Experts believe that terrorist use of chemical agents is an event with low probability, but potentially high consequences. While terrorist groups may or may not have an increased interest in chemical agent acquisition and use, the domestic vulnerability of the United States to chemical attack remains an issue. Both the United States and Russia have signed and ratified the Chemical Weapons Convention (CWC), and are reducing, and eventually eliminating, their chemical weapon stockpiles. The possibility that terrorist groups might obtain insecure chemical weapons led to increased scrutiny of declared Libyan chemical weapon stockpiles following the fall of the Qadhafi regime. Experts have expressed similar concerns regarding the security of Syrian chemical weapons, reportedly including stocks of nerve (sarin, VX) and blister (mustard gas) agents, and their potential use. For analysis of chemical weapons possession and use in Syria, see CRS Report R42848, Syria's Chemical Weapons: Issues for Congress , coordinated by [author name scrubbed]. Policy approaches to reducing chemical agent vulnerability have generally treated them as a group, rather than addressing specific agents. Additionally, military and civilian chemical agent detection has developed with little coordination, so that civilian toxic industrial chemical kits and military chemical weapons detectors have varying sensitivities and detection capabilities. Treatments for chemical exposure vary on a chemical by chemical basis. Because comparatively few individuals have been exposed to modern chemical weapons, health care providers have limited practical experience in treatment of chemical casualties, especially among civilians. While national efforts to reduce vulnerability to terrorist chemical agent use continue, it is not clear whether these efforts address the risks from those specific agents that pose the greatest danger. This report describes the different types of chemical weapons and toxic industrial chemicals, their availability, treatment, and detection. Chemical agents are, for the purpose of this report, chemicals posing exceptional lethality and danger to humans. Some chemical agents are toxic industrial chemicals used for commercial purposes, while others are chemicals developed predominantly as weapons. Different chemical weapons cause different symptoms in and injuries to their victims. Because of this range of potential effects, identifying the chemical agent is a key step to determining the most effective treatment. Also, chemical weapons may produce their effects by multiple different exposure routes, for example by skin contact or by inhalation. As a consequence, depending on the encountered chemical, those affected must employ different protective equipment and approaches; for example, a gas mask alone does not provide sufficient protection against chemicals that can damage through skin contact. Military planners generally categorize chemical agents into at least four classes: nerve, blister, choking, and blood agents. This method organizes chemical agents by their biological effects. Modern militaries have generally focused on nerve and blister agents as weapons. Several choking and blood agents are chemicals widely used in industrial processes. Chemicals categorized as nerve agents disrupt normal functioning of the nervous system. Nerve agents do not occur naturally. Rather, they are manmade compounds that require manufacture and isolation for high purity and toxicity. Most nerve agents belong to a group of chemicals called organophosphates. Organophosphates have a wide range of toxicity. Some insecticides contain organophosphates, though these are significantly less toxic compounds than those developed as chemical weapons. Nerve agents are mainly liquids. The first nerve agent, tabun or GA, was made in Germany in the 1930s. Following this discovery, a series of nerve agents similar to tabun were developed. This series, known as the G-series, includes the weapons sarin (GB) and soman (GD). In the late 1940s, another series of nerve agents, the V-series, was invented in England. This series includes the chemical weapon VX. Historically, multiple countries, including the United States and the Soviet Union, manufactured and maintained stockpiles of nerve agents. As signatories to the Chemical Weapons Convention (CWC), both the United States and Russia are reducing, and eventually eliminating, their nerve agent stockpiles. Military and terrorist use of nerve agents has been rare. Public intelligence assessments issued by the United Kingdom and the United States on August 29 and August 30, 2013, respectively, stated that the Syrian government used a nerve agent on August 21, 2013, against opposition forces outside of Damascus, Syria. During the 1980-1988 Iran-Iraq war, Iraq used nerve agents against Iranian troops and later against members of its Kurdish population in northern Iraq. In 1995, the Japanese apocalyptic cult Aum Shinrikyo used sarin on the Tokyo subway and reportedly carried out an attack in the city of Matsumoto as well. National chemical weapons programs have produced nerve agents for decades. A terrorist group might overcome technological barriers to synthesize these agents by using commercially available equipment, though the extreme toxicity of these compounds would pose appreciable danger to the manufacturer. Nerve agent production requires the use of toxic chemicals during synthesis and specialized equipment to contain the nerve agents produced. Of the nerve agents, VX is the most difficult to manufacture. An alternative to the direct manufacture of nerve agents is to manufacture certain chemicals that, when mixed, react to form the desired nerve agent. These chemical combinations are called binary chemical weapons. Binary chemical weapons have certain advantages and disadvantages when compared with the actual nerve agent. The chemicals comprising a binary chemical weapon are much less toxic than the actual nerve agent and thus are less dangerous to manufacture, transport, and handle. The nerve agent obtained through the use of a binary chemical weapon may be less pure or effective than directly manufactured nerve agent, since the conditions under which the nerve agent is manufactured are less controlled. Nerve agents are extremely dangerous and can enter the body through the lungs or by skin contact. For the G-series nerve agents, such as sarin, the inhalation toxicity is significantly greater than the dermal toxicity. Of the nerve agents, VX is the most deadly and tabun is the least deadly, though all are exceedingly toxic. Nerve agents interfere with the nervous system, causing overstimulation of muscles. Victims may suffer nausea and weakness and possibly convulsions and spasms. At higher concentration, loss of muscle control, nervous system irregularities, and death may occur. The action of nerve agents can be irreversible if victims are not quickly treated. Treatment for nerve agent exposure relies on two drugs, atropine and pralidoxime chloride, as antidotes. Atropine prevents muscle spasm and allows the body time to clear the nerve agent. Pralidoxime chloride limits the effects of nerve agents by reversing the agent's action. U.S. troops during the Persian Gulf War received both of these drugs in the form of an antidote kit. Anticonvulsants, such as Diazepam (Valium), may reduce convulsions and seizures brought on by exposure to nerve agents. The treatment window for nerve agent exposure is agent-dependent. Some agents quickly and irreversibly act within the body, while others require a much longer time. The most effective treatment occurs before or immediately after exposure to the nerve agents has taken place. For example, treatment of soman must begin within minutes to be effective, while tabun treatment can occur up to several hours after exposure. Prophylactic use of some compounds, such as pyridostigmine bromide, may allow effective treatments for some nerve agents to occur with longer delay. Chemicals categorized as blister agents, also known as vesicants, cause painful blistering of the skin. Such blistering is not generally lethal. Militarily, blister agents produce casualties and reduce the combat effectiveness of opposing troops by requiring them to wear bulky protective equipment. The most common blister agents are called mustard agents, due to their odor. Mustard agents are oily liquids that range in color from very pale yellow to dark brown, depending on the type and purity, and have a faint odor of mustard, onion, or garlic. These liquids evaporate quickly, and their vapors are also injurious. Blister agents are not naturally occurring compounds. Mustard agents, for example, were first developed in the late 1800s. During World War I, both sides in the conflict used these weapons against their enemies. Mustard-type blister agents produced the greatest number of chemical casualties during World War I, though fewer than 5% of these casualties died. Countries have stockpiled blister agents in their chemical weapon inventories. Mustard agent was also used by both sides in the Iran-Iraq war. As a signatory to the CWC, the United States is in the process of destroying its stockpile of blister agents. Production of blister agents is less complicated than that of nerve agents. Similar to manufacture of nerve agents, it requires the use of toxic chemicals and specialized equipment to contain the agent produced. The most common blister agents have many different methods for their production published in the open literature. Blister agents can enter the body by inhalation or contact with the skin or eyes. Some agents can penetrate through normal clothing material, causing burns even in cloth-covered areas. While blister agents react quickly upon skin contact, their symptoms may be delayed. In the case of mustard agent, damage occurs within one to two minutes of exposure, but symptoms do not manifest for several hours. As even low concentrations of vaporized blister agents quickly cause damage, it is unlikely that exposed individuals can remove these agents from the skin prior to injury. The initial symptoms of blister agent exposure are a reddening of the skin, resembling sunburn, combined with pain in the affected area. Swollen skin, blisters, and lesions may then develop, depending on the degree of exposure. Systemic symptoms, such as malaise, vomiting, and fever, may also develop in extreme cases. Exposure to large amounts of liquid mustard agent may prove fatal. The eyes are also very sensitive to blister agents. Following exposure to high concentration vapor, great pain, corneal damage, and scarring may occur. Liquid agent often causes the most severe eye damage.