Burn a piece of toast and you create cancer-causing dioxins. Most of us are exposed to “background” levels of this type of chemical, of which there are 20-30 different types, found in many man-made pollutants from industrial waste to cigarette smoke, explains Professor Neil Pearce from Massey’s Centre for Public Health Research.

The World Health Organisation recognised dioxin as a carcinogen in 1997.

“But the exact risk depends on the amount.

“If you smoke two packets of cigarettes a day you have a greater lifetime risk of developing lung cancer than someone who smokes one cigarette a day.”

Because of their jobs or where they live, some people have been exposed to much higher levels.

New Zealand farmers were the world’s heaviest users of the herbicide 2,4,5T, encouraged by government subsidies of up to 50 per cent.

It was sprayed all over the country to kill gorse, which had such a stranglehold on choice pasture land.

Farm production rocketed but there were early warning signs of a potential downside to the magic chemical cocktail.

From at least 1965, the company Ivon Watkins (later Ivon Watkins Dow), which made most of 2,4,5T used in New Zealand at its New Plymouth factory, knew it contained the most toxic dioxin, TCDD – a key constituent in the military defoliant Agent Orange used in the Vietnam War.

In 1977 and 1982, Ivon Watkins Dow modified the production process to reduce the contaminant levels.

Levels dropped from 950 micrograms per kg in 1971 to 4.7 in 1985, but it was impossible to fully eliminate TCDD from the product.

Production stopped in 1987.

The Health Ministry has acknowledged the exposure may have resulted in a 10 per cent increase in cancer deaths in the New Plymouth area.

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