CNS Research Story

Osmium Tetroxide - a New Chemical Terrorism Weapon?

Osmium tetroxide structure with ampoules.
[Src: Polysciences, Inc.]

By Michelle Baker and Margaret E. Kosal

April 13, 2004

A highly toxic chemical has emerged as a key component of an alleged terrorist plot in London. Serving legitimate functions in biological research and in specialized chemical industry, osmium tetroxide's suitability as a terrorist agent - a dual use compound - is limited, despite the characterizations of it generating "chemical fallout."[1]

Appearance in a Recent Terrorist Plot

On 30 March 2004, hundreds of British anti-terrorism police conducted raids throughout the London area after tracking a group of terrorist suspects over the course of several months.[2] Authorities subsequently arrested eight British citizens of Pakistani origin, who were allegedly involved in the planning stages of a terrorist attack. In the following week, reports emerged that these suspects, including a Canadian and a British-Algerian, were researching the potential of detonating a chemical bomb in a crowded, civilian location within London.[3] Authorities conducted the raids at 24 locations within London after learning from GCHQ, the British electronic eavesdropping intelligence agency, that these terrorist suspects were discussing the use of osmium tetroxide during phone calls among themselves within Britain and to Pakistan.[4] Some sources claim that the U.S. National Security Agency collaborated with its British counterpart to help intercept the phone calls implicating the suspects in planning a chemical attack.[5] The British Home Office would not, however, comment on the alleged scenario, as the case is still under investigation by authorities.[6]

The suspects reportedly were not able to acquire the osmium tetroxide before authorities were able to intercept members of the group. Those involved are allegedly sympathetic to al-Qa'ida and were preparing to target Gatwick airport, the London subway, or other enclosed high-traffic areas.[7] Al-Qa'ida has previously produced training manuals containing plans for use of choking agents as a method of attack, yet this is the first instance of osmium tetroxide being included among the list of possible chemical agents.[8] This is the first incident in the open literature in which the chemical has been connected with terrorism. Although this plot did not progress beyond the planning stages, the potential use of osmium tetroxide has raised new fears about al-Qa'ida's pursuit of dual-use chemicals as terrorist weapons and has encouraged discussion about the potential lethality of such a substance when combined with a conventional explosive.

Characteristics of Osmium Tetroxide (OsO4)

Scientists are already familiar with the use and effects of osmium tetroxide (OsO4) even as these recent reports have introduced the general public to the compound for the first time. OsO4 is a colorless to pale yellow solid at room temperature, occasionally called osmic acid. The solid readily evaporates at room temperature (has a high vapor pressure). An open canister left in an enclosed area would be readily noticeable based on the characteristic pungent, ozone- or chlorine-like smell. For the numerically inclined: the vapor pressure of OsO4 is 7 mm Hg at 20°C/68°F, compared to a vapor pressure of 17 mm Hg for water, 2.10 mm Hg for sarin nerve agent (GB), and 0.0007 mm Hg for VX nerve agent[9] (the latter three are liquids). The vapor pressure of a chemical is important in determining the inhalation hazard. Solids and liquids with no vapor pressure don't evaporate and therefore don't pose an inhalation hazard unless they are mechanically aerosolized. Liquids with very low vapor pressures, like VX nerve agent, don't evaporate readily and therefore are considered a much more significant threat for exposure via direct skin contact.

Physiological Effects of OsO4 Exposure

Osmium tetroxide is highly toxic and a rapid oxidizer. Severe reactions may result through all routes of exposure: inhalation, ingestion, contact with the eyes and other mucous membranes, and contact with skin. Because of its volatility, the vapor hazard is usually emphasized. Exposure to the vapor can cause severe chemical burns to the eyes, skin and respiratory tract. Very short-term contact with the vapor may generate a lachrymation (tear-causing) response, accompanied by coughing, headaches, and dizziness.[10] Among the most insidious effects of osmium tetroxide is its capacity to cause irreversible blindness - literally turning the corneas black. Symptoms may not be noticed until several hours following exposure, which may be an attractive feature for terrorists. People may not realize the extent of the toxic effects of a compound to which they have been exposed immediately, rather the damage will be occurring while they continue on their day. Another delayed effect of substantial inhalation exposure is a build up of fluid in the lungs (edema) leading to "dryland-drowning." Exposure to osmium tetroxide dissolved in water will turn the skin black. Painful burns or dermatitis may result depending on the concentration. It is not known, however, to be cancer-causing.

OsO4 can be compared to traditional chemical warfare agents (table below). The first appearance of a physiological response, also known as a threshold effect, is observed at a lower concentration for osmium tetroxide vapor exposure than phosgene (CG), sulfur mustard (HD), or sarin nerve agent (GB). At first glance, the inhalation hazard associated with OsO4 is comparable to that of the traditional asphyxiant phosgene and blister agent sulfur mustard based on lethal inhalation concentrations (LCt50). Phosgene is a gas at ambient conditions, so all of the material will be available as an inhalation hazard. On the other hand, sulfur mustard is a liquid with a fairly low vapor pressure (0.072 mm Hg),[11] which will result in a decreased volatility relative to OsO4 (626 mg/m3 for sulfur mustard versus 97,300 mg/m3 for OsO4 at 20°C/68°F). So there will be over 150 times more OsO4 vapor available in an enclosed area relative to sulfur mustard vapor.

While the lethal inhalation concentration of OsO4 is substantially larger than that for sarin, again the decreased volatility of the traditional warfare agent (16,090 mg/m3) should be considered in evaluating the relative threat. Under similar conditions, there will be six times more OsO4 vapor in an enclosed area compared to sarin vapor. The overall inhalation risk for osmium tetroxide is estimated to be closer to sarin nerve agent than sulfur mustard or phosgene gas.

Toxicity Comparison of Osmium Tetroxide with Three Traditional Chemical Warfare Agents

  Threshold effects
(mg / m3)
(mg-min / m3)
(mg / kg)
OsO4 0.1 - 0.6[12] 1316[13] 162[14]
Phosgene 2[15] 3200 n/a***
Sulfur mustard 12-500[16] 1500[16] 100[16]
Sarin (GB) 2[17] 70[17] 24.3[17]
*LCt50 is the vapor concentration that will cause death by inhalation in fifty percent of a population.
** LD50 is the liquid concentration that will cause death via exposure through the skin (percutaneous), in this comparison, in fifty percent of a population. Values are given in mg per kg of total body weight; a 150 lb human weighs approximately 68 kg.
*** n/a = not applicable. Phosgene is a gas at ambient conditions.

Legitimate Uses of OsO4

This substance is used primarily in the preparation of biological samples, a technique called "fixation" or "fixing," to help maintain cellular and sub cellular structures that would otherwise be damaged during further processing. Fixing is an important step in most biological applications of electron microscopy - looking at very small structures with electrons rather than light. OsO4 reacts with the olefins in fatty acids and other tissues. Fixing has some similarities to staining used in traditional microbiology - the osmium atomic nucleus helps make the biological structures more easily "seen" under an electron microscope.

Osmium tetroxide is also used in specialty organic chemistry reactions,[18] such as the synthesis of the synthetic human-hormone norestradiol[19] and industrially significant glycol compounds. These reactions using solid osmium tetroxide are most commonly done on the laboratory scale.

Commercial Availability

Osmium tetroxide is commercially available as either a solid or as an aqueous solution (less than 6% OsO4 by weight, due to limited solubility in water). Commercial quantities are typically very small and prices are high. Cost for the largest, commercially available units from a leading U.S. chemical supplier range from $118 for 1 gram of the solid compound to $195 for a 25 mL ampoules containing 2.5% OsO4 by weight, dissolved in water (0.625 grams OsO4 per vial). A terrorist attempting to use OsO4 in the creation of a chemical terrorist weapon would most likely be hindered by the high cost of the substance. There would also be a danger to the terrorist in attempting to prepare an improvised explosive device containing large quantities of the chemical compound.

In packages of five grams or more, larger quantities of material are commercially available in which osmium tetroxide is bound to a polymer backbone. The polymer backbone, or support, eliminates the vapor hazards associated with solid OsO4. If a potential terrorist were to seek to acquire large quantities of this type of immobilized OsO4, the utility as a weapon would be extremely low. Such materials were designed specifically to protect industrial workers.

A leading U.S. chemical supplier of OsO4 does not take any special precautions regarding sale of the chemical. Because of the potential dual-use nature of many chemicals with legitimate industrial and research purposes, all orders are screened prior to shipment.


If an OsO4-containing solution were to be used as a chemical terrorist weapon, it could be decontaminated with copious amounts of any "unsaturated" cooking oil or dry milk.[20] Once a solution is black, the risk of rampant oxidation (burning) is abated.

Viability of OsO4 as a Chemical Terrorism Weapon

The feasibility of using a bomb to disburse OsO4 is highly suspect. When heated OsO4 rapidly decomposes to OsO2, which is effectively a rock. OsO2 is used as a ceramic resistor in specialty electronic applications. The inhalation hazard would be destroyed with the bomb explosion rather than generating "chemical fallout" as in a dirty bomb scenario. In addition to the difficulties and hazards faced by anyone seeking to use osmium tetroxide as a dirty bomb, the effect of the compound would be minimal in an open space, and it would not leave lasting contamination in an area in the same manner as a radioactive bomb. Because it is such a rapid oxidizer, it would most likely first enhance the combustion of the materials used for the bomb. As an oxidizer for an improvised explosive device, OsO4 would be a very expensive choice and very risky for the bomb assembler. Thus, its utility in the creation of a dirty bomb, when combined with conventional explosives, is questionable.

Chemical terrorism incidents are not limited to those events involving explosives or incendiary materials. The likelihood of OsO4 to cause harm as a chemical agent alone is substantially greater than as part of a dirty bomb. The major danger from the solid is via inhalation. An enclosed space with poor ventilation would present the greatest hazard. OsO4 would not be an effective chemical terrorism weapon for a large, open air venue. The major danger in solution form is via the skin (percutaneous) or ingestion.

As a terrorist weapon, however, the biggest problem with osmium tetroxide is its nature as a rapid, indiscriminate oxidizer. OsO4 doesn't distinguish between membranes in the human eye and lungs, plants, rubber, or cooking oil. While it has the potential to inflict horrifying damage to the body in the form of chemical burns and blindness, the chemical does not specifically target a critical physiological function as nerve agents do. A second limitation as a terrorist weapon is its volatility. The persistency of both sarin and VX substantially exceed that of OsO4.


OsO4, although unquestionably a lethal compound, is not estimated to be a viable dirty bomb hazard as it will readily decompose if utilized with explosives. In comparison to traditional chemical warfare agents, OsO4 has similarities to the choking agents in its high volatility and targeting of the respiratory system. It resembles the blister agents, like sulfur mustard in that it attacks the eyes, burns the skin (by a different molecular mechanism than sulfur mustard), and some effects may be delayed. The blindness from OsO4 vapor exposure, however, may be permanent unlike sulfur mustard. Because of its high volatility combined with high toxicity, the inhalation risk of OsO4 vapor verges on that of sarin nerve agent, but it does not target critical nerve connections that control the cardiovascular and respiratory systems as the nerve agents do. Additionally, the persistency of osmium tetroxide vapor is low in comparison with the nerve agents and sulfur mustard.

The incorporation of osmium tetroxide, a fairly obscure inorganic compound, suggests some familiarization with advanced undergraduate level chemistry. The British terrorist suspects recognized the deleterious health effects, but their plan to incorporate OsO4 into a conventional explosives bomb shows a lack of sophisticated and detailed understanding of inorganic chemistry. Such knowledge might be indicative of a graduate-level individual or technician in a research or industrial biochemistry, molecular biology, or biomedical engineering laboratory with access to OsO4. Such a plot does not point to a person with graduate-level experience in synthetic chemistry or significant experience in an industrial setting. This incident may also hint at an escalating terrorist interest in pursuing non-traditional chemicals as improvised weapons. Recent events have forced British authorities to investigate such a threat, and such a possibility has caused scientists to speculate on the utility of OsO4 for use by a terrorist group.

1 Ben Taylor and Stephen Wright, "Britain foils chemical bomb plot," The Advertiser (Adelaide, Australia), 8 April 2004, accessed 8 April 2004, <>.
[2] Brian Ross and Christopher Isham, "'Very nasty,' Potential bomb plot involved deadly chemical,", 5 April 2004, accessed 7 April 2004, <>.
[3] Richard Norton-Taylor and Rosie Cowan, "Chemical bomb plot uncovered," Guardian Unlimited, 7 April 2004, accessed 7 April 2004, <>.
[4] Ross and Isham, "'Very nasty,' Potential bomb plot involved deadly chemical;" Sengupta, Kim, "Terror gas attack on Tube foiled by security agents,", 7 April 2004, accessed on 7 April 2004, <>.
[5] Norton-Taylor and Cowan, "Chemical bomb plot uncovered;" Sengupta, "Terror gas attack on Tube foiled by security agencies."
[6] Martin Williams, "Terrorism plot chemical is for sale on the internet," The Herald, 7 April 2004, accessed 7 April 2004, <>.
[7] Ross and Isham, "'Very nasty,' Potential bomb plot involved in deadly chemical."
[8] Williams, "Terrorism plot chemical is for sale on the internet."
[9] Army Field Manual No 3-9, Potential Military Chemical/Biological Agents and Compounds (Washington, DC: Department of the Army), December 1990, p. 94.
[10] National Academy of Sciences, Prudent Practices in the Laboratory: Handling and Disposal of Chemicals (Washington, DC: National Academies Press, 1995), p. 364.
[11] Army Field Manual No 3-9, Potential Military Chemical/Biological Agents and Compounds (Washington, DC: Department of the Army, December 1990), p. 31.
[12] A. McLaughlin, R. Milton, and K. Perry (1946): "Toxic manifestations of osmium tetroxide" British Journal of Industrial Medicine, vol. 3, (1946), pp. 183-186, who report workers exposed to such levels "suffered from lacrimation and disturbances of vision and in some cases, headache, conjunctivitis, and cough." The value also reflects the current NIOSH Immediately Dangerous to Life or Health Concentration (IDLH), <>.
[13] Centers for Disease Control IDLH Documentation, <>. Value derived from laboratory results done on multiple animal species.
[14] Material Safety Data Sheet (MSDS) - Osmium tetroxide. No human toxicity data reported; values based on reported animal acute toxicity data, <>.
[15] Material Safety Data Sheet (MSDS) - Phosgene, <>, and ATSDR Medical Management Guidelines for Phosgene, <>.
[16] Frederick R. Sidell, John S. Urbanetti, William J. Smith, and Charles G. Hurst "Vesicants" in Textbook of Military Medicine: Medical Aspects of Chemical and Biological Warfare (Washington, DC: Office of the Surgeon General Department of the Army, 1997), <>. Values of 12-70 mg-min/m3 are cited as threshold for eye damage and 100-500 mg-min/m3 are noted for inhalation airway injury.
[17] Material Safety Data Sheet (MSDS) - Lethal Nerve Agent Sarin (GB), <>.
[18] F.A. Cotton, and Geoffrey Wilkinson, Advanced Inorganic Chemistry (New York: John Wiley & Sons, Inc., 1998), pp. 880-881.
[19] Alaxander Kuhl, Heiko Karels, and Wolfgang Kreiser, "New synthesis of 18-norestradiol" Helvetica Chimica Acta, vol. 82 (1999), pp. 30-34.
[20] National Academy of Sciences, Prudent Practices in the Laboratory: Handling and Disposal of Chemicals (Washington, DC: National Academies Press, 1995), p. 167.

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Author(s): Michelle Baker, Margaret E. Kosal
Related Resources: CBW, Terrorism, Weekly Story
Date Created: March 30, 2004
Date Updated: -NA-
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