The events on and subsequent to 9/11 have heightened public interest in the potential for further terrorist attacks. In addition to the use of microbiological weapons, such as anthrax, the 1995 Tokyo subway attack alerted us to the likelihood that chemical agents can be used to inflict injury and death to civilian populations, especially in, but not restricted to, crowded urban areas. The use of chemical warfare agents is recorded as far back as ancient Greece but they were not extensively employed until World War I. An excellent review, which provides a valuable historical record of chemical warfare agents, was published by Brigadier-General Alden H. Wait (U.S. Army) in Gas Warfare: The Chemical Weapon, Its Use, and Protection Against It (Duell, Sloan and Pearce, New York, 1942).

There have been attempts to use many gases in warfare, either by generating the gases at the scene of battle or by encasing them in artillery shells, which were then fired at the enemy. Thus, chlorine, hydrogen cyanide, chloropicrin and a number of other chemicals were used in the war of 1914-1918.(13) Although war gases were not used in World War II, they were prepared in large quantity and held in reserve in the event of a gas attack by the other side. One such reserve was an American ship in the harbor of Bari, Italy, which was carrying 100 tons of mustard gas.(14) While much of the agent was destroyed by the bombing, the oil released by damaged ships absorbed enough of the agent to impact on personnel in the water resulting in the only reported American casualties related to mustard gas in WWII.(1) The agent was identified as a nitrogen mustard [methyl-bis(beta-chloroethyl)amine hydrochloride]. Sulfur mustard [bis-(2-chloroethyl)sulfide] will be discussed below.

At the end of WW II, the Allies captured large stocks of German nerve agents. The Americans and British, on the one hand, and the Russians, on the other, retrieved them, destroyed or discarded part of the stockpile and took the remainder back to their countries to establish stockpiles of those agents which could be used to counter future chemical threats. Thus, the chemical warfare agents known as the G agents were those obtained from Germany after WW II. In addition, the United States later developed its own agent, which is called VX.

The United States government is committed to destroying a range of chemical weapons, as described below, and that process could lead to accidents during the weapons destruction process which could have serious consequences for both the workers engaged in processing the weapons and civilian populations in the area. Furthermore, the Tokyo incident demonstrated the feasibility of a terrorist attack using chemical warfare agents. Thus, it is important that the medical community should become familiar with the likely agents of concern and therapeutic measures that might be taken to rescue affected individuals. This Cyberounds^® will feature two types of chemical warfare agents: the nerve agents and the vesicants.

Sources of Information

References for the information presented in this Cyberounds^® appear at the end. However, it is appropriate to cite some specific sources at the outset. The United States Congress issued a mandate to the U.S. Army in 1993 to destroy its entire stockpile of chemical munitions, as well as, non-stockpile chemical materiel (NSCM). NSCM included lethal wastes from previous disposal efforts, e.g., dating as far back as the post-WW I era when many of these weapons were put in landfills. Other sources were unserviceable munitions, contaminated containers used in preparing the munitions, and various facilities and sites where they were handled.

There were eight stockpile sites in the U.S. and one on an island in the Pacific Ocean. However, there were 82 sites where NSCM were located. To assist in the process, the Army developed a series of exposure standards aimed at protecting the workers involved in weapons destruction and clean-ups, as well as people who would be using the remediated site for other uses at a later time. They then asked the National Academy of Sciences, through the Committee on Toxicology of the National Research Council (NRC), to evaluate their effort and to make recommendations that might improve the work. The result was the establishment of the Subcommittee on Chronic Reference Doses for Selected Chemical-Warfare Agents and the eventual publication of a NRC report entitled Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical- Warfare Agents (National Academy Press, 1999) (NRC/COT). The NRC report has been used, in part, as a resource in the preparation of this Cyberounds^®. Note that reference doses are, in effect, acceptable exposures for a lifetime.

The National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL) (which is composed of representatives from 16 federal agencies, representatives from individual companies and trade associations, representatives from unions and public interest groups and some academics) was asked by the Army to review an additional set of standards aimed at short term exposure. The NAC issues technical documents in support of a series of air-borne exposure standards for 10 and 30 minutes, and 1, 4 and 8 hours for doses which have no effects, health-damaging effects and lethal effects. These reports, which are published in the Federal Register, are critiqued by a NRC committee, and eventually published as NRC documents, also served as a valuable resource. Reference will be made to the Technical Support Document on the "G agents" (TSD-G) and the Technical Support Document on VX (TSD-V).

[A large amount of information on chemical warfare agents and their control can be found at the The Chemical Weapons Convention web site. While much of the factual information regarding the effects of these agents can be found elsewhere and is in agreement with these documents, some of it is unique. Unfortunately, the reports are not accompanied by a bibliography. A list of references has been requested and hopefully will be added to our references at a later date.]

Much of toxicology is devoted to understanding the dose-response relationship. While the effects of chemicals are usually readily measured, the dose is a function of exposure, a phenomenon to which insufficient attention has been paid in the past. In this Cyberounds^®, Dr. P.J. Lioy will discuss some aspects of exposure including general information on the pathways and types of contacts that can lead to or enhance exposure to chemical agents.(6),(4),(11)

Chemical Warfare Agents

The agents that will be discussed here are GA (Tabun), GB (Sarin), GD (Soman), and VX, all of which are termed "nerve agents," and sulfur mustard and lewisite, which are vesicants. The G agents and VX are all cholinesterase inhibitors. The vesicants are blistering agents on the skin and other exposed tissues and may have similar effects on surfaces of the respiratory tract.

Nerve Agents

The chemical warfare nerve agents are part of a broad classification of chemicals which are known to inhibit a series of esterases including acetylcholinesterase, which is found at synapses and in red blood cells, and butyryl (or pseudo) cholinesterase, which is found mainly in blood plasma. Carboxyesterase, also known as the neuropathy target esterase, is also inhibited by many anticholinesterases, resulting in delayed neurotoxicity (OPIDN), which is discussed below.

The agents that will be discussed are:

• GA- Tabun (dimethylamidocyanophosphate)
• GB- Sarin (isopropyl methylphosphonofluoridate)
• GD- Soman (pinacolyl methylphosphonofluoridate)
• VX- O-ethyl-S-(isopropylaminoethyl)methyl phosphonothiolate

Potency of the Nerve Agents

The nerve agents are noted for their potency when compared with other cholinesterase inhibitors. The hazardous substances database, however, is incomplete, with the lack of inhalation data as the most critical deficiency. Nevertheless, some estimates of relative potency have been made based on animal studies in which the agents were administered by various routes and extrapolations were made to humans.(5),(8),(9) Thus, the estimated oral LD₅₀ values for the nerve agents are shown in Table 1.

Other data were available in animals given the agents by various routes. By viewing the entire database, despite its deficiencies, the NRC committee 10 recommended reference doses (RfD) for each agent. The committee defined the RfD as "an estimate (with uncertainty spanning perhaps an order of magnitude or greater) of a daily dose to the human population (including susceptible sub-populations) that is likely to be without an appreciable risk of deleterious health effects during a lifetime." The process involves selecting the most reliable study in animals or humans in which the agent in question either exerts no effect or a study in which the lowest effective level of exposure is identified. That value is then divided by the product of a series of uncertainty factors, including uncertainty in extrapolating between species, lack of homogeneity of response within the human population, extrapolation from subchronic to chronic exposure, quality of the database, etc. Using this approach, the NRC recommended a series of RfD values for the nerve agents. Table 1 shows both the estimated acute lethality and the RfD values for these chemicals in humans.

The NAC/AEGL examined the data base with respect to establishing short term exposure limits defined as AEGL values:

AEGLs represent threshold exposure limits for the general public and are applicable to emergency exposure periods ranging from 10 minutes to 8 hours. Three levels -- AEGL-1, AEGL-2 and AEGL-3 -- are developed for each of five exposure periods (10 and 30 minutes, 1 hour, 4 hours and 8 hours) and are distinguished by varying degrees of severity of toxic effects.

The three AEGLs are defined as follows:

AEGL-1 is the airborne concentration (expressed as parts per million or milligrams per cubic meter [ppm or mg/m³) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic, non-sensory effects. However, the effects are not disabling, and are transient and reversible upon cessation of exposure.

AEGL-2 is the airborne concentration (expressed as ppm or mg/m³) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.

AEGL-3 is the airborne concentration (expressed as ppm or mg/m³) of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death.

The currently recommended, but not yet fully accepted, AEGL values for the nerve gases are:

Specific Anticholinesterases(3),(12)

Physostigmine (eserine) was the first anti-cholinesterase studied intensively and indeed was first used therapeutically in 1877 in the control of glaucoma. Its basic structure contains a methylated carbamoyl group. Attempts to produce therapeutically useful congeners led to the development of the therapeutic agents neostigmine, pyridostigmine, etc., and the carbamate insecticides such as carbaryl. These chemicals inhibit cholinesterase by reacting with the active site via ionic bonding and the effect is reversible.

Ispropylflourophosphate (DFP), an organophosphate, was an early irreversible cholinesterase inhibitor used therapeutically in the treatment of myasthenia gravis. Organophosphates, such as tetraethylpyrophosphate, parathion, paraoxon, diazanon, malathion, etc., were also developed as insecticides The first organophosphate insecticides were synthesized in Germany in 1937 and from these were developed the nerve gases. The organophosphates act by phosphorylating the active site of cholinesterase via the formation of a covalent bond. If no therapeutic measures are taken, the enzyme is permanently inactivated by the process known as "aging," where the enzyme-inhibitor complex undergoes chemical changes which serve to fix the covalently bound species in place. Early treatment with pralidoxime may displace the inhibitor from the reactive site. In the absence of intervention, new enzyme must be synthesized to replace the irreversibly inhibited enzyme. The most recent known incidents involving nerve agents against civilian populations were the use of Sarin in the 1988 Iraqi attack on Kurdish villagers and the Tokyo subway attack in March of 1995.

Toxicology of the Nerve Gases

Acute Toxicity

GA, GB, GD and VX are all cholinesterase inhibitors. The effects of exposure to these agents includes any or all of those described in Table 3 (adapted from Ecobichon, 1995).

Delayed Effects

Although physicians are usually taught to recognize the acute effects of anti-cholinesterases as they might be encountered in an emergency room situation, the longer term effects have not been emphasized.

Ecobichon(3) reports that in patients who survive exposure to nerve gases, residual effects may include a syndrome manifested as persistently lowered vitality and ambition; defective autonomic regulation resulting in headaches, gastrointestinal and cardiovascular symptoms; decline in potency and libido; intolerance to alcohol, nicotine and various medicines; and what appears to be premature aging. Alternatively, others display many of the above with the addition of a variety of neurological and/or neuropsychological effects which may include, but are not limited to, depression, syncopal attacks, amnesia or dementia.

Reports of pesticide applicators exposed for 10-15 years suggest that these people may exhibit tinnitis, nystagmus, pyrexia, ataxia, paresthesia, polyneuritis, paralysis, slurring of speech, amnesia, insomnia, drowsiness, lassitude, emotional lability, mental confusion and some more serious psychological manifestations.(3) This continues to be an area of some debate with many conflicting reports but suggests that the general dogma that, once the immediate effects of these agents have subsided the patient will exhibit no further consequences of exposure, requires careful re-evaluation.

Another form of toxicity produced by anti-cholinergic agents was described in Sri Lanka in 1987 and was termed intermediate syndrome. It appears that attempts at suicide with the organophosphate pesticides fenthion, dimethoate, monocrotophos and methamidophos resulted in the immediate effects described above, but within 24 to 96 hours among survivors following the acute crisis, the patients demonstrated weakness in facial muscles (palsy) and muscles of the limbs. Respiratory support was necessary to rescue the patients.

Organophosphate-induced delayed neurotoxicity (OPIDN) is another form of neurotoxicity caused by some organophosphates. The effect involves the wallerian degeneration, i.e., disruption of axons in conjunction with disassociation of axons from nerve cell bodies, of large diameter neurons and their myelin sheaths in peripheral nerves and the spinal cord. The effect appears to be related to the inhibition of an enzyme called the neurotoxic esterase. The neuropathy is observed 7-14 days after exposure to such agents as tri-o-tolyl phosphate and is characterized by initial weakness and flaccidity followed by spasticity, hypertonicity, hyper-reflexia, clonus and effects indicative of pyramidal tract and upper motor neuron damage. Animals models have been developed to study this phenomenon but there is insufficient data to determine that OPIDN is a consequence of exposure to nerve warfare agents.

Exposure to Nerve Agents

Toxic responses to exposure to nerve agents require sufficient contact of the gas with target tissues such as skin, eyes, respiratory tract, GI tract, etc. The magnitude and duration of that contact, i.e., the exposure (which is a function of concentration X time) will determine the ultimate dose presented to the target tissue. The distance between victims and the location from which the agents emanates can be an important factor. For inhalation of acute toxicants, such as chemical warfare agents, the closer the victim is to the point of release, the greater will be the toxic effects. Identification of the release point of the agent may be determined by questioning conscious patients regarding where they were when they were impacted or by examining the location of severely injured or deceased victims.

Respired agents may potentially contact tissues throughout the respiratory system. Materials can be delivered to the lung as gases or vapors, the physical and chemical properties of which will determine where they have their effects on the respiratory system. In contrast, the agent could be coated on a particle which is released as an aerosol and inhaled by the victim. The size of the carrier particles will determine the location of deposition and the site of toxic response or delivery into other organ systems. The smaller the particles, called fine particles (<1.0 μm in diameter), the farther they will be delivered into the lung but the less efficient the deposition. The larger particles will contact the surface of the respiratory system closer to the nasopharynx and will deposit with higher efficiencies. The most significant exposures can be associated with releases in enclosed spaces or within a plume of material closest to the point of release of the agent.

Dermal contact may result from either direct deposition of the toxicant on the skin (and eyes) and absorption through the hair follicles or the epidermal layers of the skin. Deposition of the agent on particulate matter, which then comes in contact with the skin, can result in absorption from the particles. Entry via the skin can also occur if the agent is released into the water supply. At the point of release from the tap water, there are three routes of exposure: direct ingestion by drinking the contaminated water, dermal absorption during showering, bathing or handling the water, or through inhalation during a shower. The effectiveness of the agent in water will depend upon its stability in aqueous media but the Bari event cited above(1) suggests that this route of exposure cannot be ignored.(6)

Diagnosis and Treatment

Diagnosis is based on observation of the symptoms described in Table 3. Although therapeutic measures should be instituted as soon as the diagnosis is made, exposure to nerve agents may be confirmed by measurements of erythrocyte acetylcholinesterase activity. Although there is no clear correlation between loss of acetylcholinesterase activity and degree of toxicity, it is usually assumed that loss of 50% of activity is associated with nervous impairment.

The following therapeutic recommendations are taken from the hazardous substances data base (HSDB 2002): At the outset, it is important to assure that the patient is well ventilated, by mechanical means if necessary. Blood pressure should be monitored and supported if necessary with the administration of 10-20 ml/kg of isotonic fluid with the patient placed in the Trendelenburg position. Administration of dopamine (5-20 mcg/kg/min) or norepinepherine (0.1-0.2 mcg/kg/min) may be required to support blood pressure.

Inhibition of acetylcholinesterase results in excessive synaptic accumulation of acetylcholine. The most direct protective mechanism is the administration of a cholinergic blocking agent. Atropine, at an initial dose of 2-5 mg, should be administered intravenously (iv), followed up with 2 mg every 10-15 min. In hypoxic patients, the intramuscular (im) route is recommended. In children, the dose is 0.05 mg/kg, given every 10-15 min, and treatment for several hours may be required depending upon the degree of poisoning.

Atropine is effective in antagonizing the muscarinic effects of anthicholinesterases but is not useful in treating peripheral neuromuscular impairment. The patient should also be given 1-2 g pralidoxime (2-PAM) iv over a 20-30 min period. For children the recommended dose is 25-50 mg/kg.

In the event that the patient undergoes seizures, benzodiazepine therapy is recommended, e.g., diazepam (adults: 5-10 mg, iv, repeat as needed; children: 0.2-0.5 mg/kg, repeat at 5 min intervals as needed) or lorazepam (adults: 4-8 mg, children: 0.05-0.1 mg/kg). Recurring seizures may be treated with phenobarbital (adults: 30 mg, children: 4-8 mg).

It is anticipated that exposure will occur via inhalation in a terrorist attack. The patient may cough and have difficulty breathing. Irritation of the respiratory tract, bronchitis and/or pneumonitis may result. Bronchospasm may require treatment with a β2 agonist and steroid treatment. Oxygen may be necessary. Chest x-ray, pulmonary function tests and blood gas measurements may be indicated.

In the event of ingestion of the anticholinesterase, gastric lavage may be performed after controlling any seizures and insuring good ventilation. Activated charcoal may be effective in absorbing the toxic agent. These procedures may be contraindicated if the agent has caused corrosive effects on the GI tract and the danger of perforation exists.

Eye exposure requires extensive washing of the eye with tepid water. Ophthalmological examination is recommended. Note that systemic signs of anticholinergic toxicity may be seen after eye exposure.

Dermal exposure requires washing of the skin directly in contact with the warfare agent and then removal of clothing and jewelry to eliminate a secondary source of the material for contact with skin. The open skin and the skin areas under the clothing, etc., require extensive washing with soap and water. Washing with household bleach (hypochlorite) has also been recommended. A Skin Decontamination Kit has been developed and is available from Rohm and Haas for military and civilian defense personnel. It contains an ion-exchange resin and activated charcoal and is applied to the affected area of skin to absorb residual agent. Some nerve agents can be absorbed through the skin and, accordingly, it is necessary to observe dermally exposed patients for systemic effect.

Vesicants

The vesicants, or "blister gases," were the most effective chemical agents used in WWI.(7),(13) More casualties were caused by sulfur mustard [bis-(2-chloroethyl)sulfide] than any other gas. Lewisite [dichloro(2-chlorovinyl)arsine] was developed at the end of WWI but not used in combat.

Acute Toxicity

Although the irritant effects of Lewisite may be rapidly appreciated, the effects of sulfur mustard may be delayed and not become apparent until one hour to as much as 12 hours later. Early signs are the development of large blisters in affected areas. Vesicants act by direct contact with exposed tissues to yield edema, ulceration and necrosis.(10) Affected areas may be skin, eyes and respiratory tract. Bronchopneumonia is commonly observed following exposure. The eyes are particularly sensitive to mustard gas with the likelihood of developing swollen lids, photophobia, blepharospasm, conjunctivitis, corneal damage and the potential for blindness (HSDB, 2002). Similar effects can occur in the GI tract following ingestion. The patient experiences burning discomfort in all affected areas. Indeed, the skin may appear to have undergone burns. Systemic toxicity is indicated by nausea, vomiting, fever and malaise.

Delayed or Recurrent Effects

Grant(15) reported that Army personnel who had been exposed to mustard gas in WWI and who were discharged from the service at a time that they were asymptomatic, subsequently developed ulcerated corneas on repeated occasions over a span of forty years. Military exposure has also been implicated in chronic lung impairment with cough, shortness of breath and chest pain. As in the case of many alkylating agents, sulfur mustard is thought to be a carcinogen. Most cancer was observed among the personnel of plants which manufactured the blistering agent and were chronically exposed. In epidemiolgical studies, frequently observed cancers were related to the upper respiratory tract and included the larynx, pharynx and trachea, but there were also reports of cancer of the lung and pleura. The lung data have been challenged because of the prevalence of cigarette smokers in the control group.(10)

Lethality and Reference Doses

Table 4 shows the results of lethality studies on sulfur mustard and lewisite [taken from HSDB (2002)].

Recommended reference doses (RfD) for sulfur mustard and lewisite are shown in Table 5.(10)

AEGL values for vesicants have not been established.

Range of Toxicity Values in Humans

For sulfur mustard, it has been estimated that 65 ug can cause skin damage; 10 μg can cause a blister. It has been calculated that exposure at the 50 mg-min/m³, i.e., the product of exposure concentration X time of exposure, will produce vesication. Eye injuries can be observed at 200 mg-min/m³.

For lewisite, 0.5 ml on the skin can have systemic effects and 2 ml may be lethal. An inhaled dose of 6 ppm may be lethal.

Diagnosis and Treatment

Diagnosis is based on observation of the symptoms described above. Note that there is no antidote. Once contact has been made with skin or other tissues the blistering process will be initiated.

The following therapeutic recommendations are taken from the HSDB (2002):

If the patient has been exposed orally, it is important to assure that the patient is well ventilated, by mechanical means if necessary. Protect the airway with patient placed in the Trendelenburg position. Do not induce emesis. Administration of dopamine (5-20 mcg/kg/min) or norepinepherine (0.1-0.2 mcg/kg/min) may be required to support blood. The agent may be diluted by administering either milk or water (120-240 ml, 4-8 oz.; in children do not exceed 120 ml.) The use of a flexible nasogastric tube to aspirate stomach contents should be considered, recognizing the potential for hemorrhage or gastric perforation in a patient whose GI tract has been exposed to a corrosive agent.

In the event of a lewisite exposure, the patient should be chelated using dimercaprol (BAL), succimer (DMSA), or 2,3-dimercapto-1-propane sulfonate (DMPS).(2) Continue until 24-hour urine reveals less than 50 μg/l of arsenic.

In the event of inhalation exposure, medical personnel should not enter the area without full body protection. Any part of the body exposed to these agents will be impacted and respond with blisters.

Rapid treatment is imperative. Note the patient's respiration. HSDB states: "stabilize the hemodynamic status, provide oxygen and manage the airway aggressively." Treatment may include a b2 agonist by inhalation for bronchospasm, a nebulized mist of sodium thiosulfate, promethazine and dexamethasone. In the event of exposure to lewisite, arsenic chelation should be initiated as described above.

In the event of noncardiogenic pulmonary edema, maintain oxygenation and monitor frequently. Maintain electrolyte balance. Monitor for hypotension and administer catecholamines as needed.

In the event of eye exposure, wash the eye with large amounts of tepid water for at least 15 min. For sulfur mustard, a 2.5% solution of sodium thiosulfate should be used. For lewisite, apply BAL. Monitoring for systemic effects after eye exposure is critical.

In the event of dermal exposure, decontamination of skin as rapidly as possible is important. For sulfur mustard, a 2.5% solution of sodium thiosulfate should be used. For lewisite, use a 5% solution of sodium hypochlorite (or diluted household bleach). Then wash with soap and water followed by dermal application of 5% BAL ointment. US military decontamination kits employ resins including Ambergard XE-555 and XE-556. They also recommend the use of talcum powder, flour or Fuller's earth to absorb and remove residual agent. Lewisite carries with it the additional possibility of kidney damage from the arsenic. Hemodialysis may be necessary in the event of kidney failure.

Note to Medical Personnel

In the event of inhalation exposure, medical personnel should not enter the area without full body protection. Any part of the body exposed to these agents will be impacted and will respond with blisters.

Furthermore, all clothing or effects of patients should be treated as hazardous material. Residues of the vesicants may impact on personnel treating the patient either by direct contact or by inhalation of vapors.