This article presents two methods for estimating
infection risk among individuals wearing
air-purifying respirators against airborne pathogens, with the overall aim of selecting appropriate respiratory protection. Necessary data inputs are the parameters for the ambient pathogen concentration distribution, the
respirator penetration distribution, and the infectious dose distribution, along with the breathing rate, duration of a
respirator use period, and the number of use periods. The first method assumes that the pathogen does not exhibit a cumulative dose effect, whereas the second accounts for a cumulative dose effect. The methods are illustrated with hypothetical scenarios involving Bacillus anthracis (
anthrax) spores. Available data suggest that
anthrax spores would exhibit a cumulative dose effect for multiple exposures occurring close in time, as would likely affect personnel responding to a bioterrorist release. The analysis shows that failure to account for a cumulative dose effect when present leads to underestimating
infection risk. Three types of
air-purifying respirators are compared for their predicted efficacy in reducing the risk of
inhalation anthrax. Although uncertainty analyses are not performed, a general conclusion is that a full-facepiece powered
air-purifying respirator would be the best air-purifying device for responding to an
anthrax spore release. Because such
respirators would not prevent all personnel from inhaling an infectious dose, it would be advisable for users not previously vaccinated against
anthrax to receive post-exposure prophylactic
therapy.