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What are dioxins?

Niki Law

A five-minute primer on an issue making headlines

The government has said heavy metal contaminants and organic chemicals were found in seabed soil samples collected at the Tamar site in 2003. But they did not test for dioxins at the time.

On Friday, environment minister Sarah Liao Sau-tung said studies in 2002 revealed dioxin levels 30 to 50 times lower than the internationally permissible level. The Civic Party is calling for a Legislative Council meeting this week to discuss claims by environmentalist Laurie Wan Shek-luen that the site is highly contaminated with dioxins from its previous role as a British military dockyard.

What are dioxins?

They are a family of chemical compounds created though combustion processes such as waste incineration. Similar to DDT, dioxins work their way up the food chain to accumulate in humans and wildlife. The most toxic - and unfortunately, the most common - is 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD). Most people mean TCDD when they refer to dioxin contamination.

How toxic is it?

This dioxin builds up in the body's tissues. Too much of it will disfigure a person's face by causing a persistent acne called chloracne. In animals, it results in birth defects, miscarriages, mutations, damage to the immune system, diabetes and cancer.

Why all the fuss if it only causes acne in humans?

The US put TCDD on its list of known carcinogens in 2001 and the National Institutes of Health believes there is a clear relationship between exposure to TCDD and cancer in humans. Agent Orange, which was dispersed over Vietnam by the US, contained dioxins.

Can you poison someone with dioxins?

People have tried. In 2004, Ukraine's President Viktor Yushchenko became the first known person to be poisoned by a single high dose of TCDD - he had 100,000 units per gram of blood fat, or about 6,000 times the normal amount. Someone laced his food or drink with the dioxin in September 2004. He became seriously ill, but recovered.

Where can dioxins be found?

More than 90 per cent of the dioxins people encounter come from food, herbicides and the burning of organic material near chlorine. Trash-burning barrels, incinerators, sewage sludge, coal-fired utilities, wood-burning fireplaces and stoves, metal-smelting machines and diesel trucks are all major sources.

How can they be avoided?

Unfortunately, they cannot. Dioxins are found in tiny amounts in everything made of plastic, resins or with bleach. But since it takes concentrated amounts to cause damage, changing to a low-fat diet helps. Quitting smoking makes a difference too, since many cigarettes contain dioxin-based pesticides in their tobacco.

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