Radiation risk in Japan: an update

Peter Wehrwein

Contributor, Harvard Health

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.


  1. IT Consultancy Services

    It is good to read about the radiation risk . It is a very informative post. You have provided a wide range information it will help a lot.

  2. Debora Browy

    Hello, I am an American in Japan and was here during the March 11 earthquake/tsunami (in fact it was my birthday) anyway, I am concerned about the air quality here with the cooling ponds and reactors continuing to be cooled manually and quite a lot of steam being released into the atmosphere here and then we have daily rain during June…does this carry the radiation further across Japan? I am in the kanagawa prefecture where some hot spots were found and should I still be using bottled water?

    • David Konn

      You should definitely be concerned about radiation. Become more educated about the actual status of the continuing radiation problem (now its August 5 months later!) I have found the most reliable information comes from a nuclear engineer with over 20 years experience.

      There are some things that you can do to help protect you.

  3. PK

    Hello, thanks for the useful information. I was wondering about how long might any contamination of car parts last, assuming it is inside or under the vehicle, where it cannot easily be washed (by, say, rain water…)?


  4. Donna

    How serious would exposure to safe or low levels be over a period of time? I guess what I am asking is it cummulative? either by exposure or ingestion?


  5. B. Bierck

    In my view, there’s a need in much of the reporting and analysis to keep in mind that emission of radiation is not the same thing as emission of radioisotopes. Radioisotopes are the radioactive elements which emit radiation, and it’s important to follow where they can go after they are emitted.

    Transport of particles containing these radioisotopes is one of the big issues here because people can be exposed to radioisotopes through various routes, such as ingestion of water or food containing them, or inhalation of particles they can be “riding” upon in the air. Ingestion of very minute quantities of some radioisotopes can be a serious matter if these elements are incorporated into body tissue or bone mass. From within the body, emissions of very low doses of radiation can be have significant health consequences.

  6. lew liggett

    Could you please comment on the opinions of Dr John Gofman, Cal Berkeley and Lawrence Livermore Lab regarding radiation exposure. He worked on the development of the atomic bomb, was one of the world’s foremost radiation experts, and wrote in his book ” Radiation and Human Health ” that any exposure to radiation, even backround, would increase the risk of cancer. Dr. Gofman was a leader of the anti-nuclear power movement. Thanks.

  7. Saori Harada

    Hi Peter,
    Radiation spread through air worries us,
    and at the same time, the risk of radiation intake through tap water and food is becoming another issue.
    The Japanese government announced that at most 5200 becquerel (about 17 times as high as the government’s safety limits for food) of radioiodine
    was detected from a liter of milk near the Fukushia Daiichi nuclear power plant.
    Also, they announced that the excess level of radioiodine and radioactive cesium were detected from spinach near there (note;it was measured without washing the spinach).

    Japanese experts say even if we drink the milk, human body take in about 120 microsieverts of radiation, and that number is far less than the dangerous level,
    and about the spinach, the radiation intake can be about one-twentieth of a CT scan when we cook it.
    However, the agricultural cooperative association began to collect those food, and it is heartbreaking that milk cows and vegetables are disposed of…

    What is the exact level of radiation we should be careful of, regarding food and tap water?
    Are there any kinds of safety limits similar to this in States?

    I’m in Tokyo now, but I was in Boston when the earthquake and nuclear crisis occurred in Japan.
    Through watching the news in both countries, I realize there is a differnce between them…especially about the radiation problem.
    For example, America set the evacuation area within 80 kilometers from the nuclear power plant, on the other hand, Japan set it within 30 kilometers.
    All the people from other countries are now gone even from Tokyo, which really scares us.

    So, I’m looking forward to hearing your opinion. Thank you!

    • Peter Wehrwein
      Peter Wehrwein

      Sorry, this won’t answer your questions about what’s going on Japan. But today [Monday, March 21, at 6 p.m. EDT], the FDA updated what it is saying about food safety situation here in the United States as it relates to Fukushima. Click here to link to the agency’s question-and-answer page.

  8. Shigeru Suzuki

    Hi Peter,
    Thanks for more details about the clear explanations for the numerical values of radiation with regard to Fukushima Nuclear Plants.
    Although we have some sources of information about the radiation safety in general, there seems no reliable reference written in Japanese.
    I appreciate your further post about the radiation safety if possible, so that I could understand the true danger and safety of Fukushima’s case.

Commenting has been closed for this post.