The two major threat classes of chemical weapons are mustard gas and the nerve agents, and this has not changed in over 50 years. Both types are commonly called gases, but they are actually liquids that are not re ly voatile. These agents were designed specifically to harm people by any route of exposure and to be effective at low doses. Mustard gas was used in World War I, and the nerve agents were developed shortly before, during, and after World War II. Our perception of the potency of chemical weapons has changed, as well as our concern over potential effects of prolonged exposures to low doses and potential target populations that indude women and cildren. Many of the toxicologic studies and human toxicity esmates for both mustard and nerve agents were designed for the purpose of quickly developing maximal casualties in the least sensitive male soldier. The 'toxcity" of the chemical weapons has not changed, but our perception of 'toxicity" has.