The best protection

The transmission of the virus causing severe acute respiratory syndrome (SARS) appears to be by aerosol droplet and possibly through other routes.1 Therefore, it is recommended that health care workers and others who may be exposed1 employ respiratory and other personal protective equipment.2^,3 The type of respirator that has typically been used by health care workers is the N95 half-mask.2^,3 As correctly stated by Richard Schabas,2 the “N95-rated mask” is 95% filtration efficient,4 but does this level of efficiency provide the best protection for those at risk of exposure? The effectiveness of the N95 respirator has been supported by a small study on prevention of occupational transmission of infection.1 However, for work with bacterial bioaerosols and chemical and biological warfare agents, some have suggested that N95 masks are inappropriate5^,6 because these respirators do not provide “absorbent capability” and because of the amount of mask leakage, which can be about 5% through the filter and 10% around the mask,7 even if properly fitted. For biological diseases like SARS, for which just a few particles may be sufficient for infection, the N95 mask may indeed be inadequate, and some health care workers may therefore become infected even if they use the respirator properly.

A better selection for respiratory protection would be an N100 respirator with an ultra-low penetrating air filter (ULPA), which would cost only slightly more than an N95 respirator. N100 respirators have an efficiency of 99.977%,8 and ULPA filters are 99.999% efficient for monodispersed particles 0.12 μm in diameter or larger.9 HEPA (high-efficiency particulate air) filters would not be the best selection for use with a respirator because their efficiency is 99.97% for monodispersed particles 0.3 μm in diameter or larger, and coronaviruses are smaller than this (at about 60 to 200 nm). For effective operation of an N100 respirator with ULPA, the user must be fit-tested. The United States and many other countries have numerous requirements for using a negative-pressure air-purifying respirator, including medical evaluation and training, as well as yearly fit-testing.

John H. Lange Environmental and Occupational Health Consultant Envirosafe Training and Consultants, Inc. Pittsburgh, Pa.