Dioxins: What are they?

The carcass pyres have raised dioxin levels

Blazing pyres of animals carcasses are raising the levels of pollutants in the air.

Communities living near the fires who express fears about health risks are being backed by environmentalists and local health authorities.

The government says the level of cancer-causing dioxins in the air has increased because of the burning but insists the public health risk is minimal.

The government has nonetheless launched an investigation into the impact of the pyres on people living nearby.

So what are dioxins and what is already known about them?

What are dioxins?

Dioxins are a group of chemicals known to increase the likelihood of cancer.

An unwanted byproduct, they are formed when heating processes create certain chemicals - chlorine is the best known.

Environmental campaign groups describe them as among the most dangerous toxins known.

Scientists are working to establish their exact toxicity, but a draft report from the US Environmental Protection Agency indicates dioxins are considered a serious threat to public health.

In 1997, a World Health Organisation group declared the most toxic dioxin - 2,3,7,8-tetrachlorodibenzo-p-dioxin, or TCDD - a class 1 carcinogen, meaning it causes cancer in humans.

They are found just about everywhere - and are present in the atmosphere, soil, rivers and the food chain.

Where do they come from?

Waste incinerators and chemical and fertiliser manufacturing plants mainly.

They have been more widespread since the introduction of a new chlorine production technique in 1900.

However, volcanoes and forest fires also produce dioxins.

What is a safe level of exposure?

In 1990 the WHO set a safe daily exposure level of 10 picogrammes per kilogram of body weight.

A picogramme is one millionth of a millionth of a gram. However, following further studies this was re-evaluated in 1998 to one to four picogrammes per day.

How great is the health risk?

The first disease associated with dioxins was the extreme skin disease chloracne. It causes acne like pustules to form across the body and can last for several years.

Most concerns now lie with the potential of dioxins to cause cancer, but they are also suspected of affecting reproductive health, lowering sperm counts, causing behavioural problems and increasing the incidence of diabetes.

There is a growing body of research indicating that dioxins can cause such diseases.

In particular, a peer-reviewed study of the population of Seveso found that, in the 10 years following the explosion, both men and women were more likely to have cancer.

It looked at people who lived in an area some distance from the plant that was not evacuated.

Both sexes were more likely to have cancers of the blood system, while women were also more likely to have cancer of the gall bladder and liver system.

Also of concern is the effect dioxins can have on unborn children and infants, as they can be passed through the placenta or carried in breast milk.

How do they get into food?

In living organisms, toxins reside in fat. This means they can persist in the food chain through a process called bioaccumalution.

They are mainly found in meat and dairy produce, but are also found in poultry, fish and on unwashed fruit and vegetables.

• Fish accumulate dioxins through exposure to water - dioxins are repelled by the water and attach themselves to the fatty fish
• Unless - as was the case in Belgium - feed becomes contaminated, animals are usually exposed to dioxins in the air settling on their food. They accumulate in the fatty tissue of animals, and the longer that animal lives, the greater the build up
• Dioxins in the air also land on fruit and vegetables, but washing can get rid of these - they are not absorbed into the plant itself

What is the outlook?

Pressure from environmental lobbyists has led to companies changing their practice and a reduction in dioxin emissions.

Following the 1998 WHO meeting on dioxins, Dr Maged Younes, head of risk assessment in the organisation's Programme for the Promotion of Chemical Safety, said such steps were proving effective.

"Recent exposure data show that measures introduced to control dioxin release in a number of countries have resulted in a substantial reduction in intake of these compounds in the past few years," he said.

"This is evidenced by a marked decrease in dioxin levels in human milk, as found in an exposure study conducted by the WHO European Centre for Environment and Health, with the highest rates of decrease being observed in areas which had the highest initial concentrations."

Campaigners continue to press for tighter controls.