The incident occurred in July 1988 after a tanker driver discharged 20 tonnes of a water treatment chemical, aluminium sulphate, into the wrong tank at the unmanned Lowermoor water treatment works.
The chemical passed into water supplies serving some 20,000 people over the next two days, exposing some individuals to high concentrations of aluminium. The acidity of the water was also high enough to dissolve copper, lead and zinc from domestic water pipes and tanks, exposing some people to a cocktail of metals. Elevated aluminium levels persisted well into 1989.
In the first few days of the incident, when the pollution was at its peak, the then South West Water Authority reassured people that the water was safe to drink. By then, however, some were reporting symptoms such as mouth ulcers, skin irritation and severe diarrhoea. Later, people began to complain about a variety of chronic conditions - notably impaired memory and joint pains.
The official response was incoherent. Typical were the SWWA's efforts to monitor pollution levels at customers' taps and elsewhere in the water supply zone.
The sampling was neither systematic nor based on a standard methodology. The most common of the three methods used, with samples being taken after taps had been run for two minutes, guaranteed that there would be no record of metals dissolved from domestic plumbing by the acid water. Moreover, no records were kept of the sampling methods used at different points and times. Together, these failings precluded accurate estimation of individual exposures to the cocktail of pollutants in the water supply - in turn confounding subsequent health impact studies.
Officialdom was none too anxious to have the Camelford incident probed too deeply. The water industry was only months away from privatisation, and the incident was capable of alarming investors about the sector's liabilities. So the chairmanship of a review of its health impacts was handed to a scientist, Barbara Clayton, who had a record of conservatism - and she produced what Ministers would have been hoping for.
In its first report, issued in 1989, the Clayton committee concluded it to be "unlikely" that the pollutant exposures experienced by people in the Camelford area were capable of causing long-term effects on health, and attributed complaints of persistent symptoms to "sustained anxiety".
The report caused profound offence in Camelford, not least because the committee had spent barely half a day in the area talking to complainants and medical practitioners.
When the committee reconvened to consider fresh evidence it somewhat tempered its language. Reporting a second time in 1991, it concluded that lasting physical harm from the toxicity of the contaminated water itself was unlikely - but still suggested that "the psychological harm could last a long time for some people." It conceded, however, that the pollutant doses received by people were unknown, and so long-term health impacts could not be completely excluded.
Pressure for a more thorough investigation persisted, prompting the Labour Party to promise a public inquiry while in opposition. After taking office in 1997, however, Labour took four years to decide on a fresh inquiry. The move followed publication of a study in 1999 which concluded that residents exposed to the polluted water had suffered "considerable damage to cerebral function, which was not related to anxiety" (ENDS Report 297, pp 28-32 ).
It has taken four years for the study to be completed. It was carried out by a sub-group of the Government's Committee on Toxicity of Chemicals chaired by Frank Woods, a retired professor in clinical pharmacology.
Unlike the Clayton committee, the COT sub-group took pains to consult local opinion, making five visits to the Camelford area and receiving evidence from 113 individuals as well as health professionals.
The inquiry also dug up some new evidence, including additional data on pollutant levels in the water supply and modelling carried out by South West Water of the distribution of pollutants through the network over time. It also commissioned its own modelling, and produced "worst case" estimates of adults', children's and babies' exposures to the pollution.
The outcome is likely to have come as a disappointment to many of those who gave evidence to the inquiry. The report acknowledges that symptoms reported by individuals fell into "a recognisable pattern". But it then proceeds to discount their significance in much the same way as the Clayton committee: "The information is not sufficient to provide conclusive evidence that the frequency of symptoms and illnesses reported to us is higher in individuals exposed to the contaminated water than in those who were not."
The sub-group even struggled to acknowledge that the acute symptoms reported during the period of peak pollution could be related to ingestion of drinking water. For instance, it says that, according to existing toxicological knowledge, levels of copper, zinc and sulphate were too low to cause the acute gastrointestinal response experienced by some Camelford residents - before grudgingly conceding that "it may be that consumption of a mixture of several such contaminants together resulted in an effect at lower concentrations."
The report goes on to discount the possibility that people's exposures to aluminium were sufficiently high or long to cause long-term health effects. However, it recommends further research on the bioavailability of aluminium, threshold toxicity levels for different aluminium salts and the metal's role in neurological disease. Until this is done, the report says, a definitive hazard assessment will not be possible.
Another key recommendation is for further studies of the neuropsychological status of individuals exposed to the contaminated water, with proper controls. The sub-group was persuaded that the 1999 study of 55 individuals who claimed to have been affected by the pollution, though lacking proper controls, provided sufficient evidence of "subtle neuropsychological effects" to warrant further investigation.
The report also recommends a study of the cognitive and behavioural development of children who were less than one year old at the time of the incident, and whose exposure to toxic metals if bottle-fed had the lowest known margin of safety. An investigation is also advocated of the prevalence of joint pains among people who received polluted water.
The report is out to consultation until 22 April before the sub-group submits its final conclusions and recommendations to Ministers.