Decontamination is a process whereby hazardous material is removed from the environment, buildings, objects or people. The nature of the hazard may be biological, chemical or radiological. Usually the material will pose a direct threat to humans, but in some cases the threat may be indirect or economic — for example, insect pests or diseases of plants or animals. Sometimes the decontamination process will involve the physical removal of the material of concern, while in others it may be neutralized or rendered harmless by chemical means.
Biological hazards include microbial pathogens such as bacteria, viruses and parasites. The decontamination procedures employed will depend on both the nature of the contaminant and on what is to be decontaminated. Small objects, such as surgical instruments, can be decontaminated by heating — for example, in an autoclave. Decontamination of rooms and buildings will usually involve the use of powerful disinfectants: strong oxidizing agents, such as chlorine, chlorine dioxide or hydrogen peroxide, or toxic organic chemicals such as formaldehyde or phenol. Following the anthrax attacks in the USA in 2001, premises that might have harbored anthrax spores were decontaminated using chlorine dioxide gas, a highly effective biocide.
Dealing with soil containing human pathogens is more problematic. During World War II, anthrax spores were released on Gruinard Island — a small island off the northwest coast of Scotland — in an experiment to monitor the effects of a potential biological weapon. As a result, the island remained closed to the public for nearly 50 years. In 1986, the island was decontaminated using huge amounts of a solution of formaldehyde in seawater. The island was finally declared safe in 1990.
Due to their toxicity and corrosiveness, strong disinfectants cannot be used to decontaminate people. Human decontamination will generally involve removal and safe disposal of clothing, followed by washing of exposed skin with a mild disinfectant, such as a 0.5% aqueous solution of sodium or calcium hypochlorite. If, however, the agent is infectious or contagious, people exposed to it may be required to be quarantined to prevent the spread of the disease.
Where the hazard is chemical in nature, buildings may be decontaminated using chemicals that render the hazardous agent relatively harmless. As with biological hazards, toxic organic chemicals can often be decomposed using strong oxidizing agents. A mild hypochlorite solution may be applied to the skin to decontaminate people exposed to these toxins; in the case of human exposure it is important that the chemical be removed as quickly as possible.
In the case of toxic heavy metals, physical removal of the substance by thorough washing and safe disposal of waste water may be more appropriate. Where heavy metals have been ingested, chelating agents may be employed. These substances bind to and trap the metal ions, rendering them inactive and removing them from the system.
The decontamination of soils containing toxic metals, usually as a result of industrial pollution, can be divided into in-situ and ex-situ methods. In-situ methods can involve electro-kinetic and electro-acoustic methods, where metal ions in moist soil are mobilized by an electrical potential, aided by vibration to increase porosity, so that they migrate across a hydraulic gradient and can be pumped out via a well. Another method is phytoextraction, in which plants that can absorb the metal, while remaining relatively unaffected by it, are used to remove it from the soil. Ex-situ methods involve removing the soil in bulk, treating it chemically or by washing, and returning the clean soil to the site.
Some of the above methods are also effective for radioactive substances. There is, however, no practical way of rendering a radioactive element non-radioactive, so decontamination methods must concentrate on the physical removal and safe disposal of the element. Radioactivity is easily detected, and where the contamination consists of relatively large particles, these may be individually removed. Otherwise, for contaminated soil, phytoextraction and electro-kinetic techniques are sometimes effective.
Decontamination of humans exposed to radioactive materials again normally involves removal of clothing and thorough washing, combined with safe disposal of clothes and waste water. Where radioactive materials have been ingested, chelating agents may help to remove the radioactive element, however; this may not be an option if it is a radioactive form of an essential element. In some cases, chemical means may be used to help prevent absorption of a radioactive element into the system. For example, in the event of a leak of radioactive iodine from a nuclear reactor, potassium iodide tablets may be issued to nearby residents; these provide a source of non-radioactive iodine, reducing the amount of the radioactive element that is absorbed.