Decontamination Methods
Decontamination Methods
Decontamination refers to the efforts to safeguard property and people that have been exposed to chemical, nuclear, or biological agents. The intent of decontamination is twofold. The first objective is to make the individual free from the contaminant, or, if complete removal of the agent is impossible, to reduce the concentration of the contaminant to a level that is safe for survival. The second objective is to make property safe for habitation.
Human decontamination can involve removal of a contaminant from the skin. Usually such decontamination must be done quickly, since the contaminant may be absorbed through the skin where it can cause internal damage. In a setting such as the home, laboratory, or factory, permanent decontamination facilities can be present. For example, washrooms equipped with arm-activated water taps and antiseptic soap allow for the rapid removal of personal spills. Decontamination is also possible “in the field,” courtesy of emergency response personal decontamination kits, which can be carried with workers or soldiers.
Chemical decontamination
There are a variety of decontamination methods and strategies that can be brought to bear on a chemical problem. Often, the method selected depends on the nature of the contaminant. For example,
vacuuming up a spill of a powdery chemical can be a prudent step, while the same technique would be inappropriate for a liquid spill.
There are three general chemical decontamination methods. These methods involve physical, chemical, or thermal processes.
Physical methods
Liquid chemicals can be removed from inert surfaces or living surfaces (i.e., skin) by the use of sorbents. The sorbent can be a natural material, such as soil, diatomaceous earth, or activated charcoal, or can be synthetic (i.e., Amberlite XAD-2 and XAD-7 resins). In general, the natural materials absorb, or suck up, the liquid contaminants, while the synthetic materials adsorb contaminants. Adsorption involves the concentration of a substance from the liquid phase onto the surface of the adsorbent material due to the chemistry of the surface molecules.
The most recognizable solid absorbent is a clay material known as Fuller’s Earth. This material is commonly found in kitty litter. When solid absorbent materials like Fuller’s Earth, soil, or diatomaceous earth are used, the contaminant is usually not altered. For example, petroleum products are readily absorbed but are not changed in their character. Thus, the sorbent material becomes toxic and so must be collected and disposed of afterwards. Caution needs to be taken during the collection process, as fine dust or particles can be inhaled or stuck to exposed skin.
A different type of physical decontamination involves washing the contaminant away using another fluid like water, an alcohol, or freon. The aim here is to dilute the contaminant in the wash fluid, which should itself be collected for proper disposal. Washing is not a complete decontamination. Residual contaminant can remain behind in cracks or other hiding places. However, the use of high-pressure sprays can be an effective and rapid means of decontaminating surfaces like walls and floors.
Chemical methods
Chemical decontamination goes further than merely removing a contaminant from the environment. Rather, in chemical decontamination the adsorbing chemical neutralizes a contaminant. One example of chemical neutralization is the adsorption of a contaminant by material that is impregnated with an alkaline chemical. Another general example is the use of chemically reactive compounds that interact with the contaminant and change its structure into a form that is non-toxic.
A popular chemical decontamination strategy relies on the use of oxidizing agents. Bleach is a well-known example of an oxidizing agent. The use of oxidizing compounds such as calcium hypochlorite or sodium hypochlorite inactivates a variety of chemical compounds as well as dangerous microorganisms such as bacteria and viruses.
Oxidizing agents can be wiped onto a spill and collected in an absorbent material. As well, some oxidizing agents can be incorporated into topical lotions, which are smeared onto the skin to help inactivate a chemical or biological spill.
A recent innovative example of an oxidizing agent is L-Gel. Developed at Lawrence Livermore National Laboratory, L-Gel uses potassium peroxymonosul-fate to deactivate a variety of biological agents, including anthrax spores and Yersinia pestis (the bacterium that causes plague). The thick gel is able to cling to surfaces better than water, especially to steeply sloping surfaces like walls, which keeps the decontaminant in contact with the target longer than using a straight water-based decontaminant. It is hoped that a powdered formulation of the product will soon be available for use in ventilation ducts, where clean up of chemical and biological agents is especially difficult.
During the fall of 2001, L-Gel was successfully used to decontaminate offices of Congress and at ABC News following the receipt of letters that were laced with anthrax spores.
Strong bases, such as hydroxide forms of calcium, sodium hydroxide, and potassium are other useful chemical decontaminants. These agents disrupt chemical bonds in the contaminant and so destroy the offending compounds’ noxiousness.
Water is an ideal fluid for decontamination because a variety of chemically different detergents and soaps readily dissolve in water. These compounds can loosen or bind contaminants and so remove them from a surface. The friction of scrubbing also aids in decontamination of the skin during hand washing.
The different tendencies of chemicals to dissolve in water (a property known as solubility) affects the efficiency of a decontaminant. For example, a longer period of decontamination is needed when using a compound that is not readily soluble in water. This problem can be somewhat overcome by the use of microemulsions, which are essentially very small droplets of the decontaminant. The droplet coat is a material that is less water-soluble. The effect is best seen when oil is added to water. Then, a sheen of oil appears on the water, rather than a homogeneous oil-water mixture. If a contaminant is not water soluble, it will quickly partition into the hydrophobic (“water-hating”) decontaminant portion of a microemulsion. This can speed up the action of a decontaminant. Microemulsions can be applied to a contaminated surface as a spray, which can be washed off later.
Thermal methods
Thermal decontamination is the use of heat to vaporize those chemical contaminants that will readily convert from a liquid to a gas in the presence of heat. Both water- and alcohol-based chemicals can exhibit this behavior.
Water can also be heated, even to the extent of being converted to steam. Hot water or steam treatment can be an efficient means of decontamination of greasy or oily contaminants. The use of moist heat, as in the laboratory sterilization unit called an autoclave, disrupts chemical bonds in many microorganisms, killing them. Unfortunately, certain noxious bacteria that form spores (i.e., Bacillus anthracis, Clostridium species) can, under some circumstances, survive autoclaving.
Hot air is another useful decontaminant for compounds that can be volatilized. This method is useful for situations where a spill can be isolated and treated over a longer period of time. In a battlefield situation, other more urgent methods are preferable.
Nuclear decontamination
Nuclear decontamination in a battlefield site, to date only applicable in the Japanese cities of Hiroshima and Nagasaki in the waning days of World War II, necessitates the removal, burial, or storage of the contamination. However, in sites such as decommissioned nuclear power plants or weapons manufacturing facilities, the less concentrated amounts of radioisotopes that are encountered can be more systematically decontaminated.
Nuclear decontamination consists of the removal of the contaminating radioisotope. Removal can be accomplished by the use of water-soluble chemicals (i.e., alkaline permanganate, citric oxalic acid), fire-fighting foam, and even the electrochemical treatment of the contaminated surface.
See also Biological warfare; Weapons of mass destruction.
Resources
BOOKS
Boss, Martha J., Dennis W. Day, and Roger F. Jones. Biological Risk Engineering Handbook: Infection Control and Decontamination. Boca Raton: Lewis Publishers, Inc., 2002.
Brian Hoyle