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The Daily Need

How much radiation is too much? A handy guide

Japan’s nuclear crisis has understandably induced a panic over leaking radiation and the potential danger it poses to human health. The Japanese government has interrupted food shipments of tainted milk and spinach, and radiation has been found in the seawater near the Fukushima plant. Although health authorities have stressed that much of this radiation poses minimal danger to human health, the idea of any radiation emanating from a nuclear accident is worrying. Some Americans have been requesting potassium iodide pills, and Geiger counters have sold out in Paris.

People safely absorb small levels of radiation every day. Plants, rocks and even human bodies give off radiation. But how much radiation is normal? Randall Munroe, the mind behind the brilliantly nerdy stick figures in the web comic XKCD, has tried to answer that question. He recently drew an extremely helpful graphic comparing the radiation levels of common activities like getting a medical scan or taking a transcontinental flight with large-scale nuclear accidents like those at Three Mile Island or Chernobyl. Although Munroe, a former NASA roboticist, takes care to mention that he is no radiation expert, he provides an open list of his sources, which includes the United States Nuclear Regulatory Commission and MIT’s Nuclear Science and Engineering department.

One sievert, the unit measurement for a dose of radiation, will cause illness if absorbed all at once, and 8 sieverts will result in death, even with treatment. According to the chart, the average person safely absorbs about 3.65 millisieverts (or 0.00365 sieverts) of radiation annually, through simple activities like living in a brick or concrete building (70 microsieverts a year) or sleeping next to another person (0.05 microsieverts). A person living within 50 miles of a nuclear power plant absorbs 0.09 microsieverts of radiation per year, which is less than the amount absorbed by eating a banana.

Although the chart does not contain extensive information about the radiation leaking from the Fukushima power plant, it does note that spending a day in a town near the Fukushima plant will expose a person to an extra 3.5 microsieverts of radiation – slightly less than that of a dental X-ray. To make a few more comparisons, a mammogram will give off about 3 millisieverts (0.003 sieverts) – three times more than the maximum dose of radiation absorbed from Three Mile Island’s 1979 nuclear accident.

While some of these revelations are reassuring, the chart also shows that when things get bad, they get very bad. Spending just 10 minutes next to the post-meltdown nuclear reactor core of the Chernobyl power plant – the site of the worst nuclear catastrophe in history – a person would have taken in 50 sieverts of radiation, nearly seven times more than a fatal dose.

Of course, although a person can absorb many nonlethal doses of radiation without a noticeable effect, overall long-term absorption definitely contributes to the risk of cancer. For that reason, many of the health concerns for those living near the site of nuclear accidents are entirely valid. But a quick reality check on the safe levels of radiation we absorb every day might at least help some people save a few dollars on a Geiger counter.

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