the Canadian Center for Disease Control reports that the infectious dose in humans is 1300 organisms through inhalation. This equates to one-billionth of a gram (about the size of a speck of dust), which would be lethal to a single person. The U.S. military reports that the lethal dose is 8,000-10,000 spores. Once infected, the incubation period for anthrax is 7 days and, on average, symptoms begin within 2-5 days. When inhaled at the proper dose, the disease runs a swift sequence of events, and death can occur within 24-48 h. Spore size is also an important factor in determining their effectiveness. Particles from 2 to 5 µm would tend to be filtered out by the upper airway. It is expected that heavy smokers might be more susceptible to large particles.