Decontamination of Chemical Warfare Agents

Introduction

In protection against chemical warfare agents the decontamination is an important unavoidable part. The aim of decontamination is to rapidly and effectively render harmless or remove poisonous substances both on personnel and equipment. High decontamination capacity is one of the factors which may reduce the effect of an attack with CW agents. In this way, it may act as a deterrent.

The need for decontamination should be minimized to the extent possible by contamination avoidance and early warning. Equipment can be covered, for example, or easily decontaminated equipment can be chosen by means of suitable design and resistant surface cover.

Decontamination is time consuming and requires resources. Nerve agents and substances causing injury to the skin and tissue are easily soluble in, and penetrate many different types of material, such as paint, plastics and rubber, all of which renders decontamination more difficult. If CW agents have penetrated sufficiently deep, then toxic gases can be released from the material for long periods. By adding substances which increase the viscosity of a CW agent, its persistence time and adhesive ability can be increased. These thickened agents will thus be more difficult to decontaminate with liquid decontaminants since they adhere to the material and are difficult to dissolve.

The need for decontamination can only be established by means of detection. If detection is not possible, then decontamination must be done solely on suspicion of contamination.

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Decontaminants

All decontamination is based on one or more of the following principles:

  • to destroy CW agents by chemically modifying them (destruction),
  • to physically remove CW agents by absorption, washing or evaporation,
  • to physically screen-off the CW agent so that it causes no damage.

Most CW agents can be destroyed by means of suitable chemicals. Some chemicals are effective against practically all types of substances. However, such chemicals may be unsuitable for use in certain conditions since they corrode, etch or erode the surface. Sodium hydroxide dissolved in organic solvent breaks down most substances but should not be used in decontaminating skin other than in extreme emergencies when alternative means are not available.

Decontaminants that have effect only against a certain group of substances can be an alternative in favour of a substance with general effect. The condition is that they will have a faster and better effect against the substance in question and/or a milder effect. Examples of such substances are chloramine solutions which are often used to decontaminate personnel. These have good effect against mustard agent and V-agents but are ineffective against nerve agents of G-type (sarin, soman, tabun). A water solution of soda rapidly renders nerve agents of G-type harmless but when used in connection with V-agents, it produces a final product which is almost as toxic as the original substance. This does not prevent V-agents being washed-off with a soda solution, provided a sufficient amount is used. However, the final product will always be poisonous.

The disadvantage of specifically-acting decontaminants is partly that it is necessary to know which CW agent has been used and partly that access to several different types of decontaminating substances is required.

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Decontamination methods

CW agents can be washed and rinsed away, dried up, sucked up by absorbent substances, or removed by heat treatment. Water, with or without additives of detergents, soda, soap, etc., can be used, as well as organic solvents such as fuel, paraffin and carburettor spirit. Emulsified solvents in water can be used to dissolve and wash-off CW agents from various contaminated surfaces.

When decontaminating by washing, consideration must be taken to the poisonous substance remaining in the decontaminant unless the CW agent has first been destroyed. The penetration ability of a CW agent can be enhanced when mixed with solvent. Today, there is an international development towards chemically resistant paints and materials, which implies that water-based methods will become more effective. However, the need for penetrating decontamination methods will remain for many years.

When washing with water - particularly with hot water and detergent - the CW agent will often be decomposed to some extent through hydrolysis. Detergents containing perborates are particularly effective in destroying nerve agents. Without an addition of perborates in the detergent, the hydrolysis products of V-agents may still remain toxic unless the pH is sufficiently high. Mustard agent is encapsulated by the detergent and, consequently, the hydrolysis rate decreases in comparison with clean water. However, the low solubility of mustard agent makes it difficult to remove without the addition of detergent, but the water used will still contain undestroyed mustard agent.

Small areas of terrain, e.g., first-aid stations may be decontaminated by removal of the top-soil. Another alternative is to cover the soil with chlorinated lime powder (sludge), which is a decontaminant with general effect and which releases active chlorine. CW agents which have penetrated into the soil, from where they release toxic vapour, are screened-off since the gas and liquid is destroyed by the chlorinated lime.

The physical screening-off of CW agents by covering them can be done in the terrain by spreading a layer of soil or gravel over the contaminated area. The effect will be improved if bleaching powder is mixed into the covering material. Another example of covering is to use special plastic foil to cover contaminated areas inside vehicles. In this way, the personnel will be protected against transfer of liquid.

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Individual Decontamination

he most important decontamination measure naturally concerns the individual. If it is suspected that skin has been exposed to liquid CW agents, then it must be decontaminated immediately (within a minute). All experience confirms that the most important factor is time; the means used in decontamination are of minor importance. Good results can be obtained with such widely differing means as talcum powder, flour, soap and water, or special decontaminants.

In complete decontamination, clothes and personal belongings must also be decontaminated. If clothes have been exposed to liquid contamination, then extreme care must be taken when undressing to avoid transferring CW agents to the skin. There may be particular problems when caring for injured since it may be necessary to remove their clothes by cutting them off. This must be done in such a way that the patient is not further injured through skin contact with CW agents. During subsequent treatment it is essential to ensure that the entire patient is decontaminated to avoid the risk of exposing the medical staff to the CW agents.

In most countries, such equipment includes means for individual decontamination, generally a mixture of chlorinated lime and magnesium oxide. This decontaminant works by absorbing liquid substances and also by releasing free chlorine which has a destructive effect on CW agents. The dry powder also has good effect on thickened agents since it bakes together the sticky substance which makes it easier to remove. Personal decontaminants containing chlorinated lime have, however, an irritating effect on the skin. Consequently, comprehensive use should be followed by a bath or shower within a few hours.

Liquid personal decontaminants are common in some countries. Sodium phenolate or sodium cresolate in alcohol solution are used for individual decontamination of nerve agents. Chloramines in alcohol solution, possibly with additional substances, are commonly used against, e.g., mustard agent. Instead of liquid individual decontaminants, it is possible to use an absorbent powder such as bentonite ("Fuller's Earth"). In the U.S.A. the wet method formerly used was replaced by a decontaminant powder based on a mixture of resins, which decompose CW agents, and an absorbent.

A factor common to all individual decontaminants is that they can effectively remove CW agents on the surface of the skin. However, they have only limited ability to remove CW agents which have become absorbed by the skin, even though very superficially. CW agents that have penetrated into the skin therefore function as a reservoir which may further contribute to the poisoning also after completed decontamination.

In some cases, a wet method may give a better result in decontaminating deeply penetrated agents than a dry method. Reports from France indicate that a solution of potassium permanganate gives effective destruction of CW agents on the surface of the skin and also a certain penetrating effect. There are also individual decontaminants which can simultaneously function as a protective cream for use as a prophylactic. Canada has developed a mixture of a reactive substance (potassium 2,3-butadion monoximate) in polyethyleneglycol, which has both these properties. It can be applied to the skin either as a cream or with a moist tissue.

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Decontamination of Equipment

Immediate decontamination of personal equipment and certain other kinds of smaller equipment is generally done with individual decontaminants. However, these substances are only capable of decontaminating liquid CW agents covering the surface. The decontamination is mainly done to prevent further penetration into the material and to decrease the risk when handling the equipment.

CW agents easily penetrate different materials and into crevasses and will thus be difficultly reached by methods only designed for superficial decontamination. When a CW agent has penetrated into the surface, it is necessary to use some kind of deep-penetrating method. If such a method cannot be used, then it must be realised that the equipment cannot be used for a long period. Depending on the type of CW agent used and prevailing weather, i.e., temperature, wind velocity and precipitation (water solubility), the "self-decontamination" may take many days or even weeks. The absorption into the surface and natural chemical degradation are important factors influencing the self-econtamination period.

The diffusion and evaporation rate of CW agents from material is speeded-up considerably when heated. The decontamination tent used by Swedish army is heated with a mixture of hot exhaust gases and air from a small jet-pulse engine. The tent is used for decontamination of lighter articles, e.g., personal equipment. The decontamination container used by the civil defence forces is a development of the tent and heated with heat-exchanged hot air from a diesel burner.

The temperature in the tent is kept at about 130 oC and in the container at 80-130 oC, depending on the type of material to be decontaminated. Decontamination time varies between two and five hours depending on the temperature.

Other methods utilize heat of steam hot air stream which is blown against the contaminated surface. Decontamination by boiling is also an effective method. The advantage in comparison with heat is that hot water hydrolyzes and renders harmless many types of CW agents. The method may be of some interest in small-scale decontamination of rubber material, e.g., protective masks.

Decontamination of CW agents which have penetrated deeply into the surface can also be done with decontaminants which are capable of penetrating the contaminated material. There are different substances with varying properties. A modern decontaminant is the German emulsion which consists of calcium hypochlorite, tetrachlorethylene, emulsifier ("phase transfer" catalyst) and water. Instead of tetrachlorethylene, the more environmentally harmless xylene is sometimes used.

Decontamination of vehicles and other large objects sometimes is done with steam and suspension and/or emulsion systems. A German company has developed special equipment, C8-DADS (Direct Application Decontamination System), with which the emulsion is prepared and then dispersed onto the vehicle or the terrain.

Generally, it is an advantage to give the material an initial flushing with water before the chemical solution is added. A Swedish development of this approach is ongoing, where the intention is to spray water on, e.g., a vehicle which passes through a flushing arch. The flushing arch has several jets which are supplied with water from a powerful pump. Another type of equipment which can use water from lakes, etc., has been developed by a Norwegian firm. This is used for both flushing with cold and hot water/steam and also as a field shower.

In order to facilitate decontamination and decrease the risk when touched, the material can be painted with chemical resistant paint systems, e.g., polyurethane paint. Design of the equipment is also of major importance for ease of decontamination.

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Example of self-decontamination times for contamination on metal surfaces and on a typical (non-resistant) paint at +15C, 4 m/s and 2 mm large droplets

Note. The times for "liquid" only indicate when the surface is free of liquid, e.g., no liquid is transferred when touched. There is still a risk involved in contact and inhalation through release of gas from surfaces where the CW agent has penetrated deep
       
Substance     No contact risk
  Liquid Gas  
Untreated metal surface      
Soman < 5 h < 5 h  
Mustard agent < 20 h < 20 h  
VX 6-8 days 6-8 days  
Painted metal surface      
Soman 3-4 h 1,5 days  
Mustard agent
1 day 3 days  
VX 6 days 12-15 days  

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