So... And... If I wanted to, say, determine whether or not I could give Western Europe the flu on election day, I can think of worse ways than spraying a carefully metered amount of Iodine 131 up in the North Sea and waiting for somebody to give me a handy dispersion map like this. The black areas aren't necessarily places with no iodine. They are places where there are no Ring of 5 members, therefore the detection might not have been measured. Thus, the WC-135 flights.
It must be pointed out that only particulate iodine was reported. When detectable, gaseous iodine is usually dominant and can be estimated to be 3 to 5 times higher than the fraction of particulate iodine.
By far, the most efficient and effective mode for applying biological weapons is creation of an aerosol cloud. Such a cloud is made up of microscopic particles and is therefore invisible. It can be produced in several ways, all of which involve either an explosion (a bomb or a bomb within a missile) or spraying (usually involving a special nozzle on a spray tank). The effectiveness of the cloud is determined by numerous factors, such as the amount of agent that survives the explosion or spraying, and the wind and weather conditions. The primary result of an effective cloud is simultaneous infections among all those who were exposed to a sufficiently dense portion of the cloud. In addition, agents that can survive for a long time in the environment will eventually settle, contaminating the ground, buildings, water and food sources, and so on. In some cases, these sediments can form another dangerous aerosol cloud if they are disturbed.