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

Peter Wehrwein

Contributor, Harvard Health

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.

Millisieverts Millirems
Chest x-ray 0.1 10
Two-view mammogram 0.36 36
Average annual background exposure in the U.S. 3 300
Cardiac nuclear stress test 9.4 940
CT scan of the abdomen 10 1,000
Coronary angiogram 20 2,000
Average exposure of evacuees from Belarus after 1986 Chernobyl disaster 31 3,100
Annual dose limit* for nuclear power plant workers 50 5,000
Spike recorded at  Fukushima Daiichi nuclear power plant 400 per hour 40,000 per hour
Acute radiation sickness begins 1,000 (or 1 sievert) 100,000

*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


  1. jose fernandez

    Now I’m really confused; I thought I’d read in several places that exposure from a chest x-ray was much greater than from a mammogram, but your chart shows just the opposite. Is there a difference in exposure between the “old” type of mammogram using film and the newer digital mammograms that could explain that?

    • Peter Panayi

      The Sievert is a measurement of effective dose (ie taking into account the type of radiation) PER KILOGRAM. So if you want to compare a mammogram value to a whole body value you have to multiply by the weight of the breast divided by the weight of the body. No jokes about big boobs please.

  2. Steven

    Gracias por esta informacion.

  3. Alan Williamson


  4. susaijeyaseelan

    hi peter ,your clear explanation helps us to understand the real concern about the radiation . could you reply the following #wheather koodankulam atomic power plant proximity to the fault level ? #how do these levels compare to known levels by an ordinary illiterate man living near to atomic power plant ? , #

  5. fatkhul amin

    its realy help me

  6. muchlis

    whether the effect of radiation in Japan can make to get diseases such as mesothelioma victims ? you know what i mean?

  7. Sean

    Another really sad thing about the Fukushima Daiichi facility is that two General Electric engineers warned against building the facility in the first place because of its proximity to the fault line. (From what I understand, the two engineers resigned over the issue.)

    [URL removed by moderator]

  8. Egi

    It’s scary. I read here, nuclear radiation has reached the sea with awful level. RHow’s next, I do not know, but I believe Japan can resolve this problem properly. They are brave and strong man.

    And time for us to seek alternative sources than nuclear.

  9. Andrew

    Very informative! I’ve always been curious about the measurements of nuclear radiation but I’ve never taken the time to actually understand it, but you have presented it in such a clear and concise manner that I now understand it much better.

    [URL removed by moderator]

  10. Micha

    Hi Mike,

    I have a friend here in the US that is trying to decide if it is safe to go to Tokyo next week and stay for ten days for an important family reunion. When taking into account consumption of food and water within Japan, would she be risking her long term health if she goes?

  11. Brian Morely

    One of the better explanations of this emotionally charged issue. Trouble is no one has bridged the gap between the Becquerel and Sievert. The measured radiation of an mass (Bq) and the equivalent bio risk of absorbed ionizing radiation (Sv). Have a go …

  12. Carol

    Interesting post. What scares me the most is that if they won’t be able to contain the leakage. I’m afraid that the whole world could be affected by all of these. Everyone’s health would be at risk.

  13. Carlton Jones

    This is a good discussion of radiation levels. How do these levels compare to the discussed microscopic levels, say 1 millionth of a gram, of plutionium particulate pollution? Has any data shown if particulate uranium or plutonium dust has reached the US?

  14. Trina Bashore

    Check here for radiation readings in Japan


    It is possible to pull up a 3 month, 4day and last 24hr chart


  15. Paul Schnake

    This article is dumb. As a retired scientist, I have follow radiation and it’s effects for at least 50 years. The incidence of cancer in all forms has followed the increased introduction of radiation. As high definition of X-rays was introduced, cancer spiked. When the a-bombs were dropped in Japan to end WW2, cancer spiked again. When we had an increase in nuclear energy plants, cancer spiked again, etc. etc.

  16. New Perspectives

    With our advancing technology, you would think that safety is the bottom line. Other than that, you would be under the mercy of our traditional knowledge for any innovative tactics.

  17. alfonso negapatan

    thanks for this. very informative.

  18. Maimon Mons

    This post on the xkcd blog should be talked about anytime anyone tries to compare radiaction exposure: http://xkcd.com/radiation/

  19. Oluwole

    Hi; nice tip, please is the exposure measurement constant for every human being in our different climatic environments.

  20. Dr. James R. Marzolf

    An excellent article. However the concept of background radiation needs to be understood from the historical perspective. The White Sands tests contaminanted most of the lower 48, the pacific H bomb tests launched a stratospheric cloud that rained radionucleotides across the planet for years. Very good maps are available of how much landed where. So the current “background” radiation is not a natural level. In terms of Japan, now that the spent fuel rods appear to have ignited it is being termed a level 6 accident (3 mile Island was a 5 and Chernobyl a 7) Check out this animation of the Chernobyl accident based on French data. It will give you an idea of what can happen.


  21. Roger Brewer

    Also – What is the normal (average) background radiation in mSv/hour if someone were to take a measurement outside of their house? After atmospheric dispersion, what would 400 mSv/hour at Fukushima look like by the time it hit the west coast of the US? Would it be detectable above normal background?

  22. Roger Brewer

    Nice summary of acute effects but the people on the US west coast are worried about long-term, excess cancer risk due to days/weeks/months of exposure to radioactive particles from Fukushima. Could you put this into perspective also?

  23. Lani

    Now I’m really confused; I thought I’d read in several places that exposure from a chest x-ray was much greater than from a mammogram, but your chart shows just the opposite. Is there a difference in exposure between the “old” type of mammogram using film and the newer digital mammograms that could explain that?

  24. Robert Mandell

    Dear Peter,

    Nice explanation. Of interest to me is the amount of medical radiation one can receive from a CT scan of the Abdomen and the Coronary Angiogram. What is the amount of radiation one receives with a CT scan of the Brain? Wasn’t there a recent directive by radiologists concerning the overuse of CT scans on patients?

  25. Ulrich Decher

    Most of the values given in the table are time integrated doses, but the second to last row:

    “Spike recorded at Fukushima Daiichi nuclear power plant 400 40,000”

    I suspect is per hour. I may be wrong, but suspect that it is the dose if you stayed there for one hour.

    Ulrich Decher Phd Nuclear Engineering

  26. Peterkar

    Thanks! The first coverage I’ve found which discusses uom’s in radiation and what’s presently happening in Japan.

  27. Naomi

    Just to clarify some misunderstanding of non-Japanese mass media these days.

    Almost all the measurements of radiation are posted on-line at the homepages of municipal governments in Japan from this week.
    Some offices (such as several Wards of Tokyo 23) report hourly fluctuations of radiations as well.

    Medical information is also a plenty in internet, traditional mass media, and local clinics.

    You just need to understand Japanese though.
    (Sorry, Japan is right now too busy to translate things.)

    Based on these numbers, the situation is to some extent stabilized.
    We are grateful for our heroes who’s struggling to extinguish fires.

  28. lew liggett


    I would like take issue with your assertion that the levels of radiation received by those in Tokyo are of no risk to health. Please review the work of Dr. Gofman, whose book, RADIATION AND HUMAN HEALTH, documented that any radiation, even background,is a risk, however small, to human health. The higher the dose, the greater the risk. He exposed the fallacy that nuclear power is ” clean ” energy. Dr. Gofman, who workrd on the building of ” The Bomb ” at Lawrence Livermore Labs, was one of the world’s foremost experts on radiation .

  29. ICAN ADD

    3 is the average annual “background noise” millisievert. The chart compares that with 400 millisieverts at the plant — but that is per hour. In less than 3 hours of that level of exposure, acute radiation sickness would set in. I think the chart should make that more clear.

  30. James VanOpdorp

    I was in the army during the Cold War and we were told that 160 “rads” was the level where more than 24 hours of exposure was something we had to worry about. Does anyone know if this was simple slang for rems?

    This was more than 30 years ago and I stand to be corrected (or further enlightened).

  31. T

    Hi Peter,

    Thanks for the article. The exposure table was particularly useful — and allows the reader to draw their own conclusions. Just to make sure that I understand everything correctly, anyone in Japan who was exposed to the reported 400 millisieverts spike, would have been exposed to more than 130 years worth of the average annual background exposure in the U.S. Is this correct?


  32. Matthew

    Hi Peter,

    Thanks for the clear description! I’d be interested to know how many millisieverts are predicted by experts to “drift” to the U.S. West Coast. All I’ve heard so far are qualitative (e.g. “minor”) rather than quantitative descriptions. Surely there must be an expected range?

  33. Elsa Rosenberg

    The chart showing relative doses is very helpful. Because the USNRC has an annual dose limit for its workers, I was wondering where the new full body scanners at airports would rank in terms of rems or Svs in this comparison.

  34. Shigeru Suzuki

    Hi Peter,
    Your clear explanation helps us to understand the real concern about the radiation occurred at Fukushima Nuclear Plants.

    I wonder if there would be any thing people who live around Tokyo and in its vicinity need to pay paricular attentions other than the values of radiation, in order to keep us from being exposed to the radiation.

  35. Rupa Chinai

    Hi Peter, Neat explanation! Im wondering why no one is talking about contamination of the sea water and impact on marine life. Which way is that flowing?

  36. Alan

    I think it’s cool that you explain this! I’m a student of biomedical ebgineering but as a student sometimes you don’t get a lot of credibility when it comes about serious public health care issues and forget about when it is combined with radioactivity.. Finally I got some good sources to back me up and create consiousness that this is not a “nuclear apocalypsis” as the media in my country (latinamerica) has named it.

Commenting has been closed for this post.