TiO
2
at 46.7
µg m
−3
), Mg at 28 µg m
−3
, Ca at 25 µg m
−3
, and Al at 26 µg m
−3
. Knowing these parameters
it was possible to estimate a TiO
2
mass emission factor of 0.51 g h
−1
, an amount of 1.7 mg
TiO
2
released per kg of TiO
2
poured, and a total amount of 0.9 kg of TiO
2
released per year
to outdoors (assuming annual TiO
2
consumption of 500 ton).
The average emission factor measured in the stack emission from PS (pouring stack)
during 3 paint batch productions was 11.3 g ton
−1
in TSP at a concentration of 4–9 mg m
−3
(Table 4). Measured particle sizes by using ELPI were generally higher and coarser than
in the mixing stack varied from 50 nm to 3 µm (Supplementary Figure S16). An average
of two assays were collected where the relevant chemical elements found from the TSP
filters were Si at a flow of 0.6 g ton
−1
, Ti at a flow of 0.2 g ton
−1
, Mg at an emission factor of
1.3 g ton
−1
, Ca at flow of 0.2 g ton
−1
, and Al at a flow of 0.3 g ton
−1
. Mass concentrations
assessed by the use of the Nanobadge were lower than those measured in the quantitative
method by a factor 1.5–3.
Sampling on TEM grids allowed us to obtain microscope images of particles collected
in the stack emissions during pouring of TiO
2
and calcined clay. Examples images are
shown in Supplementary Figure S17. Overall, pouring of pigments or fillers was always
visible in the samples collected in the stack emissions. The pigment/filler poured or mixed
at the time of sampling is the main component visible, but other products can also be
detected due to historical cumulative effects (Supplementary Figure S17). Part of TiO
2
(fraction not available) is in the nanometric range but other products are rather in the
micrometric size range.
4. Discussion
In this study, a field campaign was conducted at a paint manufacturer, during pro-
duction of different paint batches. Work tasks with potential impacts on human health
and environment such as handling solvents and pouring of pigments and fillers were
studied in different pouring lines. The measurements included real time particle monitor-
ing, sampling of airborne organic compounds, collection of airborne particles in filter and
TEM samplers simultaneously at near field (close to the MS and PS), far field, breathing
zone (personal), stacks, and outdoor surroundings. Use of a high methodological strength
is essential to investigate potential risks on both environment and workers health [45,46].
Furthermore, this study facilitates the link of personal exposure and working tasks such
as handling, pouring small or big bags, and other types of activities, which can occur out-
side the near field stationary position (e.g., driving a forklift, moving sacks, and folding
empty bags).
4.1. Material Characteristics and Propensity to Dust Release
Considering the shapes of the current study materials and their VSSA-values, cal-
cined clay PoleStar 200P certainly classifies as NM (VSSA of plates >20 m
2
cm
−3
) while only
microspheres and dolomite can be excluded as not being NMs [74]. The rest of the mate-
rials can be possibly defined as NMs considering their shapes and uncertainty associated
in the use of the VSSA method.
Respirable dustiness mass fractions obtained for all the powders used for paint man-
ufacturing were found to be very low (< 10 mg kg
−1
) to moderate (between 50 and 250 mg
kg
−1
), with TiO
2
and calcite having the lowest and microspheres and clay OpTiMat having
the highest values. As expected, the emission patterns from the materials under study do
not follow a linear correlation with the primary particle size given by the manufacturer
[82]. Here it seems that near spherical materials with
d
50
> 20 µm have higher potential to
release respirable particles (e.g., clay OpTiMat, microspheres) when compared to materi-
als with smaller particle diameters (e.g., dolomite, kaolinite, calcined clay, and calcite).
Similar observations were previously reported for micron-sized and nanoscale powders
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[35,37,70–73]. This is probably due to the weaker cohesive forces and consequently mate-
rial dustiness is higher and particles with small diameters are more probable to be re-
leased. However, several mechanisms can influence on dustiness through agglomeration
and soft bridging between particles in powders [82].
The dustiness level and characteristics of dust release from a powder material de-
pends on their characteristics and properties (e.g., density, particle size distribution, mois-
ture content, and extent of aggregation and agglomeration [35,82–90]). Additionally, stor-
age conditions can play a role in the dustiness. Levin et al. [88] studied the effect of storage
conditions on different powders by subjecting them to constant load and different RH
levels (30, 50, and 70%) over 7 days. They observed that the higher RH—the lower is the
dustiness, but with considerable difference in the absolute values for Al
2
O
3
, HNO
3
-stabi-
lized TiO
2
, ZnO, and CeO
2
. This effect is expected to be more pronounced for hydrophilic
than for hydrophobic powder materials [83].
Hence, the difference in relative humidity (RH) of the air found in the paint factory
(varying from 40 to 43% RH by using a TSI VelociCalc) and the conditioned 50% ± 5% RH
used in dustiness testing according to settings for standard testing in EN17199 could result
in higher dustiness of the powders in the factory as compared to that measured in the test.
This aspect needs to be considered when using dustiness data as source strength for ex-
posure assessment because an underestimation of exposure and risk level is possible if the
workplace <50% RH. However, a more elaborate analysis on the specific materials is re-
quired to understand to what extent dustiness of these materials are affected by the
slightly lower RH.
4.2. Particle Emissions and Impact on Worker Exposure
Overall, particle emissions and consequently worker exposure occurred during pour-
ing of pigment/filler powders. The particle number concentrations monitored by the dif-
fusion chargers DMs at NF and BZ showed statistically significant increase in dust con-
centrations up to 4.0 × 10
5
cm
−3
during powder pouring (Table 2). An increase in concen-
trations was also seen in the FF, but at lower levels. However, the emission patterns were
markedly different between the NF DM and the ELPI measurements for all the pouring
events, except for TiO
2
. This led us to confirm that pouring activities involved in this study
were characterized by coarse particle concentrations (Supplementary Figures S10 and
S14), which were outside of the DM’s measurement range and therefore led to unreliable
current in the electrometer stages and consequently particle counting [55]. These observa-
tions are also in line with Koivisto et al. [76], which reported a noise signal during tung-
sten-carbide-cobalt (WCCo) sieving and milling attributed to the presence of particles
with modal diameter larger than 300 nm. Under these circumstances, other types of par-
ticle monitors (e.g., ELPI, OPS, and particle mass concentration monitors or samplers)
should be the preferred choice.
Even though the raw materials used in this factory were generally considered to be
conventional non-nano materials, exposure to UFPs (diameter
≤
100 nm) still occurred as
documented by measured particle size distributions (Supplementary Figures S10 and
S14). UFP were detected during TiO
2
pouring, and slightly in OpTiMat clay pouring. On
the other hand, coarser particles > 300 nm up to 7 µm were released from pouring of other
filler materials. Microscopy analysis of the samples collected during pouring events at NF
also confirms these facts (Figure 4 and Supplementary Figure S11). Here it should be noted
that clays in particular are highly anisotropic sheets, where typically only the sheet thick-
ness is in the nanoscale.
Obviously, BG concentrations dominated by nanoscale particles should not be ne-
glected. Microscopy samples from BG confirmed soot agglomerates infiltrated from out-
doors and pigment/filler particles with vapors condensed on them (Supplementary Figure
S4) with particle size diameters varying from 50 to 200 nm (Supplementary Figure S4).
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The BC time series from Supplementary Figure S5a confirmed that indoor BC concentra-
tion were less than 20% of common urban background concentrations reported for other
European cities (approximately 2000 ng m
−3
registered Paris, London, Barcelona, and Lu-
gano; [91,92]).
The study conducted by Van Broekhuizen et al. [5] during manufacturing of conven-
tional non-nano waterborne paint, measured significant UFPs concentrations in the BZ by
using a diffusion charger NanoTracer, which uses the same measurement principle as
DM. Pouring conventional pigment grade TiO
2
and fillers such as CaCO
3
and talc released
ca. 4.4 × 10
4
cm
−3
, 6.9 × 10
5
cm
−3
, and 5.7 × 10
5
cm
−3
, respectively. On the other hand, Van
Broekhuizen et al. [5] did not notice airborne release of UFPs during the pouring of nano-
TiO
2
to a nano-enabled paint (task mean ca. 1.5 × 10
3
cm
−3
).
In this study, personal exposure concentrations were consistently higher than meas-
ured from the NF. PM
10
concentrations in the stationary NF ranged from 0.3 to 4.9 mg m
−3
whereas personal exposure was consistently higher (0.3–6.5 mg m
−3
). This is likely because
the worker is closer to the source than the stationary measurement station during the
powder handling events (especially small bags of materials). Alternatively, workers’ ex-
posure could have occurred for example when empty bags were folded outside of the NF
volume so that the particle emission was not detected by instruments at NF. It has also
previously been reported during pouring processes in a paint factory that personal PM
samples exceed the values measured from the stationary measurement point up to 16
times for PM
1
and 3.9 times for respirable mass [13,35].
Pouring of talc from BBs resulted into the highest personal PM
10
concentrations
among all the materials studied (PM
10
= 6.5 mg m
−3
). This can be supported by the particle
size distributions, which was dominantly coarser >1 µm. On the other hand, pouring of
calcite resulted in the lowest PM
10
among all the materials poured (0.3 mg m
−3
), which can
be explained by the low dustiness level.
The pouring events suggested that emission levels are dependent on the amount in-
volved in the pouring activity when pouring rate is similar: The higher the number of
pouring from SBs, the higher is the respirable mass concentration (105 TiO
2
SBs = 622 µg
m
−3
vs. 59 TiO
2
SBs = 468 µg m
−3
). Similar conclusion was extracted from pouring BBs of
talc at a constant pouring rate 18.5 kg min
−1
: personal PM
10
during pouring 3.2 BBs was
1.3 times higher than 2.4 BBs of talc). Dolomite pouring from BBs also suggested that emis-
sion levels were dependent on the pouring rate: pouring of 1500 kg at 13 kg min
−1
was 1.7
times higher than pouring of 1500 kg at 12 kg min
−1
. This should be carefully considered
in occupational exposure assessment modeling [34,93,94]. Another possible exposure de-
terminant, which was not assessed under this study, is the type of process involving SBs
or BBs. However, it is foreseen that pouring SBs of a specific material would release higher
amounts of particles than BBs because usually they are connected to a pouring funnel,
which provides some process enclosure [13]. Besides the source enclosure, the local ex-
haust ventilation existing at MS and PS are also expected to be an important risk manage-
ment measure in place to reduce the potential of particle exposure [38,95].
Even though, none of the exposure limits set by the Danish Working Environment
Authority (10 mg m
−3
for total dust) and the National Institute for Occupational Safety and
Health (2.4 mg m
−3
REL for fine TiO
2
) appears to have been exceeded during any of the
work situations, additional measurements are advisable to be conducted in order to en-
sure that workers exposure are systematically lower than 10 percent of the PELs (EN
689:2018 [96]). The compliance with OELs can be guaranteed by increasing the efficacy of
the risk management measures. This can be done by improving the technical solutions for
process exhaust ventilation. Improvement of work practices and procedures or the pow-
der feeding system could also be implemented in order to minimize particle generation in
all the processes. Dust respirators should be used during cleaning procedures as described
by NIOSH [97] if technical solutions cannot be achieved.
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4.3. Environmental Emissions
No measurable impact from the paint production activities were found in the vicinity
of the plant due to the low particulate emissions at the stacks and absence of accumulative
conditions (i.e., no dispersion with winds < 1 m s
−1
). Particle release to the outdoor envi-
ronment varied from 6 to 20 g ton
−1
in TSP at a concentration of 0.6–9.7 mg m
−3
depending
on the powder. Particularly, the estimated amount of TiO
2
released to outdoors was 0.9
kg per year. Used TiO
2
and fillers were always detected by TEM analysis in the samples
collected at the stack emissions.
Due to the rapid dilution processes, it is not expected that particles released to the
environment would generate major environmental and health impacts.
5. Conclusions
A field measurement campaign was conducted at a paint manufacturer aiming to
quantify workers personal exposures and environmental release of particulate matter dur-
ing production of different paint batches, which involved pouring of different pigments
and fillers.
Airborne particle number concentration (1 × 10
3
–1.0 × 10
4
cm
−3
), respirable mass (0.06–
0.6 mg m
−3
), PM
10
(0.3–6.5 mg m
−3
), and BC (431–696 ng m
−3
) were measured during pour-
ing processes. The variations in particle concentrations at the near field and breathing
zone were found to be dependent on powder pouring rate, and number of repetitions.
Emissions from pouring activities were found to be dominated by coarser particles > 300
nm up to 7 µm. Even though the pigment/filler powders used at the paint manufacturer
were not identified as NMs by the manufacturers, release of UFPs to workplace air and
outdoor environment via the stacks during pouring events, especially for TiO
2
and clays,
was also confirmed. Background nanoscale particles were mostly soot infiltrated from
outdoors.
In this study, none of the exposure limits set by the Danish Working Environment
Authority for total dust, total TiO
2
, respirable mineral clays, and airborne organic com-
pounds were exceeded during any working day.
An interesting finding from this study was the different responses of the diffusion
charger DM when compared to other instruments (e.g., ELPI, OPS, and NanoScan) during
pouring processes. It may be concluded that the performance of DMs is not optimal when
exposure scenarios are characterized by coarse particles, and therefore that their use for
quantification should be discouraged in these cases.
Regarding the environmental release, low emission factor was found at the stacks,
depending on the powder (6–20 g ton
−1
in TSP at a concentration of 0.6–9.7 mg m
−3
). How-
ever, no measurable levels were detected in outdoor integrative samplings. Due to the
rapid dilution processes, it is not expected that particles released to the environment
would generate major impacts.
Overall, the obtained emission source terms will be valuable contribution for occu-
pational and environmental exposure assessment model development and testing. Alt-
hough no ambient pollution at the site could be detected, the atmospheric modeling could
confirm the high level of dilution before the impact of the plume on the ground.
Supplementary Materials:
The following are available online at www.mdpi.com/1660-
4601/18/2/418/s1. Figure S1. Pictures of the indoor and outdoor measurement sites; Figure S2. Trans-
mission electron microscopy images of: (a) TiO
2
pigment (Tioxid TR81); (b) functionalized Al
2
Si
2
O
5
(OpTiMat® 2550); (c) Microspheres (Expancel); (d) calcined clay (PoleStar™ 200P); (e) Calcined ka-
olinite (Ultrex 96); (f) Dolomite (Microdol 1); g) Talc (Finntalc M15); h) calcite, (Socal® P2); Figure
S3. Time series during non-working hours of (a) total particle number concentration, and (b) particle
number size distributions measured by NanoScan (NS) and optical particle sizer (OPS) at NF and
mean particle size diameter measured by DiSCmini (DM) at near field (NF) and far field (FF) in the
mixing station. The blue horizontal line show the filter collection time at NF and FF; Figure S4. Ex-
ample of microscope images of background particles collected in the mixing station during non-
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working hours; Figure S5. (a) Time series of indoor black carbon (BC) and PM
10
levels monitored in
the mixing station, and (b) Regression analysis for the BC measured with sensor BC ABCD and
AE33 based on 1-min resolution data; Figure S6. Time series during pouring alumino-silicate clay
(Al
2
Si
2
O
5
, OpTiMat) involved in the paint formulation of batch #1 at the MS of (a) total particle num-
ber concentration, and (b) particle number size distributions measured by NanoScan (NS) and OPS
at NF and mean particle size diameter measured by DiSCmini (DM) at NF and FF; Figure S7. Time
series during pouring microspheres involved in the paint formulation of batch #1 at the MS of (a)
total particle number concentration, and (b) particle number size distributions measured by ELPI at
NF and mean particle size diameter measured by DiSCmini (DM) at NF and FF. The blue horizontal
line shows the filter sampling period; Figure S8. Time series during pouring TiO
2
involved in the
paint formulation of batch #1 at the MS of (a) total particle number concentration, and (b) particle
number size distributions measured by ELPI at NF and mean particle size diameter measured by
DiSCmini (DM) at NF and FF. The blue horizontal line shows the filter sampling period; Figure S9.
Time series during pouring TiO
2
involved in the paint formulation of batch #2 at the MS of (a) total
particle number concentration, and (b) particle number size distributions measured by ELPI at NF
and mean particle size diameter measured by DiSCmini (DM) at NF and FF. The blue horizontal
line shows the filter sampling period; Figure S10. Averages of near field (NF) particle number size
distributions for background (BG), and pouring events at the mixing station; Figure S11. Example
of transmission electron microscopy images of particles collected NF during TiO
2
pouring activities
at mixing station; Figure S12. Time series during pouring activities involved in the paint formulation
of batch #1 at the PS of (a) total particle number concentration, and (b) particle number size distri-
butions measured by ELPI at NF and mean particle size diameter measured by DM at NF and FF.
The blue horizontal line shows the filter sampling period; Figure S13. Time series during pouring
CaCO
3
involved in the paint formulation of batch #3 at the PS of (a) total particle number concen-
tration, and (b) particle number size distributions measured by ELPI at NF and mean particle size
diameter measured by DiSCmini (DM) at NF and FF. The blue horizontal line shows the filter sam-
pling period; Figure S14. Averages of near field (NF) particle number size distributions for back-
ground (BG), and pouring events at the pouring station; Figure S15. Particle size distribution by
ELPI at the mixing exhaust stack; Figure S16. Particle size distribution by ELPI at the pouring stack;
Figure S17. Example of transmission electron microscopy images of particles collected in the stack
emissions during TiO
2
and calcined clay pouring; Table S1. Semi-quantitative air concentrations of
tentatively identified organic compounds in decane equivalents in different work scenarios; Table
S2. Elemental concentrations and emission factors measured in TSP using the reference method and
in PM
2,5
, PM
1
, PM
0,5
, PM
0,2
fractions obtained by the DGI impactor for the mixing stack.
Author Contributions:
Conceptualization, A.S.F., A.-K.V., T.K., A.S., O.A.-C., S.F., A.D., N.K., I.F.,
I.K.K. K.A.J. and J.K.; methodology, A.S.F., A.-K.V., T.K., A.S., O.A.-C., S.F., A.D., N.K., I.F., I.K.K.
K.A.J. and J.K.; formal analysis, A.S.F., A.-K.V., T,K., A.S., O.A.-C., S.F., A.D., N.K., I.F., S.H.N., N.S.,
P.A.C., B.X.N.L. and V.K.-S.; investigation, A.S.F., A.-K.V., T.K., A.S., O.A.-C., S.F., A.D., N.K., I.K.K.
and J.K.; resources, A.-K.V., T.K., A.S., O.A.-C., I.K.K., C.D., A.V.V. and S.V.-C.; data curation, A.S.F.,
A.-K.V., T.K., A.S., O.A.-C., S.F., A.D., N.K., I.F., I.K.K. and J.K.; writing—original draft preparation,
A.S.F. and A.D.; writing—review and editing, A.S.F., A.-K.V., T.K., A.S., O.-A.-C., S.F., N.K., I.F.,
I.K.K., C.D., A.V.V., S.V.-C., A.C.Ø.J., S.H.N., N.S., P.A.C., B.X.N.L., V.K.-S., K.A.J. and J.K.; visuali-
zation, A.S.F., A.-K.V., T.K., O.A.-C., S.F., A.D., N.K., I.F. and A.C.Ø.J.; supervision, J.K.; project ad-
ministration, K.A.J.; funding acquisition, I.K.K. and K.A.J. All authors have read and agreed to the
published version of the manuscript.
Funding:
This work was supported by caLIBRAte Project, which was funded by the European Un-
ion’s Horizon 2020 research and innovation program under grant agreement No 686239. Smart
Monitoring of Infrastructure and the Environment (DigiMON), cofinanced by the Danish Agency
for Institutions and Educational Grants.
Institutional Review Board Statement:
Not applicable
Informed Consent Statement:
Not applicable
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Acknowledgments:
The authors acknowledge the paint manufactory in which measurements
were carried out for their technical support.
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Conflicts of Interest:
The authors declare that they have no known competing financial interests
or personal relationships that could have appeared to influence the work reported in this paper.
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