Though dioxins have acquired a reputation as "the most deadly chemical on earth", the scientific evidence gathered over the past 30 years is equivocal. For humans, it appears, the risks are not obvious and where they exist, they are not wide-ranging.
Although dioxins have acquired a reputation as "the most deadly chemical on earth", the scientific evidence gathered over the past 30 years is equivocal. For humans, it appears, the risks are not obvious and where they exist, they are not wide-ranging.
Chemists say the reputation of dioxins is overblown. Professor John Emsley, science writer in residence at the University of Cambridge, said people are worrying unnecessarily about risks from dioxins produced by the pyres of animals slaughtered to prevent the spread of foot-and-mouth.
He said: "Humans have lived with dioxins since they first lit a fire. These are a by-product of combustion you get them from matches, cigarettes, forest fires, volcanoes. There are actually 210 different compounds which are classed as 'dioxins', but only a couple have unpleasant effects, and only one called TCDD is really dangerous. And even that isn't definitely proven."
Professor Jim Bridges, who specialises in toxicology and environmental health at the University of Surrey, said the risks would only be noticeable for people who, after the fires, live entirely off local products produced downwind of the pyre.
He said: "Even for somebody living near these fires ... unless they're a smoker, they will get 99 per cent of their dioxin intake through diet principally through fatty foods and oily fish. The airborne amount of dioxin isn't an issue. It would only be important to a farmer producing milk or meat, who might find their product contaminated."
While dioxins have a reputation as being "cancer-causing", there is no clear data to show this, although long-term studies of workers who have received high exposures have indicated an elevated general risk of cancer.
Dioxins are also suspected of causing foetal abnormalities although this is not proven either and have been shown to affect the nervous system and to suppress the immune system.
Professor Bridges said: "These are subtle effects. The most important point is that they persist in the environment." This means that animals further up the food chain can accumulate dioxins from animals and plants that they eat.
The World Health Organisation noted in 1999 that while dioxin can be classified as a "known human carcinogen ... TCDD does not affect genetic material" and that "there is a level of exposure below which cancer risk would be negligible."
The only data on birth effects comes from a study of people living in the Italian town of Seveso. In 1976, 4,000 grams of TCDD rained down on them after an explosion at a chemicals factory. Long-term studies of mothers and children did not show any clear data of foetal abnormalities. Laboratory studies on rats and guinea pigs do show effects, though so dioxins are classed as carcinogenic, largely on that basis.
A two-person team from AEA Technology is studying what products are being released from the Devon fires, and will report to the Department of the Environment, Transport and Regions (DETR).
Professor Bridges said: "For most residents near the pyre, the more important issue for health will be the amount of particles smaller than 10 micrometres known as PM10s given off. Those have a more immediate effect on health."
In fact, the Government would rather dispose of slaughtered animals via rendering plants, rather than burial or burning. Six rendering plants are being used to dispose of animals, turning them into a fine powder that can then be burnt in commercial power stations.
The advantage of this method over burial is that there is no risk of infection, whether from foot-and-mouth disease or BSE, leaking into the water supply.
Compared to burning, rendering results in fewer emissions including dioxins and produces some energy. But limited capacity means only 25 per cent of the 1.62 million animals slaughtered so far have been disposed of via rendering plants.
The rest are being buried or burnt. The balance of disposal methods is not expected to change significantly for the remaining 371,000 animals waiting to be slaughtered and disposed of, Maff said.
Alan Lawrence, of the UK Renderers' Association, said: "Normally [our plants] just receive the parts of an animal that an abattoir doesn't take. Here we're getting whole cows, and sheep which come with a fleece attached, which presents its own problems."
Rendering treats the carcass at high temperatures and pressures to remove water and fats. Themeat and bone meal is then ground up, and can be stored in warehouses.
The Government is giving priority to the disposal of cattle more than 30 months old via rendering, as these cannot be disposed of by burial because of the risk that they could be incubating BSE. Burning is also not ideal because it might not destroy any infectious prions, reckoned to carry BSE.
A DETR spokeswoman said pyres were being used in Devon because the water table there was relatively high compared to Cumbria, where landfill disposal was largely used.
Mike Owen, director of public health for North and East Devon Health Authority, said: "Everybody is worried about combustion ... it is relatively uncontrolled. I would prefer to use landfill sites but we have a vast number of carcasses."
* The disposal of liquid waste from rotting carcasses at the Great Orton mass grave has been approved by the Environment Agency.
A spokeswoman said: "[The waste] is treated to change the acidity level, which kills the virus before it leaves the site. It is then treated along with normal sewage at a works in Workington."Reuse content