Nuclear crisis: how safe is japan's food and water
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.