A number of letters allegedly containing anthrax have been sent to health clinics, government offices, and other locations in the United States. Incidents to date have been hoaxes, but all must be treated as real until proven otherwise. The first Canadian anthrax letter incident occurred on January 30, 2001. Defence Research Establishment Suffield undertook a series of experiments attempting to determine the extent of the hazard. Envelopes containing (non-pathogenic) Bacillus globigii (BG) spores (a simulant for anthrax) were opened in a mock mail room/office environment. Dispersion of the spores was measured and data were used to try to determine what might occur in an office, mail room, or central registry if an envelope containing Bacillus anthracis (anthrax) spores were opened. Dispersion was far more effective than initially suspected. Inhalational anthrax is virtually 100 percent fatal if left untreated. The study showed that a lethal dose could be inhaled within seconds of opening an envelope filled with anthrax spore. A person who remained in the room for 10 minutes could inhale 480 LD50s from a 0.1 gram filled envelope and 3,080 LD50s from a 1.0 gram filled envelope. In addition, the aerosol would quickly spread throughout the room so that others would likely inhale lethal doses. Envelopes with the open corners not specifically sealed could also pose a threat to individuals in the mail handling system. Figures, table, references, annex