Perfluorooctane sulfonate, or PFOS, is used in fighting major fires but is so dangerous for humans and animals that, last year, the Government drew up legislation to make it illegal to import it to the UK. They have been forced to change tack because of last December's fire at the Buncefield oil depot in Hertfordshire, the largest in peacetime Europe.
Firefighters poured such vast quantities of the chemical on the fire, as they fought for several days to bring it under control, that the water table has been contaminated. Until now, water companies had been barred from pumping the contaminated water in the ground below Buncefield into people's homes. However, the current water shortage has prompted inspectors to relax the rules in an attempt to ease the pressure on supplies.
PFOS is an organic compound that makes foam spread rapidly at high temperatures, cutting the time it takes to put out a fire. But it has also been linked to bladder cancer, and large doses can kill any mammal, including humans.
It is also known that if PFOS escapes into the environment, it stays around for years.
If it is in water, it accumulates rapidly in fish. Because of the health dangers, the main US manufacturer of PFOS agreed five years ago to stop producing it and, last year, the Environment Secretary, Margaret Beckett, drew up legislation to make it illegal to import to the UK any firefighting foam that contained PFOS, to prevent the substance from "causing pollution of the environment or harm to human health or to the health of animals and plants". Offenders would have faced prison sentences up to two years and hefty fines.
Mrs Beckett delayed putting the legislation into effect only because EU commissioners in Brussels were working on an EU-wide ban and asked the UK not to go it alone. When the Government's Advisory Committee on Hazardous Substances met on 6 December, just six days before the Buncefield fire, they were told by a senior civil servant present that Britain was going to object that the planned EU ban was not tight enough.
Now the chemical, which the Environment Directorate of the Organisation for Economic Co-operation and Development has described as "persistent, bioaccumulative and toxic to mammalian species" will be allowed to enter Britain's domestic water.
A spokeswoman for the Drinking Water Inspectorate said: "There was no original standard for what level of PFOS is acceptable in water, because it was not anticipated that there would be any at all. But, in response to Buncefield, we have now taken advice from health authorities.
"That advice is that up to three micrograms per litre of water is not detrimental to human health. We will, of course, try to keep below that advisory figure."
Until now, water companies have been barred from pumping the contaminated water into people's homes but with a summer drought approaching, inspectors have relaxed the rules.
Mrs Beckett will come under pressure next week to overrule the water inspectors and ban the supply of any water containing any measurable quantity of PFOS.
Mike Penning, the Conservative MP for Hemel Hempstead, which includes Buncefield, said: "I cannot see the logic that says that, on the one hand, this stuff is so dangerous that it should be a crime to import it into the country at all and, on the other hand, it's all right for my constituents to drink it, albeit in very limited quantities.
"I don't want to spread scare stories. No polluted water has been supplied to householders yet and I'm very glad this information has become public before it's too late to stop it.
"But I'm asking the Government to take the long-term view. Whether or not there is a drought coming, the safety of the water supply - and the perception that it is safe - has got to be the first priority."
Firefighters used more than 250,000 litres of foam to contain the Buncefield fire, which destroyed 20 huge oil storage tanks and created a black cloud 200 miles wide, visible from France.
They tried to prevent pollutants getting in the local environment by directing all contaminated water into man-made lagoons on the site. But it was later found that the lining of one of the lagoons had been damaged by the explosion early on the morning of 11 December which set off the fire.
Pollutants including PFOS seeped into the water table. Two rivers were found to have been contaminated.
Environment officers ordered the closure of a large water pumping station at Bow Bridge, near Buncefield, which has been out of use now for six months, denying the water company access to millions of gallons of water below ground, in case it is contaminated. Relaxing the rules on water purity will increase the chance of getting the pumping station back into action before the summer shortages.
The Health and Safety Executive, which has been conducting an inquiry into Buncefield, is expected to produce its third report on Tuesday. It will set out what inspectors believe to have been the cause of the fire.
Witnesses at the time spoke of seeing a large vapour cloud half an hour before the explosion that set off the fire. Inspectors have been trying to establish why the vapour leaked from a storage tank, and what caused the explosion.
The report will not go into the issue of whether there should be charges brought against any managers from the Buncefield depot, or inspectors from the HSE itself who were responsible for its safety.
Danger in firefighting foam
PFOS, or perfluorooctane sulfonate, is a chemical used in some firefighting foams which does not break down in the environment. It accumulates in organisms and works its way up the food chain.
PFOS accumulates heavily in humans and animals, and in humans has a relatively long half-life in the body. It has been found to be bioaccumulative and toxic to mammals. Studies have linked it to bladder cancer, although further work is needed to understand this association.
It appears to be of low to moderate toxicity to aquatic organisms but there is evidence of high toxicity to honey bees.
No information is available on effects on soil and sediment dwelling organisms.
The chemical is believed to interrupt the body's ability to produce cholesterol, a necessary building block of nearly every system in the body.
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