Abstract
A comprehensive literature review and associated new analysis of human exposure to different doses of respirable spores of Bacillus anthracis, and subsequent development, or not, of inhalational anthrax were conducted. The objective of the review was to evaluate actual human data collected at low doses (∼600 respirable particles under 5 microns in diameter per day).
The key findings of the paper are: (a) Exposure of individuals to 600 or more spores was common in U.S. textile mills before 1960; (b) If 600 spores were sufficient to induce disease, inhalational anthrax would have been common; (c) Inhalational anthrax was very rarely observed; and (d) Therefore, 600 spores may not be sufficient to induce disease in a healthy individual who is not egregiously predisposed to anthrax, lung disease, or is immuno-compromised. The upper confidence limit for the probability that disease would result from exposure to 600 spores or less is 0.000 000 06. Furthermore, it is not appropriate to characterize 600 or fewer spores as LDx for any value of x, however small.
Only nine “adventitious” cases of inhalational anthrax in the U.S. have been documented in the past 107 years for which there are no reasonable prospects of identifying evidenced-based explanations. For risk management, occupational safety, and related public-health concerns, the rarity, not the mystery, of these cases is the key point. The presence of such a small number of unexplained cases is a moot point for risk assessments and planning studies.