Radiation 101: Radiation and what it means for Japan

I was getting sick of basing my decisions on media hype, gossip forums and what my friends tell me, and decided to do a little factual research on some real scientific sites for myself. What I discovered almost made me laugh at the amount of rubbish that is being banded around by various media and gossip chains. These are the facts – this is all independent, verifiable data. I hope it helps to allay some fears at least about the current situation. I am working on another research topic on the reactor itself and will post again when that is ready.

Many thanks

Nicky Washida


Radiation is measured in several ways, the most common being sieverts and rems, a bit like the metric or imperial systems.
1000 microsieverts (µSv) = 1 millisievert (mSv) = 100 millirem (mrem).
The data below is all presented in mrems. To convert to millisieverts divide by 10, to convert to microsieverts multiply by 10


Radiation comes from several sources: rocks, soil, electrical goods, cosmic rays are a few of the main ones.

The average US citizen gets a total yearly exposure of about 360 mrems a year. This varies widely depending on the geology of the area you live in, and lifestyle. People who fly more often, for eg flight attendants, get more exposure because they are higher in the atmosphere where the cosmic rays are stronger. This is why there is a theoretical risk to a fetus from radiation on a plane.
Here in Japan, background radiation levels appear to be generally lower.

Other sources of radiation include:
X ray – anything from a few to 80-100 mrems, the average being around 60.
CT scan – this can be as low as 150 mrems for a barium meal, through to 1000 mrems for a pelvic scan.
Other nuclear medicine – eg kidney function checks that can range over 1000 mrems.

Nuclear power plant workers can add a further 300 mrems to their yearly total, taking a US power plant worker up to 660 mrems per year.


Exposure is both chronic and acute. Acute means a sudden exposure, and chronic means exposure built up over time.

Acute is easier to measure in terms of it`s impact on the human body:

Anything over 100,000 mrems will give you symptoms of radiation sickness. This is because the radioactive particles entering the body interfere with the body`s main areas where cells are dividing rapidly. The first area to be affected therefore is the intestinal tract. This is why nausea, vomiting and diarrhoea are the first symptoms and can occur anything from 10 minutes to several hours after acute exposure. How quickly symptoms occur is a diagnostic indicator of the levelof exposure.

At 300,000 mrems immune system damage occurs. This is where people can easily succumb to infections and cancers in the longer term.

At 400,000 mrems death from infection occurs within 60 days in 50% of cases

At 1,000,000 mrems the death rate is 100% due to cardiovascular damage.

The damage occurs because the radioactive particles in your body give off “waves” that interfere with cell function. If you imagine that your body is your apartment, the microwave is a radioactive particle and your cellphone is a body cell. When the microwave is running, it interferes with cellphone function. Same thing inside your body. Cell`s can`t function properly and go “haywire”.

Chronic exposure (including acute exposure) is much harder to measure because there is no “safe” limit that can be identified. This is why it is better to limit exposure as much as possible, eg by keeping away from the microwave when it is in use.

One study suggested that exposure to 250,000 mrems accumulated over 80 years (so roughly 3100 mrems per year or around 9 times normal exposure in hte US) leads to a 10% increase in cancer risk, but it is really very difficultto quantify. It has been suggested that over-exposure may explain certain cancers in those with an otherwise low risk.


To put all this scary-sounding stuff in context now, Tokyo`s background radiation levels rose temporarily yesterday from roughly .0045 mrems (daily average per hour) to .089 mrems – about 19 times the average. Yes, this sounds scary, but when you consider that it was for less than an hour, that the US hourly average for a years dose of 360 mrems is .04mrems (so it briefly went to double the US average) and levels in Tokyo are generally 10 times less than you would be receiving living in some parts of South West UK, this puts it somewhat in perspective. So we had a blip of higher than normal radiation temporarily – it was still over 600 times LESS than that of an average xray!

Hopefully this should show that the situation AWAY from Fukushima is currently no cause for concern. Note (courtesy of Sean Marsula.): Doubling your distance means the intensity of radiation drops to 1/4 its previous value. So if at one meter away the value is 1000 microsieverts, at 2 meters it would be 250 microsieverts (known as the Inverse Square Law). The data I obtained on radiation levels in Tokyo is from an independent source. Having said all that, the situation at the plant itself can`t be predicted, so the next post will turn attention to the situation at the plant itself, but right now in and around Tokyo, radiation levels are of no cause for concern at all.

See also the following articles for further reading:

Radiation and Health Effects

Toxic misconceptions about Japan

Educating ourselves: Becquerels and base figures what do they all mean?

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