Wrongful conviction that could cost lives

BSE might never have happened if we had not misjudged a solvent, says John Emsley
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Paint stripper very nearly prevented the British epidemic of mad cow disease and the subsequent panic that people may have contracted brain disease from eating beef products contaminated with the causative agent of Bovine Spongiform Encephalopathy. Chemists in Britain had discovered that the solvent dichloromethane (DCM) was ideal for extracting fat from the so-called grieves - animal offal that has been pulped and heated under pressure at 120C. When the grieves have been de-fatted, the high-protein residue is sold as cattle fodder.

The fat used to be removed from grieves with either hexane, a highly flammable solvent, or trichlorethylene, which was safer but contaminated the product. In the early 1980s, the rendering industry had already built a pilot plant using DCM as the solvent. It was producing high-grade fat and cattle cake, free of the BSE agent. However, before the process could be scaled up, a report from the US Environmental Protection Agency reported that DCM caused cancer in mice. British firms that processed abattoir waste abandoned the new solvent, and went over to a non-solvent process instead. This used lower rendering temperatures, and pressed the grieves to extract the fat. Unfortunately, as we now know, the BSE agent survived the new treatment.

DCM was also attacked from another quarter: environmentalists accused it of damaging the earth's atmosphere because, like CFCs, it contains ozone-depleting chlorine atoms. Throughout these scares DCM continued to be the active ingredient in DIY paint strippers. Ironically, research has since shown that DCM does not cause cancer in humans, nor does it damage the ozone layer.

DCM, also known by its older name of methylene chloride, is a clear, volatile, non-flammable, colourless liquid with a not unpleasant odour. It has the molecular formula CH2Cl2, with two hydrogens and two chlorines attached to a carbon atom. DCM has a remarkable ability to penetrate hardened paint films and lift them off. It is used industrially on a large scale to clean metal surfaces and to dissolve oils, fats, waxes, resins, rubber and tar. It is essential for the manufacture of viscose yarns, cigarette filters and cellophane, which are made from DCM solutions of cellulose acetate.

ICI's Chlor-Chemicals plant at Runcorn, Cheshire, is Britain's largest producer of DCM, which is made from methanol. Worldwide production is around a million tons a year, with ICI producing a fifth of this. Dr Martin Smith, ICI's Safety, Health and Environment Manager, said: "DCM was first introduced as a safer alternative to ether, an equally volatile, but dangerously flammable liquid that was common in hospitals and laboratories until the 1960s. Although DCM was also tried as an anaesthetic it was not widely used, but it has proved very popular in other ways. A high- purity grade of DCM is used extensively by pharmaceutical and cosmetic manufacturers."

As with all volatile solvents, DCM is tightly regulated by the UK Health & Safety Executive. The safe working level in air is 100 parts per million (ppm), well below the 2,000 ppm level that causes headaches and vomiting, and the 20,000 ppm that will cause death. Most DCM that enters the body is expelled on the breath, but some is converted to carbon monoxide and this could affect people with a heart condition. Splashes of DCM on the skin can be alarmingly painful, but the effect soon wears off if the affected area is bathed with water, and there is no permanent damage.

More alarming was the development of cancers by mice who were exposed to high levels of vapour. Yet research on rats and hamsters showed no increased risk of cancer, and epidemiological studies on 6,000 people who had worked with the solvent over many years showed no increased susceptibility either.

Dr Trevor Green, senior scientist at Zeneca's Central Toxicology Laboratory at Macclesfield, Cheshire, has been researching DCM for 10 years and believes there is a scientific explanation for the special sensitivity of mice: "They have high levels of an enzyme, glutathione, in the nucleus of each cell which can activate the DCM to form a metabolite. This mutates the cell's DNA and triggers off cancer." Although rats, hamsters and humans also have this enzyme it is not located in the cell's nucleus and so does not act as a carcinogen.

There are no natural sources of DCM, apart from small amounts given off by erupting volcanoes, and the current atmospheric level of 0.00005 ppm can be attributed almost entirely to human activity. Even if more is manufactured, this level is unlikely to rise because DCM is destroyed by light and oxygen, and has a life-span of only nine months in air. It is no threat to the ozone layer, nor does it cause photochemical smog over cities, and the Department of the Environment concludes that it has little effect as a greenhouse gas.

The earlier conviction of DCM as a dangerous pollutant now looks to have been a miscarriage of justice. Indeed, had it not been wrongly convicted, it might have prevented BSE in Britain, thus saving the British beef industry and, it may yet turn out, human lives.

Dr John Emsley is science writer in residence at Imperial College, London.