Terrorists have additional hurdles to overcome. Once released, an aerosol cloud gradually dissipates over time and as a result of exposure to oxygen, pollutants, and ultraviolet rays. If wind conditions are too erratic or strong, the agent might dissipate too rapidly or fail to reach the desired area. Indoor dissemination of an agent could be affected by the air exchange rate of the building. In addition, terrorists risk capture and personal safety in acquiring and processing materials, disposing byproducts, and releasing the agent.
Many agents are dangerous to handle.
Much of the intelligence information that can be incorporated into a national-level risk assessment already exists. The U.S. foreign intelligence community has issued classified National Intelligence Estimates and Intelligence Community Assessments that discuss the foreign-origin chemical and biological terrorist threat in some detail. These intelligence assessments identify the agents that would more likely be used by foreign-origin terrorists. The FBI is responsible for assessing domestic-origin threats. However, FBI analysts' judgments concerning the more likely chemical and biological agents that may be used by domestic-origin terrorists have not been captured in a formal assessment.