|There have long been fears in the US that terrorists could use crop dusting planes to distribute a biological agent. In an analysis for the US government it was estimated that up to three million people could be killed in one attack.
Fears were compounded when FBI agents reportedly found a crop duster manual among the belongings of one of those involved in the World Trade Center attacks. But dispersing anthrax from a plane would be unpredictable. Experts are divided over how effective it would be. Biological weapons are often not very efficient at killing people, but their main power is to create widespread panic.
Anthrax spores tend to stick together and form a liquid sludge. To use anthrax as a weapon it must be converted into a powder which can be inhaled. The spores have to be very tiny - between 1 and 5 microns - and present in the air to be absorbed in the lungs. Aerosolising spores in this way is difficult and costly.
The anthrax found in the US attacks belongs to the 'Ames' strain, common to northwest America. It was originally isolated in the 1930s and is widely used for laboratory cultures.