Indecent exposure

 作者:杜桃帅     |      日期:2019-03-08 05:16:01
By Debora MacKenzie THERE was uproar in Belgium when the public learnt that it may have been eating chickens, eggs and dairy produce containing cancer-causing dioxins. Angry citizens took their revenge last week by ousting the government in federal elections. But despite the health warnings, Belgians continued to buy butter, eggs and meat. This ambivalence is understandable. People are angry about the Belgian government’s failure to issue immediate warnings, but the incident’s impact on public health is unclear. Depending on which report you read, dioxins are the deadliest known chemicals or relatively innocuous. In addition, it is not clear who received how much of the toxin, or how much damage this will do in future. And the chances are scientists never will get these answers—not because so few people will be affected, but because so many are already suffering from earlier exposure, including the much larger amounts of dioxins routinely pumped into the atmosphere by incinerators. The resulting “background” poisoning due to dioxins and related chemicals called PCBs could mask any additional disease in Belgium. Evidence is mounting that many more people have been exposed to potentially dangerous levels of these chemicals than is generally admitted. Last year, the French ministry of the environment estimated that anywhere between 1800 and 5200 French people die each year from cancer caused by dioxins. Meanwhile, information on mishaps far worse than the Belgian scare is only now coming to light. It was the US that reacted most strongly to the Belgian scare, placing a blanket ban on all eggs and meat imported from the European Union—though it would be naive to think that trade hostilities between the US and the EU weren’t partly to blame. Nonetheless, in a little-publicised event in 1997, a one-off national survey of dioxins by the US Environmental Protection Agency found high levels of TCDD, the most toxic type of dioxin, in two chickens from New Orleans grocery stores. Later, the scientists found similar levels in chickens for sale near their laboratory in Mississippi. The dioxins were traced to bentonite clay from a mine in the area. The clay was used as an anti-caking agent in soya meal fed to chickens and farmed catfish. “We don’t know how the dioxins got into the clay,” admits Doug Hayward of the US Food and Drug Administration. But his measurements of dioxins in chickens and eggs, made after the feed was banned—when levels had probably started to fall—reached 25 parts per trillion (measurements of dioxins are adjusted to reflect how toxic the different dioxins are). Levels of dioxins in Belgian chicken reached 956 ppt, nearly 40 times higher. But whereas the Belgian scare affected only enough feed for 16 million chickens for a day, the EPA estimates that feed contaminated by the clay went to 1.7 million chickens over several years. Exactly how long is unknown, but researchers found up to 43 ppt of dioxins in catfish fed similar feed in 1994. One third of farmed catfish, which are popular in the southern states, were affected. The fact that this contamination happened over a long time period is significant because dioxins accumulate in body fat. This new information about the clay incident will be published later this year in the journal Environmental Research. The FDA allowed moderately contaminated chickens to be sold as food, as long as most of the fat was removed. In Germany, dioxin levels in milk had been falling since 1990, after tighter smokestack controls cut the amount of dioxin-laden incinerator ash falling on pastures. But in 1997, levels started to climb again. Intense detective work traced the contamination to pulp from squeezed Brazilian citrus fruit used for animal feed. Rainer Malisch, of the Baden-Württemberg State Chemical Research Agency in Freiburg, says the pulp contained 7 ppt dioxins. Similar levels were found in milk. These are also much lower than levels measured in Belgium—but again, the contamination spread farther and lasted longer. The dioxin came from contaminated calcium hydroxide used to neutralise the pulp’s acidity. The chemical is thought to have been a waste product sold to pulp producers by a Brazilian plant owned by Solvay, a Belgian chemicals firm. Eleven EU countries seized about 100 000 tonnes of Brazilian pulp after the German discovery—some in Britain contained 23 ppt dioxins. But the EU imports 1.3 million tonnes of citrus pulp a year from Brazil for feed, says Axel Singhofen at Greenpeace’s EU office in Brussels, and the Brazilians may have used Solvay calcium hydroxide for six years. The European Commission admits that meat was also affected, but no numbers have been published. The likelihood is that they don’t exist. There is little routine checking for dioxins and PCBs in food in either Europe or the US. Germany has Europe’s most ambitious monitoring programme for dioxins, with monthly assessments of milk, says Wim Traag of RIKILT, the Dutch laboratory that measured dioxins during the Belgian scare. But it’s still not fast enough to identify contaminated food before it has been eaten. This lack of urgency is surprising given the results of recent reappraisals of dioxin and PCB toxicity. In 1997, the International Agency for Research on Cancer in Lyons classed the most toxic dioxin, TCDD, among the worst-known carcinogens. The highest levels measured, in workers at herbicide factories in Germany, the Netherlands and the US, were linked to increases in all kinds of cancers, depending on the dose of dioxin. And in 1976, an explosion at a plant owned by Swiss chemicals firm Hoffmann-LaRoche spewed high levels of dioxins over the northern Italian town of Seveso. After two weeks of official dithering, 724 people were evacuated from the worst-hit area. Thousands more received smaller doses. Evacuees had increased incidences of heart disease and several tumours, including those of the lung and liver, and non-Hodgkin’s lymphoma. Last year, a panel of 40 scientists advised the WHO to cut its recommended maximum daily intake for dioxins and similar compounds tenfold, based upon these chemicals’ ability to cause brain damage rather than cancer. “The levels of dioxins needed to cause cancer are a hundred to a thousand times higher than the levels that cause cognitive and hormonal damage,” says Martin van den Berg of the University of Utrecht in the Netherlands, a panel member. The most worrying impact dioxin-like PCBs have is on brain development. In 1996, 11-year-olds in Michigan exposed before birth to only slightly higher levels of PCBs than the US average were found to have measurably lower IQs. In the Netherlands, where body burdens of PCBs are among the highest in the world, Caren Lanting at the University of Groningen measured maternal PCB levels then three years later gave the women’s offspring standard tests for cognitive skills including vocabulary, face recognition, number recall and simple riddle-solving. Those who received breast milk with higher levels of PCBs did worse than those who received lower levels. Will any of these effects turn up in children who were in the womb earlier this year when their mothers ate Belgian chicken? It will take a very close scrutiny of the statistics to reveal them. Unfortunately, little effort is being made to preserve potentially contaminated food to tell us who’s been exposed. But Belgians may comfort themselves that their scandal, while it captured headlines, crippled exports and toppled a government, is not excessive compared with other,