International Journal of Theoretical and Applied Nanotechnology (IJTAN)

ISSN: 1929-1248

1. Introduction

The presence of airborne nanoscale particles (ANP) in workplaces primarily results from the aerosolization of engineered nanoparticles or ultrafine particles generated during various processes, such as mechanical machining, 3D printing and wood processing [1-5]. Health and Safety agencies recommend the use of type 5 chemical protective clothing (CPC) as a final barrier against ANP exposure [6, 7]. According to ISO 13982-1 and ISO 13982-2 standards, type 5 CPC, such as coveralls, are designed to protect against airborne solid particles [8, 9]. Generally, these CPCs are effective against ANP; however, some studies have reported varying degrees of protection depending on the overall structure of the CPC, including the materials used and potential weak points.

Gao et al. evaluated particle penetration through ten different nonwoven fabrics and found that penetration increased with particle sizes ranging from 300 to 500 nm. They also noted significant differences in fabric performance due to structural variations [10]. Ben Salah et al. tested four type 5 CPC materials against airborne polydisperse sodium chloride particles (with electrical mobility diameters ranging from 14 to 495 nm, centered at 50 nm). While three of the nonwoven fabrics demonstrated efficiencies (1 – penetration) above 99%, one showed a penetration level as high as 8.5% [11]. Similarly, Vinches et al. confirmed these findings, recording a penetration level of 13.5% for the same type of particles [12]. Their study also highlighted that penetration could increase significantly in the presence of seams, reaching up to 90% depending on the seam design.

In another study, Vinches and Hallé assessed the impact of particle size (30 and 300 nm spherical silicon dioxide) and morphology (30-50 nm titanium dioxide in anatase and rutile forms) on the filtration efficiency of a type 5 CPC [13]. They concluded that the particle shape significantly affected penetration levels. They also measured the clogging effect during a 3-hour exposure to ANP, observing significant clogging for 30 nm silicon dioxide and rutile titanium dioxide particles.

All these studies were conducted under controlled laboratory conditions (20-25°C and 30-50% relative humidity, RH). However, some workplaces experience conditions that differ significantly, with temperatures reaching up to 40°C and RH levels climbing to 80%. Under these conditions, ANP behaviour could change markedly, potentially affecting filtration processes and, consequently, the effectiveness of CPC. Temperature could play a critical role in the thermal agitation of particles smaller than 100 nm, while RH could influence the agglomeration state of ANP [14].

Further research has been conducted on the effects of air RH and clogging on the performance of fibrous filters used in industrial applications and filtering facepiece respirators [15-18]. For instance, Mahdavi et al. investigated the filtration efficiency of an electrostatic N95 filtering facepiece respirator under varying humidity conditions (10%, 50%, and 80%) [15]. Their tests, using airborne polydisperse sodium chloride particles over 6-hour periods, showed that penetration levels were affected by both loading time and RH, with higher RH levels leading to increased penetration. Joubert et al. examined the effect of humidity on the clogging of HEPA filters, observing changes in particle size distribution with varying air humidity and confirming the significant influence of RH on filter efficiency [16].

Despite these findings, to the best of our knowledge, no studies have specifically examined the impact of the unique temperature and RH conditions encountered in certain workplaces on the effectiveness of CPC materials, such as fibrous nonwoven media exposed to ANP.

This study aims to experimentally investigate the filtration processes through nonwoven CPC material exposed to ANP under different temperature and RH conditions. To achieve this, the authors measured the penetration of airborne sodium chloride (nNaCl) and titanium dioxide (nTiO₂) nanoparticles through a widely used type-5 CPC material, varying the temperature from 20°C to 40°C and RH from 40% to 80%. This work serves as a preliminary step towards a theoretical study of filtration mechanisms considering the effects of environmental working conditions.

2. Material and Method

2. 1. CPC materials

A widely used model of nonwoven chemical protective clothing (CPC), known as SMMMS, was selected for this study due to its previously reported reduced effectiveness against airborne nanoscale particles (ANP) [11-13]. The CPC material is made of polypropylene and consists of five layers, giving it the name SMMMS. The inner layers are composed of meltblown (M) material, which provides filtration, while the two outer layers are made of spunbond (S) material, which ensures the structural integrity of the clothing. Figure 1 presents a scanning electron microscope (SEM) image of the surface of the CPC material. In the foreground, fibers from the S-layer are visible, while fibers from the M-layer, responsible for filtration, can be seen in the background. Even at higher magnifications, the three M-layers appear indistinguishable from one another. On the left side of the image, a solid mass indicates a calendering point, a junction that maintains cohesion between the five layers. The thickness of each tested sample was measured using a caliper, yielding a mean thickness of 249 µm with a standard deviation of ± 9 µm.

2. 2. Preparation of nNaCl and nTiO₂ solutions

High purity grade (99.5%) sodium chloride (Fisher Scientific, ON, Canada) was diluted in Milli Q water (18.2 MΩ·cm and 25°C, Organic Carbon < 2 µg C·L^{- 1}) to obtain a concentration of 0.05 g/L. Two allotropic forms (anatase and rutile) of nTiO₂ nanopowder dispersions in water were also employed from US Research Nanomaterials, Inc. Both had a 30-50 nm size range in Transmission electronic microscopy particle diameter. The stability of the suspension is guaranteed by the supplier for 6 months and all suspensions are stored at 4°C, away from light. For each kind of nTiO₂ dispersion, the commercial suspensions were not used directly. Dilutions in MilliQ water were performed to obtain concentrations of 200 ppm. These new dispersions were also stored at 4°C, away from light, but not more than 48 hours. Before the dilutions, the nanoparticle dispersions were sonicated for at least 15 minutes (80 kHz, FB11207, Fisherbrand). Airborne wet particles of these different suspensions were generated using a nebulizer (Collison 3 jets, BGI by Mesa Labs, Colorado, USA). Dry and clean compressed air at 0.69 bar (10 psig) was employed for the nebulizer.

2.3. Penetration tests: test bench and protocol

A proven test bench, developed for previous works, was designed to measure the penetration of ANP through CPC material samples. More details are available in [11-13]. The experimental protocol for penetration tests is almost the same as that presented by Vinches and Hallé [12,13]. An ultrafine particle counter (p-Trak Ultrafine particle counter 8525, TSI Inc., Shoreview, MN) was employed to measure the concentration in number of ANP. The total flow rate was 0.1 L/min. The concentration range of the p-Trak was 0 to 5× 105 particles/cm3, that was the reason why the nNaCl and nTiO₂ dispersions were diluted previously. In addition to the p-Trak, a Scanning Mobility Particle Sizer (SMPS) - SMPS Platform 3936 (with DMA 3081, CPC 3775, Neutralizer 3077) was used to determine the size distribution of the ANP [13]. Unfortunately, the lack of availability of this device only allowed us to check the particle size distribution of ANP and not use it for all the other tests. The experimental penetration, P, of ANP through the CPC material samples was defined as P = (C_down/C_up) × 100 where C_down was the downstream concentration and C_up the upstream concentration. The test bench was enclosed in a hermetic glove box to ensure a temperature and HR controlled environment and of course the safety of researchers. All the samples were taken from the front and the back of the coverall into free seams zones and handled with antistatic tweezers to prevent contamination or electrostatic bias. Moreover, before each test, all samples were conditioning at the constant ambient lab temperature and humidity (~ 20°C and ~ 40% HR) for 24 hours. After reaching the equilibrium conditions of the system (steady upstream concentration), within 5-10 minutes, the downstream concentration was measured (indicated by 0 h). For clogging measurements, the data were collected each thirty minutes of exposure up to three hours (maximum estimated wearing time of CPC). To ensure statistically significant results, all the tests were replicated five times, and the measurements were recorded as mean (M) ± standard deviation (SD).

Table 1 summarizes the entire cross conditions used for the penetration tests.

Table 1 Set of conditions for penetration tests.

	nNaCl		nTiO2 (A)		nTiO2 (R)
Temperature effect RH = 40%	20°C 30°C 40°C		20°C 30°C 40°C		20°C 30°C 40°C
RH effect T = 20°C	40% 50% 60% 70% 80%		40% 50% 60% 70% 80%		40% 50% 60% 70% 80%
Clogging effect T = 20°C	40% 60% 80%		N/A		N/A

(A) =      Anatase and (R) = Rutile - N/A = Not available

3. Results and discussion

3. 1. Airborne nanoscale particles characterization

The size distributions of airborne nNaCl and nTiO₂ were displayed in Figure 2-a and Figure 2-b. The size distribution of nNaCl was centered at 51.4 nm and nTiO₂ size distributions were smaller to the diameter indicated by the manufacturer’s data (27.9 nm and 25.0 nm for anatase and rutile respectively).

3. 2. Effect of the temperature

The temperature was the first environmental parameter studied. According to Table 1, three temperatures were monitored (20°C, 30°C and 40°C) keeping a constant RH at (38.7 ± 1.2). Figure 3 displays the penetration for nNaCl and nTiO₂ (anatase and rutile allotropic form) through CPC material as a function of the temperature. Firstly, concerning nNaCl, at 20°C, the penetration was evaluated at (22.5 ± 1.3) %. This result can be compared to its obtained in a previous work [10]. At 30°C and 40°C, nNaCl penetration increases significantly up to (45.1 ± 4.2) % and (46.2 ± 5.9) % respectively. However, this observation is contrary to what we might expect. Indeed, for particles less than 100 nm in size and under low airflow velocity, the diffusion mechanism is the main mechanism of collection of particles on a fiber [19]. It results from the Brownian motion, also called thermal agitation, which is directly related to the temperature. The higher the environmental temperature the most important is the thermal agitation which leads to a higher probability for an ANP to be collect by a fiber [20,21]. Concerning nTiO₂ airborne particles, the trend seems to be the same with a more pronounced effect for rutile particles. If the penetration is around 3% for both at 20°C, it increases at (5.0 ± 0.6) % and (8.7 ± 1.0) % for anatase and rutile at 30°C. Finally, (3.9 ± 1.1) % and (10.0 ± 1.4) % at 40°C respectively. The reason why rutile penetration is higher than anatase was mentioned by Vinches and Hallé in a previous work [11] and explained by Boskovic et al. [22]. The rod particles (rutile) slide and tumble contrary to the spherical particles (anatase), which slide or roll before coming to a stop. This behavior is even more important for cubic particles (nNaCl) because the surface contact between the fibers and particles is not optimal and decreases the possibility of the particles staying hooked onto the fibers [23].

The experimentally observed increase in penetration at 40°C is counter-intuitive from a purely diffusive perspective. Indeed, if we refer to the Stokes–Einstein relation  to highlight that the diffusion coefficient of ANP increases proportionally with the temperature. This behaviour could be explained by considering several factors beyond simple thermal agitation. Higher temperatures may reduce the adhesion between particles and the fibers of filter material due to increased thermal motion, which diminishes the Van der Waals forces and other weak intermolecular forces responsible for holding particles on the fiber surfaces [24]. As the temperature rises, the energy of the particles and fibers increases, causing a decrease in the likelihood of particles remaining adhered to the fibers. The same explanation can be proposed on the reduction in electrostatic attraction. As temperature increases, the electrostatic forces between the particles and the fibers in the CPC material may diminish [24].

Higher temperatures can increase the kinetic energy of particles, causing them to move more rapidly and potentially overcome the electrostatic forces that would otherwise attract and capture them on the fibers. This reduction in electrostatic attraction could lead to higher penetration rates for both nNaCl and nTiO₂ particles at elevated temperatures. This interpretation is supported by previous studies like Beckman et al. (2023) and Abdolghader et al. (2018) showing that filtration efficiency in fibrous media can decrease with increasing temperature when electrostatic capture plays a significant role [20, 24].

Another explanation consists in the changes in particle dynamics. While Brownian motion indeed increases with temperature, the assumption that this should lead to greater capture rates is primarily valid under conditions where diffusion is the dominant capture mechanism. In practical filtration scenarios, especially for particles in the range of 20-100 nm, other mechanisms like interception and inertial impaction can also play a significant role [25]. Increased temperatures can alter the balance of these mechanisms, potentially reducing the overall efficiency of the filtration process, especially if the fibers are not optimized to capture particles solely by diffusion.

To summarize, the unexpected increase in particle penetration with rising temperatures can be attributed to a combination of reduced electrostatic forces, decreased adhesion and changes in particle dynamics. Moreover, the unique shapes and behaviors of the particles (cubic for nNaCl, rod-like for rutile, and spherical for anatase) further influence how they interact with the fibers under different thermal conditions. These factors collectively explain why penetration rates increase with temperature, contrary to the initial expectation based purely on diffusion theory.

3.3. Effect of relative humidity

In a second phase, the effect of the RH was investigated with an ambient temperature fixed at (25.3 ± 1.4) °C. The results showed in Figure 4 were measured just after reaching the equilibrium conditions, within 5-10 minutes to eliminate a possible clogging effect. Concerning nNaCl airborne particles, the penetration seems to increase stepwise. At 40% RH, the penetration was (22.5 ± 1.3) % while for 50% and 60% RH, it was (34.9 ± 3.3) % and (35.1 ± 2.5) % respectively. And for 70% and 80% RH, the penetration reached (49.5 ± 7.0) % and (55.1 ± 5.7) %. The observed stepwise increase in penetration of nNaCl airborne particles with rising RH levels at a constant ambient temperature of 20°C may be explained by several factors related to the behavior of particles and the interaction between particles and fibers in the CPC material under varying humidity conditions. Firstly, sodium chloride (NaCl) is a hygroscopic substance, meaning it can absorb water from the surrounding air that induces hygroscopic growth through the growth factor G(RH) = d_RH/d₀, which typically ranges from 1.3 to 2 as relative humidity increases from 40 % to 80 % [26].

We are aware that the effect of particle size as a function of RH should have been investigated. The particle size distribution measurements were periodically performed using a Scanning Mobility Particle Sizer (SMPS) to verify the stability of the generated aerosols. However, due to equipment availability, it was not possible to systematically measure distributions before and after humidification for all conditions. This limitation is now explicitly acknowledged as a perspective for future work, where continuous size distribution monitoring will be implemented to better characterize hygroscopic growth and its effect on filtration performance.

Moreover, the absorption of water on the nNaCl particles leading to alter the surface properties of both the particles and the fibers of the CPC material. The presence of water molecules can act as a lubricant, reducing the friction between particles and fibers, allowing particles to pass through the filter more easily.

 Secondly, since electrostatic attraction is one of the mechanisms by which particles are captured by filter fibers, its reduction at higher RH levels would result in decreased filtration efficiency, thus increasing particle penetration [27]. Finally, larger particles (due to the absorption of water on the nNaCl particles surface) may have a different penetration behavior through the protective material because they are less influenced by diffusion mechanisms (which dominate for smaller particles) and more influenced by interception and inertial impaction mechanisms [19].

With rutile allotropic form, the penetration follows a progressive increase. Rutile which always has a percentage of penetration higher than anatase for each RH value, goes from (3.0 ± 1.0) % to (14.4 ± 1.6) %. The progressive increase in penetration observed with rutile allotropic form, can be explained by several factors related to particle shape, surface properties, and interaction with the filter material. As mentioned before, rutile nanoparticles typically have a rod-like or elongated shape, whereas anatase nanoparticles tend to have a more spherical or near-spherical shape. The rod-like shape of rutile particles allows them to tumble and slide more easily through the fibrous network of the filter material [13, 22]. This motion reduces the likelihood of these particles being captured by the fibers, leading to higher penetration rates. In contrast, spherical anatase particles are more likely to roll and eventually come to a stop, making them easier to capture. Moreover, the rod-like shape of rutile particles reduces the surface contact area between the particle and the fiber. With fewer contact points, the chances of the particle adhering to the fiber are lower, resulting in higher penetration. In contrast, spherical anatase particles have a more uniform and larger contact area relative to their size, increasing the likelihood of interception and capture by the filter fibers. The difference in shape and surface chemistry between rutile and anatase particles can lead to different behaviors in response to changes in RH. Rutile particles may be less affected by the increased adhesion forces that might occur with rising humidity, allowing them to maintain higher penetration rates. Conversely, anatase particles, with their more spherical shape, might be more susceptible to these forces, leading to relatively lower penetration as RH changes.

Rutile nanoparticles may tend to agglomerate less than anatase under certain conditions [28]. This could result in smaller effective sizes of rutile particles passing through the filter more readily. On the other hand, anatase particles may form more stable aggregates at higher RH, reducing their penetration due to their larger effective size and increased likelihood of being captured by the filter fibers.

Finally, the surface charge of rutile and anatase phases of titanium dioxide differs due to their distinct crystal structures. Rutile typically has a lower surface charge compared to anatase, influencing their interactions with particles and surfaces in aqueous environments [29]. This difference arises from the coordination of titanium and oxygen atoms in their respective structures, which impacts their surface hydroxylation and electrostatic potential. Rutile's lower charge leads to reduced electrostatic interactions, while anatase, with its higher surface charge, has stronger electrostatic attraction under similar conditions.

To summarize, the consistently higher penetration rates of rutile compared to anatase across all RH levels can primarily be attributed to the rod-like shape of rutile particles, which promotes tumbling and sliding through the filter fibers, reducing capture likelihood. The differences in surface interactions, agglomeration behavior, and electrostatic properties between rutile and anatase further contribute to the observed variations in particle penetration. This highlights the importance of considering both particle shape and surface properties when evaluating filtration effectiveness, especially under varying environmental condition.

3.4. Long-time exposure – clogging effect

The last part of this work was dedicated to the effect of a long-time exposure of CPC material to ANP, particularly nNaCl airborne particles. Figure 5 shows the percentage of particle penetration through a protective material over time under different relative humidity conditions: 40% RH (light gray bars), 60% RH (dark gray bars), and 80% RH (black bars) with an ambient temperature fixed at (25.1 ± 0.7) °C. At 0 hours (initial exposure), the penetration percentage is highest at 80% RH (52.2 ± 4.9) %, followed by 60% RH (36.4 ± 5.9) %, and the lowest at 40% RH (8.3 ± 1.5) %. Regarding the relative humidity levels of 40% and 60%, they remain relatively constant from initial exposure to 3 hours. After 0.5 hours of exposure, the penetration seems to increase slightly for 80% RH level. From 1 hour to 3 hours, there is a general decrease in particle penetration over time for 80% RH level. However, the penetration rates remain consistently higher at 80% RH compared to the other conditions, followed by 60% RH, with the lowest penetration observed at 40% RH. Figure 5 demonstrates that relative humidity significantly affects particle penetration through protective material. Higher RH levels increase initial particle penetration, but over time, particle penetration decreases (80% RH level) suggesting a potential clogging effect where the fibers of the protective material become progressively blocked by particles, reducing their penetration as time progresses. Moreover, penetration rates remain generally higher at higher humidity levels, emphasizing the need to consider relative humidity when assessing the effectiveness of protective materials against airborne particles.

Pressure-drop measurements were not performed in this preliminary study, and they are essential for a better understanding of the clogging phenomenon. This limitation is a perspective for future work.

4. Conclusion

The study investigated the effects of temperature, relative humidity, and long-term exposure on the penetration of airborne nanoscale particles through chemical protective clothing material. The penetration of nNaCl particles significantly increased with rising temperatures (from 20°C to 40°C), which contradicts the expected behaviour based on the Brownian motion theory. This suggests that factors other than diffusion may influence particle penetration at higher temperatures. For nTiO₂ particles, a similar trend was observed, with rutile particles showing a more pronounced increase in penetration than anatase particles. This difference can be attributed to the shape and behaviour of the particles; rod-shaped rutile particles slide and tumble more than spherical anatase particles, leading to higher penetration rates.

An increase in RH led to higher penetration rates of both nNaCl and nTiO₂ particles. The penetration rate for nNaCl particles increased from 22.5% at 40% RH to 55.1% at 80% RH, indicating a clear correlation between RH and penetration. Rutile nTiO₂ particles consistently showed higher penetration rates than anatase at all RH levels, reflecting the influence of particle morphology on filtration efficiency. Over prolonged exposure, particle penetration through the CPC material decreased over time, especially at 80% RH, suggesting a clogging effect. This effect likely occurs as fibers in the protective material become increasingly blocked by particles, reducing their penetration over time. Despite the clogging effect, higher RH levels still resulted in generally higher penetration rates compared to lower RH levels, emphasizing the role of environmental conditions in influencing protective material performance. Overall, the findings highlight the significant impact of temperature, relative humidity, and exposure time on the effectiveness of CPC materials in filtering airborne nanoscale particles. These results suggest that both environmental conditions and particle characteristics must be carefully considered when selecting protective materials for use in various workplace settings to ensure optimal protection against ANP exposure.

5. Acknowledgements

The authors wish to acknowledge the contributions of Pr. Maximilien Debia for the particle counter (P-trak) and Pr. Patrice Seer for the scanning mobility particle sizer (SMPS).

6. Conflict of interest

The authors declare having no conflict of interest with the CPC manufacturers of the CPC used in this study.

References