# Workplace Exposure to Nanoparticles during Thermal Spraying of Ceramic Coatings

Apostolos Salmatonidis, Carla Ribalta, Vicenta Sanfélix, Spyridon Bezantakos, George Biskos, Adriana Vulpoi, Simon Simion, Eliseo Monfort, Mar Viana

► To cite this version:

Apostolos Salmatonidis, Carla Ribalta, Vicenta Sanfélix, Spyridon Bezantakos, George Biskos, et al. Workplace Exposure to Nanoparticles during Thermal Spraying of Ceramic Coatings. Annals of Work Exposures and Health, 2019, 63 (1), pp.91-106. 10.1093/annweh/wxy094. hal-04295063

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Original Article

# Workplace Exposure to Nanoparticles during Thermal Spraying of Ceramic Coatings

Apostolos Salmatonidis¹,²,* ,³, Carla Ribalta¹,², Vicenta Sanfélix³, Spyridon Bezantakos⁴, George Biskos⁵, Adriana Vulpoi⁶, Simon Simion⁶, Eliseo Monfort³ and Mar Viana¹

¹Institute of Environmental Assessment and Water Research (IDAEA-CSIC), 08034 Barcelona, Spain;
²Department of Chemical Engineering and Analytical Chemistry, Faculty of Chemistry, University of Barcelona, 08028 Barcelona, Spain;
³Area for Sustainability, Institute of Ceramic Technology (ITC), Universitat Jaume I, 12006 Castellón, Spain;
⁴Laboratoire de Physico-Chimie de l'Atmosphère, Université du Littoral Côte d'Opale, 59140 Dunkerque, France;
⁵Energy Environment and Water Research Centre, The Cyprus Institute, 2121 Nicosia, Cyprus;
⁶Institute of Interdisciplinary Research in Bio-Nano-Sciences (ICI-BNS), Babeş-Bolyai University, 400000 Cluj-Napoca, Romania

*Author to whom correspondence should be addressed. Tel.: 0034-934006100-1543; e-mail: apostolos.salmatonidis@idaea.csic.es

Submitted 5 June 2018; revised 1 October 2018; editorial decision 5 October 2018; revised version accepted 24 October 2018.

## Abstract

Thermal spraying is widely used for industrial-scale application of ceramic coatings onto metallic surfaces. The particular process has implications for occupational health, as the high energy process generates high emissions of metal-bearing nanoparticles. Emissions and their impact on exposure were characterized during thermal spraying in a work environment, by monitoring size-resolved number and mass concentrations, lung-deposited surface area, particle morphology, and chemical composition. Along with exposure quantification, the modal analysis of the emissions assisted in distinguishing particles from different sources, while an inhalation model provided evidence regarding the potential deposition of particulate matter on human respiratory system. High particle number ($>10^6$ cm⁻³; 30–40 nm) and mass (60–600 µgPM₁ m⁻³) concentrations were recorded inside the spraying booths, which impacted exposure in the worker area (10⁴–10⁵ cm⁻³, 40–65 nm; 44–87 µgPM₁ m⁻³). Irregularly-shaped, metal-containing particles (Ni, Cr, W) were sampled from the worker area, as single particles and aggregates (5–200 nm). Energy dispersive X-ray analysis confirmed the presence of particles originated from the coating material, establishing a direct link between the spraying activity and exposure. In particle number count, 90% of the particles were between 26–90 nm. Inhaled dose rates, calculated from the exposure levels, resulted in particle number rates (n) between $353 \times 10^6–1024 \times 10^6$ min⁻¹, with 70% of deposition occurring in the alveolar region. The effectiveness of personal protective equipment (FPP3 masks) was tested under real working conditions. The proper sealing of the spraying booths was identified as a key element for exposure
reduction. This study provides high time-resolved aerosol data which may be valuable for validating indoor aerosol models applied to risk assessment.

**Keywords:** exposure assessment; inhalation exposure; inhalation model; modal analysis; nanoparticles; occupational health; process-generated nanoparticles

## Introduction

In numerous industrial sectors, such as petroleum (Moskowitz, 1993), naval (Baiamonte et al., 2015), automobile (Gérard, 2006), or aeronautical and space (Strangman, 1985; Pawlowski, 2008), metal structures, and mechanical parts are exposed to highly corrosive environments, and subject to mechanical and chemical abrasion. In these cases, protective ceramic coatings are widely used to prevent corrosion and wear, as well as to restore damaged surfaces (Tan et al., 1999; Lima and Marple, 2006; Toma et al., 2010). Such coatings are frequently applied using thermal spraying techniques, where the feedstock (the coating material) is projected at high temperature and velocity onto the surface to be protected or repaired. Different types of spraying torches (electric arc, plasma, flame) define the types of thermal spraying techniques (Pawlowski, 2008).

This industrial process has been reported to generate high concentrations of nanoparticles (NP, with diameter < 100 nm) at pilot-plant scale (Viana et al., 2017), which may result in occupational hazards (Hériaud-Kraemer et al., 2003). Previous studies of thermal spraying emissions using offline measurements focussed on the chemical characterization of particles collected on filters (Petsas et al., 2007; Huang et al., 2016), showing also that these sources are associated with high particle mass concentrations. Furthermore, high NP number concentrations (>10⁹ cm⁻³) were monitored directly from electric arc guns (Bémer et al., 2010). At pilot-plant scale, Viana et al. (2017) showed that NP released during atmospheric plasma spraying may impact worker exposure significantly. Despite the widespread industrial application of thermal spraying, emissions generated under real-world operating conditions are not well studied. Similarly to thermal spraying, the emissions of NP from other industrial processes and their impact on worker exposure is a growing topic of research (Kuhlbusch et al., 2004; Fujitani et al., 2008; Losert et al., 2014; Fonseca et al., 2015, 2016, 2018; Koponen et al., 2015; Ding et al., 2017; Viitanen et al., 2017; Koivisto et al., 2018; Salmatonidis et al., 2018), and requires further investigations due to the large variety of processes and exposure scenarios. Awareness for

nanomaterial release was raised (Maynard et al., 2006; Poland et al., 2008; Seaton et al., 2010) since the use of manufactured nanomaterials (MNMs) in workplaces started. However, NP emissions can arise from multiple processes that neither produce nor use nanomaterials, which are referred as process generated nanoparticles (PGNP) (Van Broekhuizen et al., 2012).

Pope et al. (1995) associated air pollution with adverse health effects based on epidemiological evidence. Further epidemiological and toxicological studies showed that PM₂.₅ (particles with diameter ≤2.5 µm) is a health hazardous pollutant (Pope et al., 2002; Pope and Dockery, 2006; Gakidou et al., 2017; WHO, 2017; Landrigan et al., 2018). More specifically, aerosol particles <100 nm (ultrafine) are linked to adverse health effects mainly due to exposure through inhalation (Oberdörster, 2001; Ibald-Mulli et al., 2002; Araujo et al., 2008; Hoek et al., 2010; Knibbs et al., 2011; Landrigan et al., 2018). In indoor air and more specifically in industrial settings, PGNPs pose an occupational risk, since they are unintentionally generated and released to the worker area, and they are potential health hazards (Koivisto et al., 2014). Health risks need to be dealt with by means of both technological and nontechnological mitigation strategies (Shaffer and Rengasamy, 2009; Hallé et al., 2015; Ganser and Hewett, 2017; Hewett and Ganser, 2017).

The aim of this work was to assess NP emissions during thermal spraying of ceramic coatings onto metallic surfaces and their impact on inhalation exposure, under actual operating conditions in a real-world industrial setting. A near field (NF)/far field (FF) approach as used by Koivisto et al. (2015), among others, was applied. Workplace exposure monitoring strategies applied followed good practice as described by Asbach et al. (2016) and Brouwer et al. (2014), which were the base for internationally recognized standard procedures (OECD, 2017). Modal analysis based on time-resolved particle number concentrations was used to quantitatively understand the size distribution and modal dependence of the aerosols generated during thermal spraying (Hussein et al., 2005). Finally, a case study testing the effectiveness of mitigation strategies is presented.
# Methods and materials

## Work environment

Monitoring of thermal spraying was carried out at an industrial-scale precision engineering workshop (T.M. Comas) located in the vicinity of Barcelona, Spain. Measurements were carried out over a 5-day period in April 2017, and did not interfere with the usual operating conditions in the plant. The thermal spraying facilities are schematically depicted in Fig. 1 (top). Three thermal spraying booths were located in an area of ~240 m² (14 m wide and 17 m in length), including also a storage and a central corridor. In all cases, the operators of the thermal spraying equipment worked inside and outside the booths during spraying, and each booth had doors which were not always closed. Two small-scale sand blasting boxes for polishing the final pieces were located next to the spraying booths, but they were not operated

simultaneously to the activities reported in this work. A general ventilation system operated constantly in the central area, with three extractors having a flow of 11 800 m³ h⁻¹ each (data provided by the company, not measured directly in this work). Additionally, in each of the plasma booths individual localized extraction systems were available (Fig. 1a, Table 1). Information regarding the individual extraction air flowrates was only available for Booth 3 (6500 m³ h⁻¹). The operational characteristics of each of the spraying activities and booths are summarized in Table 1. Because the duration of the individual spraying activities varied according to the parts to be coated, mean values of spraying duration and repetitions were different for each booth (Table 1).

## Thermal spraying techniques and feedstock

Two different types of thermal spraying techniques were assessed:

<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Schematic representation of the thermal spraying section of the investigated facility. (Extraction*: data not available). (b) Instrument deployment in the near field location. (c) Booth #3, showing the thermal spraying robot and the operator processing one of the work-pieces.</figcaption></figure>
<table><caption>**Table 1.** Operational characteristics of each of the spraying booths and locations in the thermal spraying facility.</caption><thead><tr><th>Characteristics</th><th>Booth 1</th><th>Booth 2</th><th>Booth 3</th><th>Central area</th></tr></thead><tbody><tr><td>Thermal spraying technique</td><td>APS</td><td>HVOF</td><td>HVOF</td><td>None</td></tr><tr><td>Spraying duration</td><td>20–30 min</td><td>5–10 min</td><td>5–10 min</td><td>None</td></tr><tr><td>No. of repetitions/half-day</td><td>3</td><td>2</td><td>7–9</td><td>None</td></tr><tr><td>Type of parts coated</td><td>Large, single part</td><td>Large, single part</td><td>Small, several parts</td><td>None</td></tr><tr><td>Door</td><td>Mostly closed</td><td>Open</td><td>Open/closed</td><td>None</td></tr><tr><td>Volume (ca., m³)</td><td>84</td><td>114</td><td>68</td><td>465</td></tr><tr><td>Local exhaust/ventilation flow</td><td>NA</td><td>NA</td><td>6500 m³ h⁻¹</td><td>3 × 11 800 m³ h⁻¹</td></tr><tr><td>Personal protective equipment</td><td></td><td></td><td></td><td></td></tr><tr><td>Respirators</td><td>Pressurized helmet<br>helmet, FPP3 mask</td><td>FPP3 mask</td><td>FPP3 mask</td><td>None</td></tr><tr><td>Cloths</td><td>Protective jacket</td><td>None</td><td>Protective jacket</td><td>None</td></tr><tr><td>Gloves</td><td>High temperature<br>gloves</td><td>None</td><td>High temperature<br>gloves</td><td>None</td></tr></tbody></table>

NA, not available; APS, atmospheric plasma spraying; HVOF, high velocity oxy-fuel coating spraying.

- Atmospheric plasma spraying (APS), characterized by high temperatures (5000–20 000°C) and projection velocities of 200–500 m s⁻¹. This technique was applied in plasma Booth 1 (Fig. 1). APS is generally used to project oxides and metals, which in this case of our measurements were a TiO₂-Al₂O₃ blend and a Cr-Ni blend (two different feedstock materials that were applied separately).

- High velocity oxy-fuel coating spraying (HVOF), characterized by high velocities (425–1500 m s⁻¹) and lower temperatures (2900°C). This type of spraying was applied in Booths 2 and 3 (Fig. 1b, bottom). Due to the lower temperatures compared to APS, HVOF is frequently used to project carbides and metals. During our measurements a WC-Co-Cr-Ni blend feedstock was used in Booth 2, and a WC-CrC-Ni blend in Booth 3.

Additional information about the feedstock powders is provided in Supplementary Table S1 (available at *Annals of Work Exposures and Health* online).

## Particle monitoring and sampling

Particle number concentration (N), size-segregated mass concentrations (PMₓ), size distribution, mean diameter ($D_p$) and lung-deposited surface area (LDSA) were monitored at NF and FF locations. The NF location was inside each of the spraying booths, while the FF was next to the storage area in the case of Booth 3 and in the middle of the central area in the case of Booths 2 and 3 (Fig. 1a). The monitoring instruments in the FF and their inlets were located between 0.7 and 1.5 m above ground and were not placed directly inside the breathing

zone (Ojima, 2012). The resulting measurements were considered representative of worker exposure, by assuming that the concentrations in the NF and FF were well-mixed. The online particle monitors deployed were:

* A miniature diffusion size classifier “DiSCmini” (TESTO AG), that can measure particles having sizes from 10 to 700 nm, and can report total particle number (N), mean particle diameter ($D_p$), and LDSA with a 10 s time resolution. This instrument was deployed both in the NF and FF locations.

* A mini laser aerosol spectrometer “Mini-LAS 11-R” (GRIMM), that was used to measure particles in the size range 0.25–32 µm, and report total and size-segregated particle mass concentrations in 31 channels with a 6 s time resolution. This instrument was deployed both in the NF and FF locations.

* An electrical mobility spectrometer “Nanoscan-SMPS” (TSI model 3910), covering particles in the size range 10–420 nm, that was used to measure the particle mobility size distributions in 13 channels with a 1 min time resolution. This instrument was deployed only in the FF location.

* A mini wide range aerosol spectrometer “Mini-WRAS” (GRIMM), that can measure particles having sizes from 10 nm to 35 µm, for monitoring the particle mass concentrations across 41 channels. This instrument was also deployed in the FF location.

The mini-LAS units were operated enclosed in a protective case with a vertical stainless steel inlet (Fig. 1b). Inlet extension tubes were not used for any of the other instruments. The default (manufacturer) impactor was used for the DiSCmini (cut-off diameter
of 700 nm) and the default cyclone (cut-off diameter of 550 nm) was used for the Nanoscan. The DiscMini and MiniWRAS instruments were inter-compared before the measurements at an air quality monitoring station in Barcelona (Spain), using ambient air aerosols. The comparison resulted in R² coefficients >0.89, and as result the instruments were considered comparable for the purpose of our study (Supplementary Table S2, available at Annals of Work Exposures and Health online).

In addition to the online measurements described above, samples were collected on electron microscopy grids (Agar scientific Quantifoil 200 Mesh Au) for offline morphological and physicochemical particle characterization. A Leland pump (SKC, Inc.) with a flow of 5 l/min was connected to a cassette (SKC inlet diameter 1/8 inch and filter support pads that were 25 mm in diameter) to which the microscopy grid was attached. The morphology and primary particle size of the particles collected were analysed using transmission electron microscopy (TEM; Jeol, JEM 1220, Tokyo, Japan) and TEM/HRTEM, FEI, Tecnai F20 (200 KV, Eindhoven, Netherlands) coupled with an Energy-Dispersive X-ray (EDX) spectrometer, following a similar method as Voliotis et al. (2014).

## Modal analysis

While modal analysis has been applied to characterize the size distributions of atmospheric aerosols regarding the physicochemical processes they have been involved in (Hussein et al., 2005), this type of analysis is seldom applied to indoor or, more specifically, industrially produced aerosols.

In the present study, NanoScan measurements were expressed as dN/dLogdp distributions to apply modal analysis, because this method assumes that particle number concentrations are log normally distributed across their size range. Their distribution can be analysed as three modes: Mode<sub>10-25nm</sub>, Mode<sub>26-90nm</sub>, and Mode<sub>91-660nm</sub> which includes mainly accumulation mode particles. The algorithm for modal analysis uses a non-linear least square fitting, based on the interior-reflective Newton method (Coleman and Li, 1994, 1996). The three lognormal fitted curves and their parameters (geometric standard deviation, GSD; geometric mean diameter, GMD; and mode number concentration, N<sub>i</sub>) were calculated following the same assumptions and conditions as Hussein et al. (2005). The modal analysis was performed exclusively on the size-distribution data obtained with NanoScan.

## Inhalation model

The inhalation dose of deposited particles in the respiratory system was quantified by multiplying particle size concentrations on the worker area by the ICRP human respiratory tract model deposition probability (Cousins et al., 2011). A respiratory volume of 25 l/min was used, which corresponds to male respiration during light exercise (Koivisto et al., 2012). The regional dose was calculated for head airways, tracheobronchial and alveolar regions by using simplified deposition fraction equations for the ICRP model as described by Hinds et al. (1999). Particles were assumed to preserve their size during inhalation and the calculation was based on the mobility diameter. Background aerosol particles were assumed to be spherical and to have a density of 1.5 g cm⁻³ (Martins et al., 2015). The density of particles emitted during thermal spraying was set equal to the density of the feedstock material 4.3 g cm⁻³, given that the particles released were found by EDX analysis to have similar composition with the feedstock (see next section). It should be noted that the use of the bulk density of the feedstock material probably overestimates the resulting particle concentrations, because particles may have vaporized and condensed to smaller particles. Results from the NanoScan and MiniWras instruments were combined to obtain a 10 nm–35 µm particle size distribution for the FF (Koivisto et al., 2014). NanoScan size bins between 11.5 and 86.6 nm were used, whereas bins ranging from 139 nm to 35 µm were taken from the MiniWras. Between 86.6 and 139 nm a combined channel (108.6 nm) was created.

The particle active surface area was calculated by applying particle size distribution obtained to the equation (1) described by Heitbrink et al. (2009) and Koivisto et al. (2012):

$$s = \frac{3\pi\lambda D_b}{C_c (D_b) \delta} \quad (1)$$

where λ is the mean free path for air, 0.066 µm, and δ is the scattering parameter for air, 0.905. D<sub>b</sub> is the mobility diameter and C<sub>c</sub> the slip correction factor for the corresponding aerodynamic or mobility particle size. This calculation refers to particles <700 nm according to Heitbrink et al. (2009) and Keller et al. (2001).

The particle mass was calculated by using mobility particle diameter and effective density (Koivisto et al., 2012):

$$m = \rho_{eff} \frac{\pi}{6} D_b^3 \quad (2)$$
where ρ<sub>eff</sub> is the effective density (DeCarlo et al., 2004). The effective density used was 1.5 g cm<sup>-3</sup> for background particles and 4.3 g cm<sup>-3</sup> for PGNPs.

**Results and discussion**

Particle emissions and impact on exposure

Activities in the thermal spraying facilities were planned by the company for half-day periods. While Booth 1 was active during 2 half-days, Booths 2 and 3 were operational for 4 half-days each. The data obtained were analysed considering two phases: a pre-process period (background) consisting of the 90-min lunch break when all activity ceased, and the activity periods when particle emissions were simultaneously monitored in the NF and in the FF location. The activity periods were different for each booth (Table 1): while spraying was longer and less repetitive in Booth 1 (20–30 min, 3 repetitions/half-day), it had a shorter duration and higher frequency in Booth 3 (5–10 min, 7–9 repetitions/half-day). In Booth 2 the spraying activity duration was highly variable (0.5–10 min) and, due to technical problems, there were deviations in the frequency of spraying applications. Hence, the dataset obtained was not representative of regular working days in Booth 2.

In Table 2, average N, Dₚ, LSDA, and PM₁ values for each of the spraying activities and booths monitored are shown. A representative time series during the process in Booth 3 is depicted in Fig. 3 (NF) and Fig. 4 (FF) during one of the days (26 April 2017), to thoroughly analyse the emission patterns and impacts on exposure of the released NP. The time series for other days and booths are shown in Supplementary Figs. S1–S4 (available at *Annals of Work Exposures and Health* online). In the case of Booth 3 each thermal spraying activity lasted for 5–10 min (Fig. 2); subsequently, the worker entered the booth and swapped the metal part which was coated with a new one. The process was repeated to 7–9 times/ half-day.

In Fig. 2, N, D<sub>p</sub>, and size segregated mass concentrations (PM<sub>1</sub>, PM<sub>2.5</sub>, PM<sub>10</sub>, alveolar) values monitored inside Booth 3 (NF) are presented. The start of the spraying activity is evident by the rapid increase of N, which exceeded the instrument's (DiSCMini) monitoring range during all repetitions (maximum N = 4 × 10<sup>6</sup> cm<sup>-3</sup>). Particle number emissions were directly correlated with decreased D<sub>p</sub>, which ranged between 30 and 35 nm during spraying and increased to 45–55 nm between repetitions. The patterns observed for the different spraying activities within each half-day showed good repeatability. N and D<sub>p</sub> values during

the process (3.8 × 10⁶ cm⁻³, 28.6 nm) were markedly
different from background aerosols (N < 3.0 × 10⁴ cm⁻³,
Dₚ = 57.5 nm) monitored during the inactivity period
on that day (Fig. 2, Table 2). The same was true for
particle mass concentrations (Fig. 2b), which reached up
to 4 × 10³ µg m⁻³ for the alveolar size fraction (approxi-
mately similar to PM₄) while they remained <30 µg m⁻³
during the inactivity period. This result may be attributed
to the release of fine and coarse particles (>1 µm), in
addition to NPs, during thermal spraying. The presence
of fine and coarse particles was also confirmed by TEM
analysis (Supplementary Fig. S5, available at *Annals of
Work Exposures and Health* online).

NP emitted during spraying in Booth 3 were analysed by TEM (Fig. 3), providing evidence that the particles in the collected samples were generated by different mechanisms. Particles collected exhibited a diversity of morphological characteristics, varying both in shape (e.g. spherical, fractal, and star-like aggregates) and size (from ca. 5–500 nm). Possible emission mechanisms are mechanical attrition as the feedstock particles crush onto the metal surface with high kinetic energy resulting in irregular-shaped particles, but also melting–evaporation–condensation of the feedstock material which would result in spherical particles (Fonseca et al., 2015; Viana et al., 2017). EDX analysis of the airborne particles collected on TEM grids confirmed that their chemical composition was similar to that of the feedstock (WC, CrC, Ni), demonstrating that particle release originated from the feedstock and not from other secondary or confounding sources. The thermal spraying processes investigated were not intended to produce NPs. Hence, the NPs emitted from the micro-scaled feedstock (Supplementary Table S1, available at *Annals of Work Exposures and Health* online) were PGNP. In the case of fine and coarse particles, emissions probably resulted when the feedstock particles projected are in the outer plasma stream and do not reach high temperatures, they remain solid and are not deposited effectively on the metal surface.

In the FF location, the impact of NP, fine, and coarse particle emissions on exposure was assessed in terms of size-resolved N and PMₓ (Fig. 4). The same repetitive patterns which were monitored inside the spraying booth (Fig. 3) were also detected in the worker area, indicating the representativeness of the results as well as clear impacts on exposure. The patterns were different in the morning and afternoon periods because the door of the booth was kept open while spraying during the morning, and closed in the afternoon. N values that reached up to 1.7 × 10⁶ cm⁻³ (with the NanoScan instrument), were
<table><caption>**Table 2.** Mean particle number (N), particle diameter (Dₚ), lung deposition surface area (LDSA) from DISCmini and mass (PM₁) concentrations during each of the activity periods monitored. Each half-day is labelled as (M) morning, (A) afternoon, or (cs) case study. Statistical values: mean, standard deviation (SD), maximum (Max), minimum (Min) of each process calculated from non-normalized values for the total activity duration monitored during our studies (24–28 April 2017; DISCmini).</caption><thead><tr><th rowspan="2">Date</th><th rowspan="2">Feedstock</th><th colspan="4">Near field (inside booths)</th><th colspan="4">Far field (worker area)</th><th colspan="4">Inactivity</th></tr><tr><th>N (cm<sup>-3</sup>)</th><th>D<sub>p</sub> (nm)</th><th>LDSA (µm<sup>2</sup> cm<sup>-3</sup>)</th><th>PM<sub>1</sub> (µg m<sup>-3</sup>)</th><th>N (cm<sup>-3</sup>)</th><th>D<sub>p</sub> (nm)</th><th>LDSA (µm<sup>2</sup> cm<sup>-3</sup>)</th><th>PM<sub>1</sub> (µg m<sup>-3</sup>)</th><th>N (cm<sup>-3</sup>)</th><th>D<sub>p</sub> (nm)</th><th>LDSA (µm<sup>2</sup> cm<sup>-3</sup>)</th><th>PM<sub>1</sub> (µg m<sup>-3</sup>)</th></tr></thead><tbody><tr><td colspan="14" style="text-align: left; font-weight: bold;">Booth 1</td></tr><tr><td>27/04–M</td><td>Cr/Ni</td><td>2.0 × 10<sup>6</sup></td><td>31.5</td><td>4.0 × 10<sup>3</sup></td><td>5.3 × 10<sup>1</sup></td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>2.3 × 10<sup>4</sup></td><td>51.6</td><td>6.6 × 10<sup>1</sup></td><td>2.8 × 10<sup>1</sup></td></tr><tr><td>27/04–A</td><td>Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub></td><td>1.6 × 10<sup>6</sup></td><td>36.8</td><td>3.1 × 10<sup>3</sup></td><td>6.9 × 10<sup>1</sup></td><td>5.8 × 10<sup>4</sup></td><td>40.0</td><td>1.2 × 10<sup>2</sup></td><td>4.4 × 10<sup>1</sup></td><td>2.3 × 10<sup>4</sup></td><td>51.6</td><td>6.6 × 10<sup>1</sup></td><td>2.8 × 10<sup>1</sup></td></tr><tr><td>Mean</td><td></td><td>1.8 × 10<sup>6</sup></td><td>34.2</td><td>3.6 × 10<sup>3</sup></td><td>6.1 × 10<sup>1</sup></td><td>5.8 × 10<sup>4</sup></td><td>40.0</td><td>1.2 × 10<sup>2</sup></td><td>4.4 × 10<sup>1</sup></td><td>4.4 × 10<sup>3</sup></td><td>1.4</td><td>1.4 × 10<sup>1</sup></td><td>8.2 × 10<sup>1</sup></td></tr><tr><td>SD</td><td></td><td>2.0 × 10<sup>5</sup></td><td>2.6</td><td>4.3 × 10<sup>2</sup></td><td>8.0 × 10<sup>1</sup></td><td>1.7 × 10<sup>4</sup></td><td>2.9</td><td>2.8 × 10<sup>1</sup></td><td>6.6 × 10<sup>2</sup></td><td>3.5 × 10<sup>4</sup></td><td>53.4</td><td>1.0 × 10<sup>2</sup></td><td>4.8 × 10<sup>1</sup></td></tr><tr><td>Max</td><td></td><td>2.8 × 10<sup>6</sup></td><td>63.7</td><td>5.1 × 10<sup>3</sup></td><td>2.6 × 10<sup>2</sup></td><td>1.1 × 10<sup>3</sup></td><td>49.1</td><td>2.0 × 10<sup>2</sup></td><td>6.4 × 10<sup>1</sup></td><td>3.5 × 10<sup>4</sup></td><td>49.2</td><td>4.9 × 10<sup>1</sup></td><td>1.9 × 10<sup>1</sup></td></tr><tr><td>Min</td><td></td><td>5.2 × 10<sup>4</sup></td><td>12.1</td><td>4.6 × 10<sup>2</sup></td><td>3.0 × 10<sup>1</sup></td><td>3.4 × 10<sup>4</sup></td><td>33.2</td><td>8.5 × 10<sup>1</sup></td><td>3.2 × 10<sup>1</sup></td><td>1.7 × 10<sup>4</sup></td><td></td><td></td><td></td></tr><tr><td colspan="14" style="text-align: left; font-weight: bold;">Booth 2</td></tr><tr><td>24/04–A</td><td>WC/Cr/Co/Ni</td><td>3.4 × 10<sup>6</sup></td><td>37.2</td><td>5.8 × 10<sup>3</sup></td><td>1.7 × 10<sup>2</sup></td><td>3.5 × 10<sup>4</sup></td><td>75.6</td><td>1.5 × 10<sup>2</sup></td><td>NA</td><td>7.0 × 10<sup>3</sup></td><td>86.1</td><td>4.0 × 10<sup>1</sup></td><td>2.0 × 10<sup>1</sup></td></tr><tr><td>25/04–M</td><td>WC/Cr/Co/Ni</td><td>5.3 × 10<sup>6</sup></td><td>28.3</td><td>7.6 × 10<sup>3</sup></td><td>1.0 × 10<sup>2</sup></td><td>5.4 × 10<sup>4</sup></td><td>54.3</td><td>1.7 × 10<sup>2</sup></td><td>8.3 × 10<sup>1</sup></td><td>6.3 × 10<sup>3</sup></td><td>123.3</td><td>4.9 × 10<sup>1</sup></td><td>2.9 × 10<sup>1</sup></td></tr><tr><td>26/04–M</td><td>WC/Cr/Co/Ni</td><td>6.0 × 10<sup>5</sup></td><td>55.7</td><td>1.9 × 10<sup>3</sup></td><td>3.8 × 10<sup>1</sup></td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>3.7 × 10<sup>4</sup></td><td>56.8</td><td>6.7 × 10<sup>1</sup></td><td>2.9 × 10<sup>1</sup></td></tr><tr><td>27/04–cs</td><td>WC/Cr/Co/Ni</td><td>1.6 × 10<sup>6</sup></td><td>42.3</td><td>3.4 × 10<sup>3</sup></td><td>1.1 × 10<sup>2</sup></td><td>1.2 × 10<sup>3</sup></td><td>61.6</td><td>3.0 × 10<sup>2</sup></td><td>9.3 × 10<sup>1</sup></td><td>2.3 × 10<sup>4</sup></td><td>51.6</td><td>6.6 × 10<sup>1</sup></td><td>2.8 × 10<sup>1</sup></td></tr><tr><td>Mean</td><td></td><td>2.7 × 10<sup>6</sup></td><td>40.9</td><td>4.7 × 10<sup>3</sup></td><td>1.0 × 10<sup>2</sup></td><td>6.9 × 10<sup>1</sup></td><td>63.8</td><td>2.1 × 10<sup>2</sup></td><td>6.5 × 10<sup>1</sup></td><td>1.8 × 10<sup>4</sup></td><td>79.5</td><td>5.6 × 10<sup>1</sup></td><td>2.5 × 10<sup>1</sup></td></tr><tr><td>SD</td><td></td><td>1.8 × 10<sup>6</sup></td><td>9.9</td><td>2.2 × 10<sup>3</sup></td><td>4.7 × 10<sup>1</sup></td><td>3.5 × 10<sup>4</sup></td><td>8.8</td><td>6.6 × 10<sup>1</sup></td><td>3.2 × 10<sup>1</sup></td><td>1.3 × 10<sup>4</sup></td><td>28.5</td><td>1.1 × 10<sup>1</sup></td><td>3.9 × 10<sup>1</sup></td></tr><tr><td>Max</td><td></td><td>6.3 × 10<sup>6</sup></td><td>72.7</td><td>8.9 × 10<sup>3</sup></td><td>1.8 × 10<sup>3</sup></td><td>3.1 × 10<sup>5</sup></td><td>107.8</td><td>6.2 × 10<sup>2</sup></td><td>1.6 × 10<sup>2</sup></td><td>3.1 × 10<sup>5</sup></td><td>141.6</td><td>3.0 × 10<sup>2</sup></td><td>4.8 × 10<sup>1</sup></td></tr><tr><td>Min</td><td></td><td>7.6 × 10<sup>4</sup></td><td>19.8</td><td>2.1 × 10<sup>2</sup></td><td>3.3 × 10<sup>1</sup></td><td>9.3 × 10<sup>3</sup></td><td>45.7</td><td>4.3 × 10<sup>1</sup></td><td>4.1 × 10<sup>1</sup></td><td>2.8 × 10<sup>3</sup></td><td>50.5</td><td>2.1 × 10<sup>1</sup></td><td>1.9 × 10<sup>1</sup></td></tr><tr><td colspan="14" style="text-align: left; font-weight: bold;">Booth 3</td></tr><tr><td>26/04–M</td><td>WC/Cr/Cr/Ni</td><td>3.8 × 10<sup>6</sup></td><td>28.6</td><td>5.7 × 10<sup>3</sup></td><td>7.0 × 10<sup>2</sup></td><td>3.6 × 10<sup>5</sup></td><td>33.2</td><td>5.6 × 10<sup>2</sup></td><td>1.0 × 10<sup>2</sup></td><td>3.0 × 10<sup>4</sup></td><td>57.5</td><td>1.1 × 10<sup>2</sup></td><td>2.9 × 10<sup>1</sup></td></tr><tr><td>26/04–A</td><td>WC/Cr/Cr/Ni</td><td>3.8 × 10<sup>6</sup></td><td>30.2</td><td>6.1 × 10<sup>3</sup></td><td>7.4 × 10<sup>1</sup></td><td>1.1 × 10<sup>3</sup></td><td>36.6</td><td>2.1 × 10<sup>2</sup></td><td>6.3 × 10<sup>1</sup></td><td>5.8 × 10<sup>4</sup></td><td>33.9</td><td>1.0 × 10<sup>2</sup></td><td>2.4 × 10<sup>1</sup></td></tr><tr><td>28/04–M</td><td>WC/Cr/Cr/Ni</td><td>3.1 × 10<sup>6</sup></td><td>31.3</td><td>5.1 × 10<sup>3</sup></td><td>5.2 × 10<sup>2</sup></td><td>1.7 × 10<sup>5</sup></td><td>46.4</td><td>3.9 × 10<sup>2</sup></td><td>1.0 × 10<sup>2</sup></td><td>8.4 × 10<sup>1</sup></td><td></td><td></td><td></td></tr><tr><td>28/04–A</td><td>WC/Cr/Cr/Ni</td><td>2.9 × 10<sup>6</sup></td><td>32.1</td><td>4.8 × 10<sup>3</sup></td><td>5.8 × 10<sup>2</sup></td><td>1.5 × 10<sup>5</sup></td><td>42.1</td><td>3.2 × 10<sup>2</sup></td><td>8.7 × 10<sup>1</sup></td><td>4.4 × 10<sup>4</sup></td><td>45.7</td><td>1.1 × 10<sup>2</sup></td><td>2.7 × 10<sup>1</sup></td></tr><tr><td>Mean</td><td></td><td>3.4 × 10<sup>6</sup></td><td>30.6</td><td>5.4 × 10<sup>3</sup></td><td>6.4 × 10<sup>2</sup></td><td>2.0 × 10<sup>5</sup></td><td>39.6</td><td>3.7 × 10<sup>2</sup></td><td>1.6 × 10<sup>1</sup></td><td>1.4 × 10<sup>4</sup></td><td>11.8</td><td>5.1 × 10<sup>1</sup></td><td>2.6 × 10<sup>1</sup></td></tr><tr><td>SD</td><td></td><td>4.1 × 10<sup>5</sup></td><td>1.3</td><td>5.0 × 10<sup>2</sup></td><td>8.7 × 10<sup>1</sup></td><td>9.3 × 10<sup>1</sup></td><td>5.1</td><td>1.3 × 10<sup>2</sup></td><td>1.0 × 10<sup>1</sup></td><td>7.8 × 10<sup>1</sup></td><td>60.4</td><td>3.0 × 10<sup>2</sup></td><td>4.5 × 10<sup>1</sup></td></tr><tr><td>Max</td><td></td><td>6.3 × 10<sup>6</sup></td><td>52.3</td><td>8.9 × 10<sup>3</sup></td><td>1.3 × 10<sup>3</sup></td><td>1.7 × 10<sup>6</sup></td><td>69.4</td><td>2.1 × 10<sup>3</sup></td><td>2.7 × 10<sup>2</sup></td><td>7.8 × 10<sup>4</sup></td><td>29.4</td><td>5.0 × 10<sup>1</sup></td><td>2.0 × 10<sup>1</sup></td></tr><tr><td>Min</td><td></td><td>1.9 × 10<sup>5</sup></td><td>10.0</td><td>2.2 × 10<sup>2</sup></td><td>2.1 × 10<sup>1</sup></td><td>3.3 × 10<sup>4</sup></td><td>18.4</td><td>8.6 × 10<sup>1</sup></td><td>1.8 × 10<sup>1</sup></td><td>1.5 × 10<sup>4</sup></td><td></td><td></td><td></td></tr></tbody></table>

NA, data not available.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Particle concentrations monitored inside booth #3 (near field) on 26/04/2017 during spraying of feedstock WC/CrC/Ni: (a) particle number concentration and mean diameter; (b) size segregated particle mass concentrations.</figcaption></figure>

also outside this instrument's monitoring range and should be considered with high uncertainty. Size-resolved N values were highest in the range 20–85 nm ($6 \times 10^4$ particles cm$^{-3}$) during the morning working session, providing evidence that the spraying activities substantially impacted worker exposure. The fact that the personal protective equipment (FPP3 mask) was frequently removed by the worker as soon as he exited the spraying booth (between repetitions) could further burden the workers' respiratory system. As in the case of the NF, particle concentrations in the FF location were markedly higher during activity than inactivity period. In addition, the effectiveness of keeping the booth door closed as a mitigation strategy was evident: average N recorded in the worker area decreased by a factor of 0.31 in the afternoon when the door was closed, and LSDA concentrations were reduced by a factor of 0.38 (Table 2). Hence, the booth's proper sealing during activity period demonstrated a much larger influence on exposure reduction than the total duration of the activity, or its repetition frequency.

Similar findings on the importance of airtight booths had also been reported by Viana *et al.* (2017) during APS at pilot plant scale.

In terms of particle mass similar impacts and patterns were observed, although with two particle size modes: 50–500 nm and >3 µm. While the first mode corresponds to PGNP emissions, the second is attributed to direct feedstock release. Alveolar particle mass concentrations reached >300 µg m$^{-3}$ (1-min means) during the morning activity period, and 150 µg m$^{-3}$ in the afternoon.

As shown in Table 2, inside the NF concentrations ranged $1.8–3.4 \times 10^6$ cm$^{-3}$ and had a clear impact on exposure at the FF location, where concentrations in the range of $5.8 \times 10^4–2.0 \times 10^5$ cm$^{-3}$ were monitored, and were significantly higher than the background concentrations (average $3.0 \times 10^4$). Concentrations in the FF were above the nano-reference values (NRV, non-regulatory reference values for nanomaterials, based on the precautionary approach) $4 \times 10^4$ cm$^{-3}$ (Van Broekhuizen *et al.*, 2012), and orders of magnitude higher inside the spraying booths (NF). This impact was
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Particles released during spraying of W/CrC/Ni feedstock in booth #3 on 26/04/2017, analysed by TEM and showing similar chemical composition (WC, CrC, Ni) but different morphology and size. EDX spectrum confirmed the origin of the particles (WC/CrC/Ni feedstock). The spectrum is representative of particles collected on other TEM grids.</figcaption></figure>

considered statistically significant following the tiered approach as established in OECD (2015), meaning that exposure concentrations were higher than the background plus three times the standard deviation of the background concentrations. The N, LSDA, and PM values measured were comparable across the different booths, indicating that NP release is activity-dependent and not related to the specific feedstock powders applied. Although different spraying temperatures and velocities were applied in the different booths NP release was evident in all cases. This would indicate that NP emissions

are independent of these parameters or that both of the thermal spraying techniques provide the necessary conditions for PGNP emissions.

However, differences in terms of PM₁ ranged from 61 µg m⁻³ (Booth 1) to 100 µg m⁻³ (Booth 2) and 640 µg m⁻³ (Booth 3). These can be attributed to the combination of different process parameters (spraying velocity, speed) of each spraying technique and the different feedstock materials (Cr/Ni and TiO₂/Al₂O₃, Supplementary Table S1, available at *Annals of Work Exposures and Health* online). In the case of Booth 1, where two
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Size-resolved particle concentrations monitored in the worker area (far field) outside booth #3 on 26/04/2017 during spraying of feedstock WC/CrC/Ni: (a) particle number concentrations; (b) particle mass concentrations.</figcaption></figure>

different feedstock powders were applied separately, clear differences were observed in terms of PM<sub>2.5</sub> (2 × 10<sup>2</sup> versus 4 × 10<sup>2</sup> µg m<sup>-3</sup>) but not for PM<sub>1</sub> (53 versus 69 µg m<sup>-3</sup>). The feedstock powders applied in Booths 2 and 3 were similar (Supplementary Table S1, available at *Annals of Work Exposures and Health* online) resulted in higher PM<sub>1</sub> (100–640 µg m<sup>-3</sup>) than in Booth 1 (Table 2). These differences are probably associated with the physical-chemical properties of the feedstock materials rather than to the aggregate size, which as shown in Supplementary Table S1 (available at *Annals of Work Exposures and Health* online) is larger for the lowest emitter. As a result, it may be concluded that PM<sub>1</sub> emissions were independent of the feedstock and thermal spraying conditions applied, while emissions of coarse particles (>2.5 µm) were influenced by the feedstock. Further research would be necessary to interpret the different behaviour for micron-scaled particle emissions.

With regard to D<sub>p</sub> for NP, similarities were detected across booths with particles ranging between 30 and 40 nm in the NF and increasing during transport to

40–64 nm in the FF, where workers were exposed. Workers from other sectors of the facility frequently entered the central area without respiratory protection and were directly exposed to particle concentrations from the spraying booths. As expected, mean diameters during the inactivity period were larger and representative of urban background ultrafine (46–80 nm) aerosols (Reche et al., 2015).

## Modal distribution and analysis

The modal analysis was applied to the same results dataset as in Fig. 4, in FF of Booth 3. The time series of the modal particle number concentrations is plotted in Fig. 5 for three periods: (a) morning activity, (b) midday inactivity period, and (c) afternoon activity. The relative contribution from each mode to total particle number concentrations is also shown for each period.

Modal analysis of particles in the worker area provides information on the size distribution as well as on the potential sources of particles. The Mode<sub>90nm</sub> was dominant throughout the day in the FF
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Particle number concentrations for particles in each of the modes identified by modal analysis (top), for booth #3 Far field data on 26/04/2017. Absolute average number concentration and relative contribution (%) of each mode (bottom) for time intervals (a) morning, (b) inactivity period, and (c) afternoon.</figcaption></figure>

contributing 88–94% of particle number, with the time series of this mode highly resembling the one of total number concentration (Fig. 4). New particle formation took place inside the booth (NF, Fig. 2), and particles subsequently grew into the larger-sized mode (Mode$_{26–90nm}$) during their transport from the source towards the worker area (FF, Fig. 4a). The relative contributions from the Mode$_{26–90nm}$ were similar during the morning (Fig. 5a) and afternoon (Fig. 5c) spraying activities, despite the differences in total particle number concentrations resulting from the open/closed doors scenarios (Fig. 4). Mode$_{10–25nm}$ were minor contributors with 2–3% during both activity periods, while Mode$_{91–660nm}$ contributed with 4–9% of relative particle number concentrations.

The lowest contribution of Mode$_{10–25nm}$ (which includes particles formed by nucleation) in terms of particle number release was recorded during the inactivity period (Fig. 5b) as expected due to the absence of activities in the facility. Mode$_{91–660nm}$ showed the highest contribution during the morning activity (Fig. 5a) due to the influence of the open door.

Certain considerations should be taken into account in the application of modal analysis to the occupational dataset presented in this work. Particles deriving from nucleation can have sizes from 1 nm, while very few instruments are able to measure particles in this size range. The instrumentation used in this work had a lower cut-off at 10 nm (NanoScan) which results in an underestimation of the Mode$_{10–25 nm}$ contributions. The same is probably true for the Mode$_{91–660nm}$, given that the instruments had an upper cut-off size at 420 nm. The underestimation of the Mode$_{91–660nm}$ mode was probably lower since the number of particles in the range 420–660 nm emitted during thermal spraying is low.

## Calculated deposited dose during inhalation

Inhalation dose rates for particle number, surface area and mass were estimated for Booth 3 (26 April 2017 and 28 April 2017), for each half-day (morning and afternoon) and midday inactivity period for the FF location (Table 3), where workers carried out different tasks and the use of personal protection equipment was limited. Particle number dose rates (n) were higher during activity (353 ×
<table><caption>**Table 3.** Dose rates in particle number (n̄), surface area (s̄), mass (m̄), and regional deposition to head airways, trachea bronchi, and alveolar regions, calculated based on particle size-resolved concentration data (measured with NanoScan combined with MiniWRAS).</caption><thead><tr><th rowspan="2">Dose rates</th><th colspan="3">26 April 2017—Booth 3</th><th colspan="3">28 April 2017—Booth 3</th></tr><tr><th>Inactivity</th><th>Morning</th><th>Afternoon</th><th>Inactivity</th><th>Morning</th><th>Afternoon</th></tr></thead><tbody><tr><td>Mean size (nm)</td><td>76</td><td>47</td><td>49</td><td>51</td><td>57</td><td>54</td></tr><tr><td>Total: n̄, 10<sup>6</sup> [min<sup>-1</sup>]</td><td>138</td><td>1024</td><td>353</td><td>374</td><td>535</td><td>483</td></tr><tr><td>Head airways: n̄, 10<sup>6</sup> [min<sup>-1</sup>]</td><td>16.3</td><td>163</td><td>61.7</td><td>40.2</td><td>106</td><td>91.3</td></tr><tr><td>Trachea bronchi: n̄, 10<sup>6</sup> [min<sup>-1</sup>]</td><td>20.2</td><td>130</td><td>43.3</td><td>65.7</td><td>61.6</td><td>56.8</td></tr><tr><td>Alveolar: n̄, 10<sup>6</sup> [min<sup>-1</sup>]</td><td>102</td><td>730</td><td>248</td><td>268</td><td>367</td><td>335</td></tr><tr><td>Total: s̄, 10<sup>6</sup> [µm<sup>2</sup> min<sup>-1</sup>]</td><td>1.8</td><td>6.3</td><td>3</td><td>1.8</td><td>5.7</td><td>4.4</td></tr><tr><td>Head airways: s̄, 10<sup>6</sup> [µm<sup>2</sup> min<sup>-1</sup>]</td><td>0.3</td><td>2</td><td>1.1</td><td>0.2</td><td>2.2</td><td>1.6</td></tr><tr><td>Trachea bronchi: s̄, 10<sup>6</sup> [µm<sup>2</sup> min<sup>-1</sup>]</td><td>0.2</td><td>0.54</td><td>0.3</td><td>0.3</td><td>0.4</td><td>0.3</td></tr><tr><td>Alveolar: s̄, 10<sup>6</sup> [µm<sup>2</sup> min<sup>-1</sup>]</td><td>1.2</td><td>3.8</td><td>1.7</td><td>1.3</td><td>3.1</td><td>2.5</td></tr><tr><td>Total; n̄, [ng min<sup>-1</sup>] (inactivity ρ = 1.5 g cm<sup>-3</sup>; Process ρ = 4.3 g cm<sup>-3</sup>)</td><td>1</td><td>9.5</td><td>5.5</td><td>0.8</td><td>8.5</td><td>11.5</td></tr><tr><td>Head airways: n̄, [ng min<sup>-1</sup>]</td><td>0.9</td><td>8.8</td><td>5.1</td><td>0.7</td><td>7.7</td><td>11</td></tr><tr><td>Trachea bronchi: n̄, [ng min<sup>-1</sup>]</td><td>0.02</td><td>0.2</td><td>0.1</td><td>0.02</td><td>0.2</td><td>0.1</td></tr><tr><td>Alveolar: n̄, [ng min<sup>-1</sup>]</td><td>0.1</td><td>0.5</td><td>0.3</td><td>0.1</td><td>0.6</td><td>0.4</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Particle number concentration and mean diameters measured with DiSCMini inside and outside an FPP3 personal mask worn by a volunteer-member of the research team. Measurements took place in the near field during two activity periods in booth #2 (27-04-2017).</figcaption></figure>

10⁶ to 1024 × 10⁶ min⁻¹) than during inactivity periods (138 × 10⁶ to 374 × 10⁶ min⁻¹). During the spraying activity, ~70% of the total inhaled particle number concentrations was deposited in the alveolar region, 12% in the trachea bronchi, and 18% in the head airways. Hence, for workers in the FF location 82% of the total inhaled particles sourcing from thermal spraying were deposited in the deepest regions of the respiratory tract.

The same analysis was applied to the calculated active surface area concentrations of the particles deposited in

the airways (s̄) during activity periods (3.0 – 6.3×10⁶ µm² min⁻¹, Table 3): 60% was estimated to be deposited in the alveolar region, 9% in the trachea bronchi, and 31% in the head airways. Total surface area of the deposited particles during activity was higher than during inactivity (1.8 × 10⁶ µm² min⁻¹). The LSDA concentrations in this study (1.2–5.6 × 10³ µm² cm⁻³, FF, Table 2) were mostly higher than others found in the literature, where LSDA concentrations were on average 21.6 µm² cm⁻³ during handling and loading of halloysite nanotubes (Koivisto
et al., 2018), and <19 µm² cm⁻³ during WC–Co fine powder production (Koivisto et al., 2016). Regarding the total deposited mass (m̄), increases of 1–9.5 ng min⁻¹ were calculated from inactivity to activity periods. These dose rates in terms of mass were higher than those reported by Koivisto et al. v(2014) (0.03–0.53 ng min⁻¹) during a lab-scale exposure assessment to nanodiamonds.

Koivisto et al. (2012), who found comparable results to those in the current work during a high energy process (liquid flame spray), concluded that N was the most relevant metric for exposure assessment. However, biologically, the most relevant metric considering pulmonary inflammation is likely surface area (Schmid and Stoeger, 2017).

## Effectiveness of protective personal equipment

The effectiveness of a personal FPP3 respirator (Moldex Air Plus 3405, half-piece mask) was assessed on a case study basis by comparing measurements obtained simultaneously inside and outside the respirator, with two DiSCMini units, while the mask was worn by one volunteer member of the research team. The respirator assigned protection factor (APF) was 10 as specified by the 29 CFR 1910.134 (Occupational Safety and Health Admin, 2006). Programme protection factors (PPFs) were studied by Koivisto et al. (2015) for loose-fitting respirators. The measurements were carried out inside Booth 2 (NF) and during two representative activity periods. The inlet of one of the instruments was held in the breathing zone (outside the mask), and the second was connected to the interior of the mask with Tygon conductive tubing (Asbach et al., 2016).

The particle concentrations monitored (Fig. 6) show that the mask achieved a reduction of worker exposure in terms of number concentration by 87% (from 3 × 10⁶ to 4 × 10⁵ cm⁻³) on average for both activity periods. This is likely a conservative estimate since the inlet of the instrument prevented from achieving a proper fit of the mask and it is highly probable that unfiltered air entered in the mask and interfered with the measurements.

## Conclusions

Particle emissions and impacts on exposure were monitored for NPs and micron-scaled particles (fine and coarse) in terms of particle number and mass concentrations, LDSA, mean diameter, and size distributions, during the application of ceramic coatings on metal surfaces by means of thermal spraying, at industrial scale. The results obtained are representative of the industrial facility under study, which has unique

characteristics as most industrial facilities do, which highlights the need for additional studies with this kind of focus in the literature. Results may only be generalized once the body of literature regarding industrial thermal spraying emissions and the influence of different feedstock materials become available.

High particle number (>10⁶ cm⁻³) and mass (60–600 µgPM₁ m⁻³) concentrations were recorded inside the thermal spraying booths (NF), which were transported towards the worker area (FF) increasing the FF concentrations by one order of magnitude in terms of N (10⁴–10⁵ cm⁻³) and up to a factor of 4 in PM1 (44–100 µg m⁻³). NPs were generated through different mechanisms (mechanical, melting-condensation), resulting in diverse morphologies (irregular, spherical). NPs with small diameters were detected inside (31–41 nm) and outside (40–64 nm) the spraying booths. Worker exposure occurred both in the NF and FF locations, given that the workers operated equally inside and outside the spraying booths. The inhalation model applied showed that particles emitted during thermal spraying were mainly deposited in the alveolar region (70%).

The high correlation between particle concentrations in the NF and FF suggest that worker exposure was strongly impacted by NPs, fine and coarse particles emitted during thermal spraying. Whereas similar NP number concentrations were monitored irrespective of the spraying technique and feedstock material applied, coarser particle (>2.5 µm) mass concentrations showed differences as a function of the feedstock material. Additional research is necessary to understand the relationship between coarse particle emissions and feedstock physical-chemical properties.

The advantages and limitations of applying modal analysis to an occupational dataset were assessed. This analysis allowed for the identification of a dominant mode (Mode₂₆⁻₉₀nm particles, 89%), the increased contribution from Mode₁₀⁻₂₅nm (nucleation) particles during thermal spraying periods, and the influence of outdoor urban background aerosols during the inactivity period. In spite of the continuously working local extraction systems, the proper sealing of the spraying booths was identified as a key element for exposure reduction. Differences in exposure concentrations of one order of magnitude (from 10⁵ cm⁻³ to 10⁴ cm⁻³) were recorded when doors were open/closed. Optimizing the production routine to prevent the opening of doors during the spraying activity as well as a delayed door-opening protocol could reduce NP transport from inside the booths and consequently minimize impacts on exposure in the adjacent worker areas. Furthermore, worker access in the central area should be restricted during spraying operation, or carried out with adequate personal
protective equipment (PPE). Finally, the use of FPP3 masks (with APF 10) was also advised, given their high potential for reduction of particle number concentrations.

## Supplementary Material

Supplementary data are available at *Annals of Work Exposures and Health* online.

## Acknowledgements

The current work was carried out in the framework of the CERASAFE project (www.cerasafe.eu), with the support of SIINN ERA-NET (project id: 16), and was funded by the Spanish MINECO (PCIN-2015-173-C02-01) and the Romanian UEFISCDI (PN-III-P4-ID-PCE-2016-0835). Partial funding from the ‘Generalitat de Catalunya’ (project number: AGAUR 2017 SGR41) is also acknowledged. The authors kindly acknowledge TM COMAS (http://www tmcomas.com) for their committed cooperation.

## Declaration

The authors declare no conflict of interest relating to the material presented in this article. Its contents, including any opinions and/or conclusions expressed, are solely those of the authors.

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