Top 5 Chemical and Biological Weapons — Threat Analysis

Chemical warfare is much older than the modern armies. Greek and Roman writers sometimes referred to these ancient forces as ‘barbarian hordes.’ They used poisoned arrows, poisoned wells, arsenic smoke and other toxic gases. But the phrase is historically inaccurate, as organised states also used these methods.

The earliest firm archaeological evidence is from Dura-Europos in Syria, AD 256. The Sasanian Persian forces apparently burnt sulphur and bitumen in a tunnel during a siege, killing some 20 Roman defenders with the poisonous smoke. So chemical warfare did not begin with one supposedly barbarian people. It arose independently in several ancient military cultures over many centuries.

Chemical & Biological Synopsis

Toxicity is not a sufficient basis for ranking chemical and biological agents. A military assessment should also consider persistence, transmission, detection, protection and operational disruption. Thus, this analysis ranks the five most deadly chemical and biological weapons in terms of their overall strategic casualty potential. An agent is a weapon only if it has hostile intent and a delivery mechanism.

Any toxic chemical used for a prohibited purpose is defined by the Chemical Weapons Convention (CWC). In the meantime, the Biological Weapons Convention (BWC) bans biological agents and toxins that do not have a peaceful justification. This is important because many of the substances listed have legitimate medical, industrial or research uses. The following ranking is still at the level of threat assessment, historical evidence and defensive planning. It does not address the manufacture and use of weapons.

Top 5 Chemical and Biological Weapons Rank

There is no best agent in all situations. Weather. Terrain. Immunity. Protective equipment and medical preparedness can alter the expected outcome. The top 5 chemical and biological weapons, however, provide the widest combination of casualties, contamination and command disruption.

1. Smallpox: The Leading Strategic Biological Threat

The Variola virus is an orthopoxvirus, which is a double stranded DNA virus. Why it is ranked first is that the infected can propagate an incident far beyond the first exposed group. Thus, its strategic impact depends on the behaviour of the epidemic, not just on the initial release. 30% of unvaccinated patients died of variola major, historically. The incubation period was usually 10–14 days, but it varied from 7 to 19 days. During this time patients were asymptomatic and could not transmit the disease, but the delay could mask the cause while exposed personnel moved through transport hubs, bases and staging areas.

Vaccination wiped out smallpox. The last naturally occurring case was in 1977. There have been no smallpox attacks in modern times. In addition, the World Health Organization has a stockpile of vaccines for emergency use, is responsible for the two approved variola repositories, and the distinctive rash allows for clinical identification once the disease has occurred. But even those great defensive advantages can be swamped by a planned return that overwhelms medical intelligence, contact tracing, isolation capacity and continuity of command systems. Thus, in this ranking, smallpox remains the most strategically disruptive biological agent.

2. Anthrax: Durable Spores Without Contagion

Bacillus anthracis is a gram-positive, spore forming bacteria. Its spores can live in soil or on contaminated material for years. Unlike smallpox, anthrax is not easily transmitted person to person; its danger lies in its persistence, delayed recognition, and severity of the inhalational disease. The bacterium uses an antiphagocytic capsule and two binary exotoxins. The protective antigen promotes the entry of the oedema factor and the lethal factor into cells. These components then interfere with cellular processes and immune signalling. Inhalational anthrax usually starts not like ordinary pneumonia, but usually in the thoracic lymph nodes. Symptoms usually develop within a week but may take as long as two months.

Inhalation anthrax is almost always fatal without treatment. But with aggressive treatment, survival jumps to about 55%. The strategic value of forensic epidemiology is exemplified by the 1979 Sverdlovsk anthrax outbreak, when an agent was released by a Soviet military facility, causing at least 96 infections and 64 deaths. Initially, Soviet authorities blamed the contaminated meat. Later evidence revealed that airborne exposure and a serious verification weakness were involved in the incident. The 2001 letters attacks in the United States involved 22 cases and five deaths, and the response utilised more than 1,500 responders and 500 field personnel. The episode showed how an incident in a single locality can have national costs far exceeding the immediate toll of casualties.

3. VX: Persistent Toxic Terrain

VX is an organophosphorus nerve agent of extreme toxicity and inhibits acetylcholine esterase, resulting in the accumulation of acetylcholine at the muscarinic, nicotinic and central synapses. Cholinergic crisis thus may lead to bronchospasm, seizures, paralysis and respiratory failure. VX is much less volatile than sarin, and the oily liquid can persist for days or months depending on conditions. This means that contaminated ground, equipment and casualties can remain dangerous long after the initial exposure.

The United States once had a chemical stockpile of approximately 30,500 tonnes, of which more than 4,500 tonnes were VX, but verification has since established destruction of declared holdings. In the 2017 killing of Kim Jong-nam, the secret threat of VX and the depth of chemical forensics were revealed, with Malaysian authorities providing 102 samples to the OPCW. Analysts say that various samples contained VX, three precursors, four degradation products, one metabolite, and protein-bound VX, which gave investigators enough information to reconstruct exposure through the parent agent and its chemical signatures. VX is not spread from casualty to casualty. Meanwhile, it is also the most toxic and persistent chemical agent in the combined list.

4. Sarin: Speed and Vapour Exposure

Sarin (GB) is a volatile organophosphorus nerve agent which inhibits acetylcholinesterase (AChE) and evaporates readily enough to be a rapid inhalational emergency. Symptoms can develop within seconds or minutes. Sarin can inflict rapid casualties in confined spaces but is less persistent in contamination than VX or sulphur mustard. Therefore, outcomes are determined by ventilation, weather, detection and respiratory protection.

The most obvious example of mass casualties was the attack on the Tokyo underground in March 1995, in which the OPCW says 13 people died and over 1,000 were seriously injured. Furthermore, secondary contamination and large numbers of worried but unexposed people strain hospitals. In April 2017, the OPCW confirmed that sarin had been used on civilians at Khan Shaykhun in Syria and that nerve agents are still being used despite the global ban. Sarin reduces all warning and response times, but its reduced persistence is still less than VX in this assessment.

5. Sulphur Mustard: Delayed Injury and Incapacitation

Mustard gas, or sulphur mustard, is not a true gas but a persistent oily vesicant that alkylates DNA, proteins and other cellular targets. This may cause damage to eyes, skin, respiratory epithelium and bone marrow. Mustard ranks fifth because it kills less rapidly than nerve agents. However, it contaminates equipment, slows manoeuvres and places long-term pressure on medical services. Eye and airway injuries can remove personnel from duty without causing immediate death.

Several agents are said to have been involved in the attack on Halabja, Iraqi Kurdistan, on 16 March 1988. OPCW estimates about 5,000 dead and 10,000 injured. Others suffered long-term damage to their lungs, eyes and skin. Skin reddening may develop between 2 and 48 hours after exposure, and eye symptoms may develop earlier. The delayed onset allows those exposed to continue functioning until they break down. There is no antidote to reverse the cellular damage. Supportive care is provided by clinicians.

Top 10 Biological-Warfare Agents

Notable Agents Beyond the Top Five

The combined ranking does not erase other major threats; instead, it weighs contagiousness, lethality, persistence, detection and treatment across both weapon classes.

Other Leading Biological Agents

Yersinia pestis is the bacterium that causes pneumonic plague, a serious respiratory illness. It is transmitted by droplets through close contact, and without immediate antibiotics, the disease progresses rapidly. However, botulinum toxin causes paralysis and respiratory failure. It is compelling, but it is not transmittable. Another large threat group is that of viral haemorrhagic-fever agents, including Ebola, Marburg and some arenaviruses, which can cause severe disease and strain clinical care.

WHO guidance identifies anthrax, botulinum toxin and plague as agents capable of creating major public-health emergencies; moreover, transmissible agents can generate secondary epidemics and initially resemble natural outbreaks. Therefore, a biological-only strategic list would read: smallpox, anthrax, pneumonic plague, botulinum toxin and viral haemorrhagic fever agents.

Other Leading Chemical Agents

VX heads a list of chemicals-only, followed by Novichok-family nerve agents, sarin, soman and sulphur mustard. “Novichok” is a family of highly toxic organophosphorus compounds, not a single chemical compound with a single toxicity value. Soman, or GD, is also fast-acting, and some of its toxic effects can be difficult to reverse shortly after exposure. Sulphur mustard is not as strategically lethal immediately as these nerve agents but is historically far more important and capable of producing casualties.

Top 10 Chemical-Warfare Agents

Operational Impact and Defensive Priorities

Smallpox is an agent of force-wide contagion and strategic continuity; anthrax is a combination of environmental durability and delayed, severe disease. Each agent presents a different battlespace problem. VX, however, is a permanent toxic hazard. Sarin causes an immediate vapour emergency. Sulphur mustard causes delayed casualties, contamination and medical attrition. So the top 5 chemical and biological weapons require different warning, protection and care plans.

Chemical defence thus covers detection, individual protection, casualty management and contamination control while biological defence includes surveillance, laboratory confirmation, vaccination, isolation and public health command. The Nuclear and Biological section provides related research, while the Comparison section places capability rankings in a wider military context.

Treaties, Verification and the Strategic Bottom Line

By 31 December 2025, the OPCW’s verified total covered all 72,304.34 tonnes of declared chemical-agent stockpiles. The CWC has 193 member states, which comprise about 98% of the world’s population. But you can’t ‘verify’ the non-existence of undeclared assets. The BWC had 188 state parties but lacked a verification organisation like the CWC, making it more difficult to gauge biological compliance.

Credible CBRN assessment must thus incorporate treaty declarations, health surveillance, laboratory evidence, chemical forensics, and disciplined sourcing. None of the top five chemical and biological weapons share a single quality. Smallpox is contagious. It kills. Anthrax kills with durable spores . VX kills by lingering. Sarin kills quickly. Sulphur mustard kills by disabling its victims. In the end, readiness is as important as toxicity, because detection, protection, medical capacity and reliable command systems can turn an incident into a strategic crisis or contain it.

References

• Defense News Today — Nuclear and Biological
• Defense News Today — Comparison
• WHO — Public Health Response to Biological and Chemical Weapons
• OPCW — Destruction of All Declared Chemical-Weapons Stockpiles

About the Author

Farhan J. Satti (Chief Editor)

Administrator

Mr. Farhan J. Satti is a defense analyst and HR leader with 15+ years covering weapons, arms control, and operations. UN Disarmament Affairs-trained (WMD missiles, lethal autonomous weapons) and firearms-safety trained (Washington State), he explains hard problems with clarity and impact.

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