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

March 16, 2011

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Peter Wehrwein, Contributor, Harvard Health

Peter Wehrwein was the editor of the Harvard Health Letter from 1999 to May 2012. He is currently a freelance writer and editor, and contributes to the Harvard Health blog and Before editing the Health Letter, … See Full Bio
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jose fernandez
January 16, 2012

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
February 1, 2012

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.

January 12, 2012

Gracias por esta informacion.

Alan Williamson
October 26, 2011


October 23, 2011

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 ? , #

fatkhul amin
October 1, 2011

its realy help me

September 15, 2011

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

August 17, 2011

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.)

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August 10, 2011

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.

July 20, 2011

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.

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April 22, 2011

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?

Brian Morely
April 8, 2011

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 …

April 8, 2011

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.

Carlton Jones
April 4, 2011

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?

Trina Bashore
April 1, 2011

Check here for radiation readings in Japan

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


Paul Schnake
April 1, 2011

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.

New Perspectives
April 1, 2011

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.

September 24, 2011

You have the monopoly on useful information—aren’t monlopoies illegal? 😉

alfonso negapatan
April 1, 2011

thanks for this. very informative.

Maimon Mons
March 20, 2011

This post on the xkcd blog should be talked about anytime anyone tries to compare radiaction exposure:

Peter Wehrwein
March 21, 2011

I like the chart!

March 19, 2011

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

Dr. James R. Marzolf
March 19, 2011

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.

Roger Brewer
March 18, 2011

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?

Roger Brewer
March 18, 2011

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?

March 18, 2011

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?

Robert Mandell
March 17, 2011

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?

Ulrich Decher
March 17, 2011

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

March 17, 2011

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

March 17, 2011

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.

lew liggett
March 17, 2011


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 .

March 17, 2011

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.

James VanOpdorp
March 17, 2011

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).

March 17, 2011

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?


March 17, 2011

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?

Elsa Rosenberg
March 17, 2011

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.

Shigeru Suzuki
March 17, 2011

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.

Rupa Chinai
March 17, 2011

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?

Peter Wehrwein
March 17, 2011

Hi. I think they will be talking about that soon.

March 16, 2011

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.

Peter Wehrwein
March 17, 2011

You’re welcome.

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