Chemical and Biological Agents: A Tutorial

Chemical and biological (CB) agents pose a threat to U.S. and allied military forces. Unfortunately, what strictly used to be a military risk has now become a concern to the civilian population—brought about by terrorist groups and other factions with developed CB capabilities. As a result, national, state and local authorities have initiated domestic preparedness against CB terrorism. Although the probability of a terrorist CB attack is low, it’s important to have an understanding of the potentially devastating and far-reaching effects of these agents. 

There are several reasons for heightened interest in CB agents: the potential for large numbers of casualties; the psychological impact of their potential use; and the relative ease of their acquisition and/or production. Moreover, it is extremely difficult to identify the acquisition of a CB agents due to the widespread availability of the related technology and raw materials. Regardless of whether CB agents are actually used, the mere potential for use can cause significant concern and disruption.

Chemical Agents

Chemical agents are toxic substances that cause incapacitation or death upon exposure. There are four general types of “traditional” chemical agents—choking, blood, blister and nerve—that vary in their toxicity, mode of action and effect. Characteristics for several of these common chemical agents are given in the table below . . .

Agent Name Agent Type Physical Properties Physiological Effects Relative Rate of Action1
Phosgene Choking Fresh cut hay odor; heavy gas Coughing and choking, followed by chest tightness, nausea, tearing, vomiting, and headaches. Death due to fluid accumulation in the lungs. Immediate irritation in high concentrations, and delayed reaction (several hours) in low concentrations.
Hydrogen Cyanide Blood Almond odor; highly volatile gas If high concentration– violent convulsions after 20–30 seconds, breathing stops in one minute; cardiac failure within a few minutes. Very rapid; incapacitation within minutes and death within 15 minutes.
Mustard Blister Possible garlic odor, medium volatility, oily liquid Blisters or irritation to skin, eyes and lungs Delayed onset (4-6 hours)
Sarin  Nerve Colorless/odorless, low volatile liquid Difficulty breathing, miosis a, blurred vision, headache and nausea leading to respiratory distress, convulsions and eventually death. Rapid (within minutes)
VX Nerve Colorless/odorless, low volatility, oil liquid Difficulty breathing, miosis a , blurred vision, headache and nausea leading to respiratory distress, convulsions and eventually death. Relatively rapid (within 30 minutes)

1 Depending upon the concentration. a Contraction of the pupils.

Choking agents are the oldest known chemical warfare agents and have a corrosive effect on the respiratory system. Breathing these agents causes pulmonary edema where the lungs fill with fluid and choke the victim—also known as “dry land drowning.” Choking agents are heavy gases and tend to stay close to the ground but tend to dissipate rapidly in a breeze. Examples include phosgene and chlorine.

Blood agents were used to a limited extent in World War I. They are fast acting, highly poisonous chemicals. This class of agent inactivates the enzyme cytochrome oxidase which prevents the normal uptake of oxygen by the cells, causing damage to body tissues. Blood agents are highly volatile and enter the body through the act of breathing. In a gaseous state they dissipate rapidly in the air. Examples include hydrogen cyanide and cyanogen chloride.

Vesicant (blister) agents were initially used by Germany in World War I and are casualty-producing compounds. Released as liquids, these agents cause painful, debilitating blisters on exposed skin and can also affect unprotected eyes and lungs. Effects occur within a few minutes to hours of exposure. The blisters from sulfur mustard exposure heal very slowly and are more susceptible to infection than chemical or physical burns. Examples include sulfur and nitrogen mustard.

Nerve agents are the most toxic chemical agents which can cause death in minutes. They can be inhaled or absorbed through the skin. This category is further broken down into G-series and V-series.

Discovered in the 1930s by German scientists, G-series nerve agents are organophosphorus compounds that inhibit the action of the acetyl cholinesterase enzyme. As a result, large amounts of the chemical acetylcholine build up within the nervous system, which cause hyperactivity of the muscles and body organs stimulated by these nerves. Examples include sarin, tabun and soman.

V-series nerve agents were researched by the British in the 1950s and are similar to G-series nerve agents. However, they are more advanced than G-series agents and are generally more toxic and less volatile. They also pose a greater skin penetration hazard. An example of a V-agent is VX.

Industrial Chemicals

Although countries generally focus on traditional agents (choking, blood, blister and nerve), terrorist groups may use readily available toxic industrial chemicals as well. In fact, several compounds initially developed for military use in wartime, such as phosgene and chlorine, are commonly used in industry today. There are a wide variety of potential chemicals that could be used for malicious purposes including:

Organophosphate pesticides—such as malathion and parathion. Chemically related to nerve agents but are not nearly as toxic. These compounds disrupt the acetyl cholinesterase enzyme just like nerve agents. 
Carbamates—produce the same effects as nerve agents and organophosphate pesticides, but are not structurally related. An example is Sevin R. 
Metallic poisons—affect a person in a variety of ways and are usually inhalation or ingestion hazards. Arsenic trioxide is a metallic poison.

Biological Agents

Biological agents are live microorganisms or toxins that can incapacitate or kill humans and animals, and damage crops. Biological warfare is the most economical and easily concealed of the weapons of mass destruction (biological, chemical and nuclear). Members of the Japanese Aum Shinrikyo cult admitted to using biological agents in Japan during the mid-1990s, further highlighting the increased danger of biological warfare by terrorist organizations.

One of the most insidious aspects of biological agents is their extraordinary potential for covert use. Biological agents are undetectable by the human senses and can be readily released from stand off distances. Clinical symptoms usually do not appear for days to a week or more after an attack. Since the biological agent attack may not be detected, initial cases of exposure may not be attributed to it. This would make it extremely difficult to adequately respond to exposure of a large number of people and to identify the culprits. One disadvantage to the use of biological agents is that many are rapidly degraded upon exposure to certain conditions in the environment, such as ultraviolet and visible radiation, heat, drying or humidity.

Pathogens are living organisms that can cause diseases in humans. Pathogens include bacteria, viruses and fungi and vary considerably in their lethality and physiological effects. Toxins are also classified as biological agents even though they are non-living sub-stances. The table below provides characteristics for several common biological agents. 

Bacteria are single-cell organisms that can be grown and developed by terrorists. Examples include Francisella tularensis and Bacillus anthracis (see table below), the cause of tularemia and anthrax. 

Viruses are submicroscopic organisms that require living cells to produce and multiply. Variola major, the causative agent of smallpox, is a virus that could be used as a biological agent. 

Fungi usually do not affect healthy individuals but they can pose a significant hazard to plants such as crops. Cereal rust is an example of a fungal agent. 

Toxins are metabolic by-products of living organisms, such as microbes, insects, snakes and plants. They can also be artificially produced. Ricin, for example, is a toxin extracted from castor beans.

The characteristics and effects of biological agents vary; Yersinia pestis, the causative agent of the plague, has the potential to inflict epidemics while Bacillus anthracis (anthrax) spores can contaminate soil for decades. Toxins can take effect within hours and most are more deadly than the synthesized chemical nerve agents. Biological agents can be spread through the contamination of food and water supplies or via aerosol dissemination. An example of food contamination would be the use of Salmonella typhii which produces symptoms similar to that of food poisoning.

Disease (Common Name) Causative Agent Physiological Effects Time to Effect1
Anthrax Bacillus anthracis Mild fever and fatigue, worsening to severe respiratory disorders, high fever and excessively rapid pulse rate. Death can occur within 5–12 days of exposure if left untreated. Pulmonary anthrax is fatal more than 90% of the time. 1-5 days
Plague Yersinia pestis Fever, headache and rapid heart rate, followed by pneumonia and hemorrhaging of the skin and mucous membranes. Untreated plague pneumonia fatalities approach 100% but early treatment can reduce mortality to as low as 5%. 2-3 days
Smallpox Variola major Sudden onset of fever, malaise, headache, severe backache and prostration; after 2–4 days fever falls and rash appears; scabs form and fall off at the end of the fourth week. 10-14 days
Ricin Ricinus communis (castor bean plant) Initial symptoms include high fever, pain, cough and shortness of breath; after several days severe dehydration and a decrease in urine/blood pressure. If death has not occurred in 3–5 days the victim usually recovers. Several hours

1 Time to initial effect is highly variable, depending upon the dosage received.

Dissemination Devices

Terrorist groups can acquire common devices to disseminate CB agents and readily adapt them for ill-conceived purposes. Agents can be distributed using simple containers such as glass bottles or modified aerosol generators. Ease of dissemination is highlighted by the Japanese cult Aum Shinrikyo, in which ordinary plastic bags were used to release the nerve agent sarin in the Tokyo subway system.

In general, the effects of chemical agents occur more rapidly and contaminate smaller areas than biological agents (on a per weight basis). Biological agents however, can cover vast areas, resulting in large numbers of indiscriminate casualties comparable to that of nuclear devastation.

Commonly Asked Questions
Q. What is the physical state of chemical agents?
A. Chemical agents come in all states (liquid, gas, solid), however, most are liquids. Some of the original compounds used in war were gases— simply because they were readily and commercially available. Therefore, the use of terms like mustard “gas” and nerve “gas” is not necessarily correct.
Q. Do any of the chemical agents have odors?
A. Yes, depending on the chemical and its purity. For example, distilled sulfur mustard is odorless while a less-pure sample would have a faint garlic smell. The nerve agents sarin and VX are odorless; the choking agent phosgene smells like freshly mown hay, while hydrogen cyanide has an odor of bitter almonds.
Q. I have seen chemical agents referenced as letters. What does this mean?
A. The U.S. military uses digraphs to identify chemical agents. Examples include GB (sarin), HD (distilled sulfur mustard), CG (phosgene) and VX for the chemical compound O-ethyl-S-(2-diiso-propylaminoethyl) methyl phosphonothiolate. The chemical name, trade name/synonym or digraph are used interchangeably.
Q. Are toxins classified as chemical agents or biological agents?
A. The U.S. military categorizes toxins as biological agents. However, other countries or groups may con-sider toxins to be more similar to chemical agents. Toxins are in a “gray area” because they are non-living substances, similar to chemical compounds, but are produced by living organisms.
Q. What is a “causative agent” when discussing biological warfare?
A. Microorganisms are specifically identified by their genus and species. Many people refer to biological agents simply as Tularemia, Anthrax and the Plague. In fact, these terms are the diseases that the microor-ganisms cause. For example, the biological agent named Bacillus anthracis causes the disease we know as anthrax.

Sources for More Information

For further information on chemical and biological warfare issues and the efforts underway to counter these threats, please contact federal, state or local emergency management agencies, the Federal Bureau of Investigation or the Department of Health and Human Services. 

In addition, the U.S. Army's Domestic Preparedness Chemical and Biological Helpline can be reached at 1-800-368-6498. To report a chemical release or a known or suspected terrorist threat contact the National Response Center and Terrorist Hotline at 1-800-424-8802.

Additional Sources of Information

U.S. Army’s Soldier and Biological Chemical Command: 
Web site: 

Department of Defense’s focal point for CB defense scientific and technical information: 
Web site: 

U.S. Army Surgeon General’s source for nuclear, biological and chemical medical information: 
Web site:

The State Department’s Arms Control and Disarmament Agency: 
Web site:

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