Several people who read my earlier post about radiation readings at the Fukushima Daiichi nuclear power plant in Japan pointed out that the time period over which the radiation exposure occurs is important. They’re right—the radiation dose and how long you are exposed to it determine how much radiation you are receiving. That is why all most of the reports out of Fukushima have been reported on a per hour basis.

The reading of 400 millisieverts per hour inside the boundaries of the Fukushima plant mentioned in my original post is still the highest reading I have seen reported by the news media (more precisely, the English language news media) or by the International Atomic Energy Agency (IAEA).

Here are some numbers that may put that reading in perspective:

• 400 millisieverts per hour means that if someone stood in that radiation field for an hour, he or she would be exposed to 400 millisieverts. If he or she stood in that field for half an hour, the exposure would be 200 millisieverts, and so on.
• 400 millisieverts per hour works out to 6.6 millisieverts per minute. So theoretically, if someone were to have stood in that radiation field for just 7.5 minutes, he or she would have been exposed to 50 millisieverts of radiation. And 50 millisieverts is the U.S. Nuclear Regulatory Commission’s upper limit for the radiation exposure for nuclear power plant workers for an entire year.
• A chest x-ray delivers 0.1 millisievert of radiation because it’s so brief, say a tenth of a second. If a person stood in front of an x-ray machine that was left on and waited, he or she would be hypothetically exposed to 400 millisieverts of radiation after 6½ minutes.
• Edward Maher, an associate at Dade Moeller and Associates, a health physics and radiation safety consulting firm and an adjunct lecturer in the Department of Environmental Health at the Harvard School of Public Health, told me in a phone interview today that radiation and distance from the source follows what’s called the inverse square law: the dose rate (the millisieverts per hour) falls by factor of four when you double the distance from the source—and that assumes no buildings or other obstacles are in the way. So say you were a mile away from the 400 millisievert per hour “hot spot.” By the inverse square law, the reading there would be 2 millisieverts per hour. And if you were 50 miles away, it would be 0.0008 millisieverts per hour.

Of course, this presumes that there’s a hot spot, or hot spots, and that not a lot of radioactive emissions are leaving the plant. Based on the limited information we have so far, that seems to be the case.

Here are the radiation levels that the International Atomic Energy Agency reported on March 17, at 10 a.m. EDT. Note that these readings are in microsieverts (a millionth of sievert), so the units are a 1,000 times smaller than millisieverts (a thousandth of a sievert).

In some locations at around 30 km [18.6 miles] from the Fukushima plant, the dose rates rose significantly in the last 24 hours (in one location from 80 to 170 microsievert per hour and in another from 26 to 95 microsievert per hour). But this was not the case at all locations at this distance from the plants.

Dose rates to the north-west of the nuclear power plants, were observed in the range 3 to 170 microsievert per hour, with the higher levels observed around 30 km from the plant.

Dose rates in other directions are in the 1 to 5 microsievert per hour range.

And here is what The Guardian, the British newspaper, is reporting this morning:

Radiation readings taken 1km [0.62 miles] west of unit 2 offered some hope, dropping from 351.4 per hour just after midnight [March 17, 8 p.m. EDT] to 265μSv/h [microsievert per hour] at 11am [March 18, 7 a.m. EDT]. But there have been enormous variations in readings at different parts of the plant and within short spaces of times.