Performance Comparison of N95 and P100 Filtering Facepiece Respirators with Presence of Artificial Leakage (2024)

Search for other works by this author on:

Article history

Abstract

Introduction

Aerosols in the workplace that may affect worker health are mainly controlled by engineering and administrative practices. However, when these measures cannot reduce workers’ exposure to acceptable levels, the US Occupational Safety and Health Administration (OSHA) requires that respirators be provided to exposed workers (OSHA, 2006a). In the USA, there are more than 3 million workers in 282,000 establishments that are required to wear respirators (Doney et al., 2005). As the last line of defense, respirators play a key role in protecting the health of employees. Filtering facepiece respirators (FFRs) approved by the National Institute for Occupational Safety and Health (NIOSH) as N95 and P100 have a filter efficiency of at least 95% and 99.97%, respectively (42 CFR Part 84, Code of Federal Regulations 1995). Due to their low cost and light weight, N95 and P100 FFRs are widely used to protect against hazardous particulates (Doney et al., 2005).

The general performance of FFRs is strongly affected by the filter efficiency, the faceseal leakage, and the breathing resistance (R_f). There are many factors that may affect the filter efficiency, such as filter material (charged state, fiber diameter, porosity, etc.) (Balazy et al., 2006), particle size, and breathing flow rate (Boskovic et al., 2008; Eshbaugh et al., 2009; Haruta et al., 2008; He et al., 2013; Mukhametzanov et al., 2016). Currently, N95 and P100 FFRs are typically manufactured with electret filters (Balazy et al., 2006). The most penetrating particle size (MPPS) for electret filters has been shown to be below 100 nm (Balazy et al., 2006; Huang et al., 2007; Eninger et al., 2008; Rengasamy and Eimer, 2012; Mukhametzanov et al., 2016). NIOSH-approved FFRs are conventionally tested under a constant flow of 85 L/min (42 CFR Part 84, Code of Federal Regulations 1995). However, under constant test flow, the filter efficiency is higher at lower flow rates (Balazy et al., 2006; Boskovic et al., 2007, 2008; Eninger et al., 2008). At the same time, constant flow regimes do not reflect the characteristics of human respiratory flow; it was found that a sinusoidal waveform (cyclic flow) can better simulate human breathing (Stafford et al., 1973). The flow type (constant or cyclic) also can significantly affect the filtration efficiency. Only a few studies have addressed the influence of cyclic flow on the filter efficiency of FFRs (Eshbaugh et al., 2009; Haruta et al., 2008; Cho et al., 2010).

Although filter efficiency has been the focus of numerous studies, faceseal leakage has received much less attention. Based on the assumption that leakage is inevitable, the OSHA assigned protection factor for FFRs is 10 (OSHA, 2006b). Several investigators have reported that penetration due to faceseal leakage may be much higher than the filter penetration, which means that the protection level offered by a FFR may primarily depend on the faceseal fit (Eninger et al., 2008; Eshbaugh et al., 2009; Grinshpun et al., 2009; Rengasamy et al., 2009; Rengasamy and Eimer, 2011; He et al., 2014a; Mukhametzanov et al., 2016). Using human subjects, investigators have found that faceseal leaks mainly occurred at the nose, chin, and cheek areas (Crutchfield and Park, 1997; Oestenstad and Bartolucci, 2010). However, respirator testing using human subjects has its own limitations. First, it is impossible for tested subjects to keep a constant leak size and shape; furthermore, it is unethical to expose subjects to potentially hazardous particles (Wander et al., 2012).

To investigate the effect of leak shape, size, and location on faceseal leakage, artificially induced slit-like or circular leaks have been commonly used in manikin-based respirator testing experiments (Chen and Willeke, 1992; Rengasamy and Eimer, 2011, 2012). Studies have shown that the artificial leaks introduced in the manikin-based study are comparable to those leaks that naturally occur in human subjects (Lee et al., 2008; Grinshpun et al., 2009; Bergman et al., 2015). Some early tests have been conducted to address the performance of FFRs against micrometer-size (~µm) or fine (<2.5 µm) particles under constant flow when artificially created leaks were introduced (Hinds and Kraske, 1987; Chen et al., 1990; Chen and Willeke, 1992). A few studies also investigated artificial leakage under cyclic flows (Grinshpun et al., 2009; Cho et al., 2010). Very few recent studies have been conducted on the artificial leakage with nanoparticles (<100 nm) as the challenge aerosol (Rengasamy and Eimer, 2011, 2012; Mukhametzanov et al., 2016).

In addition to the effects of filter efficiency and faceseal leakage discussed above, flow resistance (R_f) is another important factor to affect the effectiveness of a respirator. Air tends to flow through a channel with the least resistance; consequently, for a respirator with presence of faceseal leakage, inhaled airflow can pass more easily through the leakage rather than through the respirator filter that has a significantly higher resistance (Campbell, 1984; Myers et al., 1991; Krishnan et al., 1994; Janssen and Weber, 2005). In reality, high-efficiency FFRs are typically associated with a higher R_f as compared to less efficient FFRs (Liu et al., 1993; Janssen and Weber, 2005), thus suggesting that an FFR with a more efficient filter (e.g. P100 FFR) may offer a lower overall protection than the one with the less efficient filter (e.g. N95 FFR) in the presence of the same leakage. Liu et al. (1993) found that the ratio of leakage airflow to total breathing flow of dust/mist respirators (2.5%) was less than the corresponding value (8%) for High Efficiency Particulate Air (HEPA) filters. Rengasamy and Eimer (2012) also reported a higher efficiency N95 FFR may provide less protection than a relatively lower efficiency N95 FFR when there were artificial leaks.

R_f in itself is an important index of the overall performance of a respirator. Higher R_f may lead to lower pulmonary ventilation, which is associated with formation of dead spaces in the respirator; the dead spaces contribute to an increase of local CO₂ concentration, resulting in the deficiency of inhaled oxygen (dyspnea) (Johnson et al., 1995). A good respirator should combine high efficiency with low resistance. However, the resistance and filter efficiency often conflict with each other. Considering both filter efficiency and R_f, the overall performance of a respirator can be assessed by the quality factor (q_f) (Hinds, 1999). A higher value of q_f corresponds to a better respirator performance.

In summary, no published investigations have compared the performance of a highly efficient P100 FFR (high R_f) with a less efficient N95 FFR (low R_f) when leakage exists. The objective of this study was to compare the performance of N95 FFRs with that of the P100 FFRs when both have the same artificial leakage. The study was conducted with respirators mounted on a manikin headform with introduction of different artificial leaks. Both constant and cyclic test flows were applied. The values of TIL, R_f, and q_f were used to compare the protection and overall performance of N95 and P100 FFRs.

Materials and methods

Tested respirators and challenge aerosols

Two pairs of NIOSH-approved N95 and P100 FFRs from manufacturers “A” and “B” were tested and compared (N95-A versus P100-A; N95-B versus P100-B) in this study. Each of the four FFRs has three principal layers with the middle layer being charged (electret filter) to enhance the filter efficiency. All the chosen respirators were of medium size. The two P100 FFRs are equipped with exhalation valves (additional experiments verified that exhalation valves did not introduce additional leakage for the P100 FFRs), while the two N95 FFRs are not. The models are commercially available and widely used at various workplaces.

This manikin-based study was conducted using the experimental setup shown in Fig. 1. Sodium chloride aerosols (count median diameter, CMD~45 nm) were produced by a constant output particle generator (Model 8026, TSI Inc., Shoreview, MN, USA) and fed into a respirator testing chamber (1.2 m × 1.2 m × 1.2 m). Four fans were placed at each corner of the chamber on the floor to assure the aerosols were well mixed. A NanoScan SMPS Nanoparticle Sizer (Model 3910, TSI Inc., USA) was used to measure both total (10–420 nm) and size dependent (d_p, centered at 15.4, 20.5, 27.4, 36.5, 48.7, 64.9, 86.6, 115.5, 154, 205.4, 273.8, and 365.2 nm) particle number concentrations. According to measurements of particle size distribution inside the testing chamber, the number concentrations of particles with size >205 nm were negligible (generally <100 particles/cm³). Therefore, the utilization of the Nanoscan SMPS was appropriate to capture most of the generated particles in this study. Before each test, the dispersion airflow from the particle generator was adjusted and operated for 15–30 min to achieve uniform aerosol concentration in the chamber at a level of 300,000 ± 3000 particles/cm³ (total). Relative humidity measured inside the chamber was very stable and consistent (32–35%), slightly higher than the relative humidity of the laboratory room air (28–31%). The particle concentration was log-normally distributed in a size range of 10–365 nm, which covers the MPPS of the electret filters such as N95 or P100 (Balazy et al., 2006; Rengasamy and Eimer, 2012; Mukhametzanov et al., 2016).

Experimental design

As can be seen in Fig. 1, the tested N95 and P100 FFRs were mounted on a medium-sized hard plastic manikin headform which represents face size/shape of about 50% of the current US workforce (Zhuang, et al., 2010), and placed inside the chamber. A copper pipe with an internal diameter of 22 mm was inserted into the headform to simulate the upper respiratory tract with one end connected to the mouth area of the manikin and the other end connected to either a vacuum pump (Model VP 2200, HFS, Covina, CA, USA) or a breathing simulator (Series 1101, Hans Rudolph, Inc., Shawnee, KS, USA). Constant or cyclic airflows were established by adjusting the three-way-valve^①. The total air into the chamber = dispersion air from the generator + dilution air from the laboratory room (makeup air introduced through an opening with a diameter of 60 mm, as shown in Fig. 1) = test airflow out from the chamber. To prevent particles from re-entering into the breathing zone of the respirator during exhalation, a plastic container with a rubber bladder and a HEPA filter inside was placed between the manikin and the breathing simulator (Rengasamy and Eimer, 2011, 2012). To take the samples outside the respirator (inside the chamber), the sampling probe was set on the top of the headform (this was done to avoid sampling the air exhaled directly from the manikin). When measuring the aerosol concentration inside the respirator, the sampling probe was connected to the breathing tube behind the manikin where the inhaled air had been well mixed, which reduced the spatial variability of the particle concentration inside the respirator. Through the adjustment of the three-way-valve^②, aerosol from either inside or outside the respirator was sampled into the NanoScan SMPS alternately (for each measurement: one minute inside the respirator, then two minutes outside the respirator, finally one minute inside the respirator). Under the constant testing flows, values of R_f were obtained using a micromanometer (Airflow^TM PVM100, TSI Inc., USA), which simultaneously measured the pressures inside and outside of the tested respirator. The experimental design was verified under the blank control condition (without mounting any respirators and no HEPA filter placed inside the plastic container). The results showed that, at various test flows (15, 50, and 85 L/min under both constant and cyclic flow types), the particle losses inside the manikin headform were negligible, and the particle size distribution inside and outside the headform (without respirator mounted) were the same.

Test conditions

Five sealing conditions were created in the following order: (i) fully sealed; (ii) 0.8-mm leak (one needle with an internal diameter of 0.8 mm and a length of 25 mm inserted between the respirator peripheral edge and the right cheek area of the headform, total cross-sectional area, TCSA = 0.5 mm²); (iii) 2 × 0.8-mm leak (one more needle inserted at the corresponding area at the left side, TCSA = 1.0 mm²); (iv) 3 × 0.8-mm leak (one more needle inserted at the nose area, TCSA = 1.5 mm²); (v) 2-mm leak (one needle with an internal diameter of 2 mm and a length of 25 mm inserted at the right cheek area, TCSA = 3.1 mm²). The needle sizes selected here were based on the faceseal leak sizes of 0.5–0.7 mm and 1.0–1.3 mm found for human subjects with high and low fit factors, respectively (Krishnan et al., 1994). Except for the introduced leaks mentioned above, all other interface between the headform and the respirator was sealed by silicone sealant (Model 49365, DAP Inc., Baltimore, MD, USA) to assure there were no additional leaks (Balazy et al., 2006). For each sealing condition, cyclic flow with a mean inspiratory flow rate of 15, 50, and 85 L/min (12, 21, and 28 breaths/min, respectively) was applied. To investigate the effects of the flow type, the tests were also performed with a constant flow at the above-specified three flow rates. Since the pressure drop would follow the airflow during cyclic flow, R_f were only measured under constant flow conditions. Thus, the tests carried out using the constant-flow settings were necessary for the overall performance evaluation using the pressure drop-dependent q_f value (see equation 4). Four replicate tests were conducted for each combination of the experimental conditions, resulting in a total of 480 measurements. A summary of test conditions is presented in Table 1. For each combination of FFR and sealing condition, the two test flow types, three flow rates, and four replicates were examined in a fully randomized order to minimize experimental errors.

Size dependent filter penetration (P_dp) and total inward leakage (TIL_dp)

The P_dp and TIL_dp values for each d_p (except for the latter two channels 273.8 and 365.2 nm because a low number of particles) were determined as the ratio of inside (C_{in_dp}) to outside concentration (C_{out_dp}):

Non-size dependent filter penetration (P_filter) and total inward leakage (TIL)

Integrating all particles between 10 and 420 nm, the P_filter and TIL were obtained as follows:

Flow resistance (R_f)

R_f was determined as the difference between the pressure outside (p_out) and inside the respirator (p_in) when constant flow was applied:

Quality factor (q_f)

Comprehensively considering P_filter, TIL, and R_f, the overall performance of a respirator was assessed by the q_f (Hinds, 1999):

Data analysis

Analysis of variance was performed to study the effects of leak size, flow type, flow rate, and particle size on the P_dp and TIL_dp, respectively. Performance comparisons of N95-A versus P100-A and N95-B versus P100-B were made by the paired T-test on P_filter, TIL, R_f, and q_f values. All statistical data analyses were conducted utilizing SAS version 9.3 (SAS Institute Inc., Cary, NC). A P-value <0.05 was used to designate statistical significance.

Results and discussion

Size dependent filter penetration (P_dp)—N95 versus P100

P_dp values found for the tested fully sealed FFRs under different breathing conditions are presented in Fig. 2. Compared with N95 FFRs, the two P100 FFRs showed much lower P_dp. ANOVA revealed that this effect is significant for all tested respirators, flow types, flow rates, and particle sizes. Under constant flow, P_dp increased with the increase of the flow rate, which is consistent with published studies (Balazy et al., 2006; Boskovic et al., 2007, 2008; Eninger et al., 2008). For a respirator with a fixed surface area, as the volumetric flow increases, the surface velocity of airflow increases, leading to a shorter residence time when particles passing through the respirator filter media. Thus, there is a lower chance for the particles (<200nm) measured in this study to be removed from the airstream by the capture mechanisms of diffusion and electrostatic attraction, resulting in a greater penetration. Contrary to the results obtained for constant flow, the increase of the cyclic flow results in a decrease of penetration for all tested FFRs. He et al. (2013, 2014a, b) have reported similar findings. Unlike the constant flow, cyclic flow includes both inhalation and exhalation cycles. During exhalation, the airflow filtered by the HEPA filter (essentially particle-free) returns back into the headform (see Fig. 1), diluting the aerosol inside the respirator measured by the SMPS. The higher the cyclic flow rate is, the more significant effect the dilution has, resulting in lower C_{in_dp} in equation (1) and correspondingly lower values of P_dp. Both N95 FFRs showed an MPPS in the range of 40 to 60 nm, which was consistent with the MPPS reported by Bałazy et al. (2006 (30–70 nm), Rengasamy and Eimer (2011, 2012) (45–50 nm), and Mukhametzanov et al. (2016) (40–70 nm) for the N95 FFRs tested at the fully sealed condition.

Size dependent total inward leakage (TIL_dp)—N95 versus P100

The TIL_dp values determined for all respirators with leaks of 0.8-mm, 2 × 0.8-mm, 3 × 0.8-mm, and 2-mm under different tested airflows are shown in Figs. 3–6. ANOVA revealed that for the four model FFRs, leak size, flow type, flow rate, and particle size all had a significant effect on the TIL_dp. When artificial leaks were introduced, for all particle sizes and tested airflows, TIL_dp increased significantly as the leak size increased. A similar conclusion has been reported in several published studies when testing respirators against nanoparticles under different constant flow rates and artificial leak sizes (Liu et al., 1993; Rengasamy and Eimer, 2011, 2012; Mukhametzanov et al., 2016).

Comparing Figs. 2 and 3, it was found that even at the smallest leak size (0.8 mm), TIL_dp was significantly higher than the P_dp in the fully sealed condition, indicating that even a small faceseal leak could substantially reduce the effectiveness of a respirator (Chen and Willeke, 1992; Grinshpun et al., 2009). A slight increase of TIL_dp was seen when one more 0.8-mm leak was added (2 × 0.8-mm), as shown in Fig. 4; however, the increase was insignificant. When Rengasamy and Eimer (2012) investigated the TIL_dp of four models of N95 FFRs with artificial leaks, the TIL_dp values obtained at the first and second leak size (2 × 0.9 mm and 2 × 1.27 mm) were similar, which agrees with the finding in this study. A significant anomaly was seen for N95-A: the TIL values with leak sizes of 0.8-mm and 2 × 0.8-mm were all significantly less than the P_filter values (see Table 2). Possible explanation is that the performance of respirators under cyclic flow is much more complicated compared to that under constant flow (various previously published papers have proved this), because the cyclic flow involves both inhalation and exhalation cycles. The inhalation phase of the cyclic flow is similar to that of the constant flow, but the exhalation brings purified air (due to a HEPA installed between the manikin and the breathing machine) back to the inside of the respirator, which pushes the residual particles (resulted from the inhalation) out of the respiratory cavity. Because airflow always finds the least-resistant pathway, due to the existence of the faceseal leak (needles inserted), it is much easier for the residual particles to escape the respirator cavity than the fully sealed condition during the exhalation cycles. Therefore, the particle concentrations measured inside the respirator during the exhalation cycles could be reduced, resulting in a lower TIL with artificial leakages compared to that of the fully sealed condition. In addition, the filter media (pressure resistance) are different between N95-A and B (see Table 3), thus the results obtained from respirator A were not the same from that of the respirator B.

When a third 0.8-mm diameter needle was inserted in the nose area (3 × 0.8-mm leak), it was found the TIL_dp increased significantly (Fig. 5). For example, the TIL_dp of 3 × 0.8-mm leak was significantly higher than the corresponding values at leaks of 0.8-mm (Fig. 3) and 2 × 0.8-mm (Fig. 4). As the leak size increased to 2 mm, the TIL_dp was much higher than that of the above three leak sizes (see Fig. 6 versus Figs. 3–5). The 2-mm leak produced the highest TIL due to having the largest leak surface area. It was calculated that the TCSA for the 2-mm leak was 6-, 3-, and 2-fold greater than for 0.8-mm, 2 × 0.8-mm, and 3 × 0.8-mm leaks, respectively. Liu et al. (1993) and Mukhametzanov et al. (2016) also reported that when the leak size exceeded a certain limit, the protection factor (PF, the reciprocal of TIL) of the tested respirators decreased rapidly.

Non-size dependent filter penetration (P_filter) and total inward leakage (TIL)—N95 versus P100

The P_filter and TIL values calculated according to equation 2 under five sealing conditions, two flow types and three flow rates are presented in Table 2 and Figs. 7–8. As expected, when a respirator was fully sealed on the manikin, P_filter was much lower for the P100 FFRs than for the N95 FFRs. When the smallest leak (0.8-mm) was introduced, the TIL for N95 FFRs was still higher than that of P100 FFRs for most of the tested flow conditions. One exception occurred when the lowest constant flow (15 L/min) was applied, the N95-B FFR showed a lower TIL than that of the P100-B FFR. For the larger leak (2 × 0.8-mm and up) and flow rates of 50 and 85 L/min, the TIL was always higher for P100 FFRs than for N95 FFRs regardless of the flow type. When the leak size increased to 3 × 0.8 mm and 2 mm, for all combinations of flow type and flow rate, the N95 FFRs consistently showed lower TIL than P100 FFRs (see Fig. 7).

It is assumed that the particles were not agglomerated or de-agglomerated when passing through the respirator filters or the artificial leaks, and that there was no particle loss during the passage of the leak, i.e. the penetration through the leak is 100% (Mukhametzanov et al., 2016). Based on the count balance of the particles inside and outside the respirator (Liu et al., 1993) and the flow conservation (Mukhametzanov et al., 2016), the following two equations are presented to associate the airflow, particle concentration and filter penetration:

where Q_t is the test flow; Q_f is the airflow passing through the respirator filter; Q_l is the airflow through the artificial leak; C_in and C_out represent the number concentration of the non-size dependent particles (across all sizes) inside and outside the respirator, respectively. According to the equations 5 and 6 and the definition of TIL (the ratio of C_in to C_out), the TIL can be expressed as:

For a specific constant test flow (Q_t), P_filter can be considered as a fixed value for a specific filter material, suggesting the TIL increases with the increase of Q_l. Since higher efficient FFRs are typically associated with higher flow resistance as compared to the less efficient ones (e.g. in this study, R_f of P100 FFRs is typically twice that of N95 FFRs, see Table 3), and air tends to flow through channels with lower resistance, when faceseal leak is present, the Q_l for P100 FFRs is likely to exceed that of N95 FFRs. Furthermore, it was reported that as Q_t increased, the ratio of Q_l to Q_t decreased (Chen et al., 1990; Cho et al., 2010; Mukhametzanov et al., 2016), suggesting that a larger portion of Q_t would leak into the respirator through the faceseal leak under lower flow rates. As shown in Fig. 7 and Table 2, at leak sizes of 0.8 mm and 2 × 0.8 mm, under the lowest test flow (15 L/min), the highly efficient P100 FFRs with higher R_f may not be as protective as less efficient N95 FFRs with lower R_f. As the leak size increased to a certain threshold, compared to the TIL, the P_filter for a respirator became negligible (He et al., 2014a; Mukhametzanov et al., 2016). Thus the expression of TIL can be simplified as:

When large leak sizes such as 3 × 0.8 mm and 2 mm were introduced, regardless of the tested flow (Q_t), the Q_l value was always higher for P100 FFRs with high R_f, resulting in higher TILs as compared to N95 FFRs (see Figs. 5–7 and Table 2).

Breathing resistance (R_f) and quality factor (q_f): N95 versus P100

R_f has been recognized as a parameter useful for evaluating the respirator performance (Hinds, 1999). For all three constant flows, the R_f values of the four FFRs at five sealing conditions are presented in Table 3 (as indicated above, the measurements of R_f can only be conducted with constant flow). Under each sealing condition, the R_f increased significantly with the increase of flow rate. In particular, the R_f values obtained at 85 L/min were 4- and 2-fold greater than those at 15 and 50 L/min, respectively. Under each constant flow rate, as leak size increased, the value of R_f decreased. However, the decrement was minimal. Under each combination of sealing condition and constant flow, the R_f of P100 FFRs was always twice that of N95 FFRs.

Based on the values of P_filter, TIL and R_f reported in Tables 2 and 3, q_f was calculated to compare the overall performance of the tested FFRs. The results are presented in Table 4. The higher q_f corresponds to a better overall performance of a respirator. It was found that for all sealing conditions, the q_f for each FFR decreased rapidly with the increase of the flow rate. Compared with the fully sealed condition, even the smallest leak of 0.8-mm caused q_f to decrease substantially (see Table 4), indicating that a small leak could significantly affect the overall performance of an FFR. The q_f value of each respirator further decreased with the increase of the leak size, which was consistent for all tested FFRs. At all leak sizes and breathing flows, the q_f values were always higher for N95 FFRs than for P100 FFRs, supporting the conclusion that the overall performance of the N95 FFRs was better than the P100 FFRs when a faceseal leakage was present.

In this study, both N95 and P100 from the two manufacturers (A and B) were tested under the same conditions using the same instruments. Theoretically, P100 FFRs should perform better than N95 FFRs. However, in reality different manufacturers may adopt different fiber material, electret charge techniques, respirator shape and filtering area, etc., resulting in the production of respirators with different characteristics and quality. For example, one manufacturer may produce a N95 FFR with filtration efficiency higher than that of a P100 respirator made by another company, even though this P100 satisfies the NIOSH certification criteria under standard testing conditions (filtration efficiency > 99.97%). The above statement may explain why the tested respirator N95-A performed better than the P100-B in most cases. In this study, we assumed that both N95 and P100 produced by the same company would have the similar filter characteristics except that higher filtering efficiency and flow resistance were associated with P100 as compared to N95. The data presented in this manuscript supported this assumption (see Tables 2–3). Therefore, we didn’t perform further data analyses on the comparison between A and B, and to be consistent, the comparisons were made (N95 versus P100) for A and B, respectively. Lastly, it needs to be pointed out that cautions should be made to avoid making a generalization about the performance of N95s versus P100s as differences exist among different FFR manufacturers, and the conclusion that a N95 FFR may perform better than a P100 FFR under faceseal leakages only applies to FFRs manufactured by the same company.

There were some limitations in this study. Firstly, the leakages were artificially introduced with fix sizes and locations, which may not be representative of actual wearing conditions. Secondly, because the mass of nanoparticles is not comparable to that of large particles (>1 µm), the number concentration of nanoparticles was chosen in this study to evaluate the performance of tested respirators. However, current permissible exposure limits for particulates are generally mass-based, thus the TIL values of large particles may be different from the results obtained from this study.

Conclusion

Under the fully sealed condition (perfect fit), the filter efficiency of P100 FFRs was always higher than that of the N95 FFRs. The TIL of each respirator increased significantly with increased leak size. When the leak size ≥2 mm, the protection performance of all tested FFRs decreased rapidly. With small leak sizes (0.8 mm and 2 × 0.8 mm) under the lowest test flow (15 L/min), the TIL of the two P100 FFRs was greater than that of the two N95 FFRs. When the leak size increased to 3 × 0.8 mm and 2 mm, the TIL of P100 FFRs was always higher than that of N95 FFRs regardless of the breathing flow rate.

The R_f of each respirator decreased slightly with increased leak size, but increased significantly as the breathing flow increased. The R_f of P100 FFRs was always two times that of the N95 FFRs. Even the small leakage could cause the q_f of a respirator to decrease significantly, and q_f decreased with the increase of the leak size and flow rate. Under the fully sealed condition and the lowest tested flow rate (15 L/min), the q_f values of P100 FFRs were lower than those of the N95 FFRs; at the same time, once artificial leaks were introduced, the q_f values of N95 FFRs were greater than for P100 FFRs regardless of leak size and test flow.

Overall, the findings suggest—perhaps, counter-intuitively—that a P100 FFR with high-flow-resistance will not be as protective as a low-flow-resistance N95 FFR especially under conditions of low (<15 L/min) mean inspiratory flow rates and high leakage areas (>1.5 mm²). This study will have an impact on future design of P100 FFRs by promoting filter materials with lower flow resistance or/and developing a better faceseal design.

Acknowledgements

The authors are grateful to Dr. Sergey Grinshpun for fruitful discussions concerning the findings and help in preparing the manuscript. This research was supported by the National Natural Science Foundation of China (No. 51904291), the Basic Research Program of Jiangsu Province (No. BK20190638), the Fundamental Research Funds for the Central Universities (No. 2017XKQY027), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the State Key Laboratory of Coal Resources and Safe Mining (No. SKLCRSM16X03).

Declaration

The authors declare no conflict of interest relating to the material presented in this article.

References

© The Author(s) 2019. Published by Oxford University Press on behalf of the British Occupational Hygiene Society.

Issue Section:

Original Articles

Email alerts

Citing articles via

More from Oxford Academic