Ancient Taint: Likely source of old dioxins identified

The burning of peat in coastal areas of Scotland could be responsible for the enigmatic concentrations of dioxins that scientists sometimes find in pre-20th-century European soil samples.

Dioxins are a class of more than 200 chlorine-rich organic chemicals that are highly toxic, trigger birth defects, and can cause cancer (SN: 5/15/99, p. 309). Presumed modern substances, dioxins typically are byproducts of the production of industrial chemicals such as polychlorinated biphenyls (PCBs) and some pesticides.

However, dioxins are also created during the incomplete combustion of organic carbon in the presence of chlorine. That’s why they can spew from municipal incinerators and residential trash fires (SN: 1/29/00, p. 70: Backyard burning is recipe for dioxin). Now, biogeochemist Andrew A. Meharg and his colleague Kenneth Killham, both of the University of Aberdeen in Scotland, have shown that dioxins aren’t just a modern problem.

For their experiments, the researchers obtained peat samples from a site on the northwestern coast of Scotland. Each kilogram of peat held about 114 nanograms of dioxins–probably from modern atmospheric contamination, says Meharg. However, the researchers found that the smoke and ash produced by burning each kilogram of peat included more than five times the original amount of dioxins. The chlorine required for this spike of dioxin production came from oceanic salt spray that had permeated the peat before it was dried and used as fuel, the team speculates.

If historians’ estimates are accurate, each household in the area burned about 20 tons of coastal peat per year. Therefore, the region produced about 1 kg of dioxin annually. For comparison, municipal incinerators throughout the United Kingdom today collectively produce only about 11 kg of dioxins annually, says Meharg. He and Killham report their results in the Feb. 27 Nature.

In the Scottish highlands and islands in the 18th and 19th centuries, residents–most of them subsistence farmers–would have been exposed to dioxins in at least two ways. First, home heating came from a peat fire in the center of the floor in windowless structures known as blackhouses, which had low entrances, no chimney, and very little ventilation. Second, the farmers used peat ash as a fertilizer, a technique that would have permitted dioxins to contaminate root crops such as potatoes and turnips.

The new measurements may solve the mystery of how dioxins came to be present in soils excavated and archived in the 19th century, says Ruth E. Alcock, a chemist with Environmental Research Solutions in Cumbria, England. As it turns out, however, blackhouse residents had more to worry about than dioxin exposure. They probably suffered from respiratory illnesses brought on by other chemicals and particulates prevalent in peat smoke, Alcock says.

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