A 1994 report in Science, pgs 1202-1208, analyzed the 1979 Sverdlovsk anthrax accident, and commented on lethal aerosol anthrax doses from other studies. In a study of 1236 cynomolgus monkeys exposed to an anthrax aerosol, investigators found the median lethal dose to be 4100 spores. A different study on rhesus monkeys found a median lethal dose of 2500 spores. However, they don’t say if this was the dosage from naturally occurring anthrax, or finely machined anthrax (probably the former). The DoD reported in 1986 that the median lethal dose for a human was 8-10,000, but the original study is probably restricted.
The Science report used the 8-10,000 median figure to fit a lognormal distribution to the Sverdlovsk data, and model the death rates. Remember that responses to any disease are probabilistic, some people live while others die at the same dosage, so that the 8-10,000 represents a median, that is, half the people that died received more than that, and half the people that died received less than that.
When they fit the Sverdlovsk data, they found that the minimum dose was nine (9) spores, a dosage at which 2% of the exposed people died, which occurred 2.8 km downwind of the source. This means that inhalation of 9 spores is sufficient to kill 2% of the exposed population. What is not reported in the Science study is the size distribution of the Sverdlovsk spores. If they had a large variation in size, it could be that all of the people that died received as few as nine spores, but that only 2% of the spores were of the right type (under 5 microns and isolated rather than clumped) to cause death by inhalation. Or, to put it in another way, that in naturally occurring anthrax, of every 8-10,000 spores, nine, on average, are under 5 microns and remain isolated so as to become airborne. IF so, the implication to weapons-grade anthrax is serious, because only a few spores would be needed to cause death.
I doubt if we can find much information about the Sverdlovsk accident, since the Soviets tried very hard to keep the entire thing under wraps. Also, the Sverdlovsk deaths continued (at a decreasing rate) for up to 8 weeks after the accident.
Another interesting finding is that all of the deaths were adults, no children. They don't know why this is, whether fewer children were exposed or children have some type of natural immunity. |
