Chemical agents differ from biological agents in a number of significant ways. To begin with, the onset of symptoms may occur instantly or within a few hours of exposure, whereas biological agents can take up to weeks to present. Chemical agents are synthesized through some type of industrial process, whereas biological agents are either self-replicating microbes or compounds formed by these microbes such as botulinum toxin (produced by Clostridia; see Chapter 17) or ricin, a natural metabolic product of the castor bean plant, Ricinus communis. Chemical agents act by disrupting normal metabolic function at varying sites throughout the body according to the type of agent used and are categorized into roughly eleven categories (Table 21–1). From a weapons standpoint, not all chemical agents pose a threat as a likely choice for terrorism or as a WMD. In large part this is because of logistical or technical considerations—some require too much volume for easy concealment, others pose difficulties of dispersal, and others require such quantity as to be impractical for small, poor groups with limited technical or financial resources.