What Does Respirator Certification Tell Us About Filtration of Ultrafine Particles?

Abstract

Recent interest in exposures to ultrafine particles (less than 100 nm) in both environmental and occupational settings led the authors to question whether the protocols used to certify respirator filters provide adequate attention to ultrafine aerosols. The authors reviewed the particle size distribution of challenge aerosols and evaluated the aerosol measurement method currently employed in the National Institute for Occupational Safety and Health (NIOSH) particulate respirator certification protocol for its ability to measure the contribution of ultrafine particles to filter penetration. Also considered were the differences between mechanical and electrically charged (electret) filters in light of the most penetrating particle size. It was found that the sodium chloride (NaCl) and dioctylphthalate (DOP) aerosols currently used in respirator certification tests contain a significant fraction of particles in the ultrafine region. However, the photometric method deployed in the certification test is not capable of adequately measuring light scatter of particles below approximately 100 nm in diameter. Specifically, 68% (by count) and 8% (by mass) of the challenge NaCl aerosol particles and 10% (by count) and 0.3% (by mass) of the DOP particles below 100 nm do not significantly contribute to the filter penetration measurement. In addition, the most penetrating particle size for electret filters likely occurs at 100 nm or less under test conditions similar to those used in filter certification. The authors conclude, therefore, that the existing NIOSH certification protocol may not represent a worst-case assessment for electret filters because it has limited ability to determine the contribution of ultrafine aerosols, which include the most penetrating particle size for electret filters. Possible strategies to assess ultrafine particle penetration in the certification protocol are discussed.

Keywords:

• certification
• filtration
• particle
• respirator
• ultrafine

ACKNOWLEDGMENTS

This research was partially supported by a NIOSH ERC Pilot Project Research Training Program grant (Grant No. T42/OH008432-02) through the University of Cincinnati Education and Research Center.

Notes

^A N-series filters require preconditioning at 85% relative humidity and 38°C for 25 hr.

^B In oil aerosol environment.

^C P-series respirator filter service life recommendations are manufacturer specific.

^A Per 42 CFR 84.181(g), the challenge aerosol CMD must be within ± 20 nm.

^B Count and mass distributions differ slightly from those predicted by the logarithmic function due to assumed 1 μ m upper particle size.

^C Calculated using Hatch-Choate equations.^{( 6 )}

^A Columns may not add to 100% due to rounding error.

^B Scatter values for 0–100 nm are theoretical; scatter from ultrafine particles is poorly detected by photometers.

^A PP = polypropylene; α = filter packing density; d _f = fiber diameter; L = thickness; q (nC/m) or σ (C/m²) = charge density; ρ _sf = surface density.

^B V _f = filter face velocity; provided where given or can be calculated from data provided in study; Q = volumetric flow rate.

^C MPPS = most penetrating particle size.