澳门金沙城娱乐客户端:Nuclear crisis: How safe is Japan's food and water?
By Andy Coghlan Alerts have been issued on radiation levels in Japanese milk, spinach, leeks and tap water. Food shipments from four prefectures around the Fukushima nuclear power plant have been suspended. So how hazardous are the radiation levels found, and should people panic about what they’re eating? Which goods have been contaminated? Today the Japanese government ordered four prefectures to stop selling spinach and leeks after levels of radiation above the legal limit were picked up in them. The affected prefectures were Fukushima, where the nuclear plant damaged by the tsunami is based, and the nearby Ibaraki, Tochigi and Gunma prefectures. Traces of radioactive iodine-131 were discovered in Tokyo tap water on Sunday. In Iiate, a village in Fukushima prefecture, there was so much in the water that the health ministry advised the village’s 3700 residents not to drink it. The water here contained 965 becquerels of radiation per kilogram – treble the “safe” legal level of 300 becquerels per kilogram. Lastly, officials have discovered iodine-131 in three milk samples from Kawamata, a town in Fukushima prefecture. Radioactive caesium-137 also appeared in one of the samples, but at levels below the legal limit. As a precaution, the government has asked farmers in the prefecture to stop selling raw milk. So how dangerous were the levels found? Not very – at least, so says Yukio Edano, the chief cabinet secretary. He is quoted in the Japan Times as saying that the contaminated milk from Kawamata contained up to 1500 becquerels of iodine-131 per kilogram, about five times the legal limit for milk. But to put this in perspective, he pointed out that if someone on a typical Japanese diet drank this milk for a whole year, the accumulated radiation would equal that from a single CT scan. Turning to contaminated spinach from Ibaraki prefecture, Edano said that it contained up to 15,020 becquerels of iodine-131 per kilogram, about seven times the safe limit for spinach – plus 524 becquerels of caesium-137, which just exceeds the 500-becquerel limit. Again, to put this into perspective, he said that eating this spinach daily for a year would inflict a fifth of the radiation from a CT scan. Nothing to worry about, then? Apparently not, according to Edano. Urging the public not to overreact to the findings, he said that “eating food with radioactivity levels exceeding provisional limits isn’t going to affect your health”. Affected farmers would be compensated, he added. How trustworthy are the government’s reassurances? Assuming the levels are being honestly reported, Edano’s attempts to prevent panic by putting the doses into perspective are justified. This impressive chart assembled by the web-based science “comic” XKCD shows how doses scale up in sieverts, the units by which absorption of radiation into living tissue is measured. As a yardstick, 8 sieverts is considered fatal, even with treatment. The chart starts from the 0.05 microsieverts you could receive by sleeping next to someone, scaling up to the massive 50-sievert doses received every 10 minutes by the heroic workers who tackled the catastrophic Chernobyl nuclear disaster in Ukraine in 1986. It reveals, for example, that the maximum average daily extra dose for people living near the Fukushima plant is estimated at around 3.5 millisieverts. The worldwide annual average background dose for a human being is about 2.4 millisieverts, according to the UN Scientific Committee on the Effects of Atomic Radiation. OK, so that helps with perspective, but what will happen to the radioactive material spreading from Fukushima into the environment? Iodine-131 is the most hazardous isotope in the material, because if breathed in or eaten it can lodge in thyroid tissue and cause thyroid cancers, as happened after the Chernobyl accident when children drank contaminated milk. But if people at risk receive tablets containing non-radioactive iodine, this reaches the thyroid first and effectively prevents the radioactive isotopes being absorbed. Also, the threat should be short-lived because half of any given amount of iodine-131 decays away weekly. What about caesium-137? It could be more of a problem. With a 30-year half-life, dangerous amounts can remain for years in pasture that might be grazed by livestock. That’s why farmers in the European “hotspots” most heavily contaminated from Chernobyl were banned from selling their produce for many years. It is not as harmful as iodine-131, but can still damage DNA and cause cancers long after iodine-131 has decayed to insignificance. So the government precautions are sensible? Indeed. The bans on sale of produce can be lifted once the scale of contamination has been fully evaluated, and once leakages from the nuclear plant have been permanently halted. When is this likely? Although the plant is still in a serious condition, the recent news has been promising. Almost all six reactor units at the plant are now stable or approaching stability, and power has been restored, enabling the operators to resume controlled cooling of the reactors. The worst contamination so far has come mainly from fires in ponds where spent fuel rods are stored. Because the ponds are open to the atmosphere, radioactive material from the spent fuel spread straight into the environment. In an update yesterday, the International Atomic Energy Agency said that radiation levels had spiked three times since the quake, but have stabilised since 16 March “at levels which are, although significantly higher than the normal levels, within the range that allows workers to continue onsite recovery measures”. More on these topics: