Radiation risk in Japan: understanding radiation measurements and putting them in perspective

There hasn’t been much good news about the Fukushima Daiichi nuclear power plant in Japan. Multiple explosions. Fires. Containment buildings breached.

This is a developing story—and it’s not going in the right direction.

When I posted this article, the highest reported radiation reading had been 400 millisieverts an hour. According to the International Atomic Energy Agency’s running account of events at the Fukushima plant, that measurement was taken on the grounds of the plant, between two of the reactors (the timing is vague in the agency’s account but the measurement seems to have been made on Monday). I haven’t seen any reports of radiation levels in the vicinity around the plant, although they’d be lower because radiation gets diluted the farther you are from the source. News reports have mentioned slightly higher-than-normal radiation readings in Tokyo, about 150 miles away, but the level there doesn’t pose a health risk.

Is 400 millisieverts a lot? And what is a millisievert? I’ve linked to good, clear explanations of radiation measurements (including millisieverts) from here and here and here.

You can check them. Or take the short course and read on.

Measuring exposure

Radiation deposits energy in human tissue. Over the years, scientists have measured the interaction between radiation and living tissue in many ways. The sievert (Sv)—the word is pronounced SEE-vert—takes into account the type of radiation emitted (alpha particles, beta particles, gamma rays, etc.) as well how much energy the body absorbs from it. In short, it’s a measure of the biologic effect radiation has on people and the possible harm it can cause.

A millisievert (mSv) is a thousandth of a sievert. You might see some reports about the Fukushima plant that mention microsieverts. A microsievert is a millionth of sievert.

American scientists and regulators sometimes use an older unit that is comparable to sieverts, called a rem. One sievert equals 100 rem and one millisievert equals 100 millirem.

Comparing exposures

So how does radiation escaping from the Japanese power plant compare to other sources of radiation? As you can see from the chart below, the 400 millisievert per hour spike is large relative to other exposures and far above the annual exposure of 50 millisieverts that regulators consider the safe upper limit for people who work at nuclear power plants. According to the World Health Organization, acute radiation sickness (hair loss, burns, skin redness) may develop after whole-body doses above 1,000 millisieverts. We’ll have to wait and see whether future readings go up or down, and how much radiation is spreading beyond the Fukushima nuclear power plant.

This is not a waiting game anyone enjoys playing.

*Set by the U.S. Nuclear Regulatory Commission

Sources: U.S. Environmental Protection Agency, Health Physics Society, U.S. Nuclear Regulatory Commission, International Atomic Energy Agency