23 March 2011

Radiation Dose Chart

The news from Fukushima and the continuing drama about whether the six reactors can successfully be brought to a safe and stable condition has, quite understandably, got people all over the world worrying about the hazards of nuclear radiation. 

The hazards from an uncontrolled event are very real. There is no safe dose, and no-one should fall for the silly and irresponsible material that has been circulating on the web and in the twittersphere to the effect that small amounts of radiation are good for you. They are not.

On the other hand, we are exposed to ionising radiation every moment of our lives, and the modest risks associated with low doses or with infrequent higher doses under controlled conditions are something we all need to live with. In a wide range of medical circumstances the risks of exposure to ionising radiation are the better part of the risks of failing to make a timely diagnosis.

Most people are aware that medical radiation can be harmful, but very few are aware of the small doses we receive every day of our lives, or the relativities of the different sources of radiation.

The Radiation Dose Chart at http://xkcd.com/radiation/ sets it out very nicely. There are many things to be discovered here; I will point out just a few. By way of background, the standard unit for absorbed dose is the sievert (Sv), hopefully measured in millionths of a sievert (microsieverts μSv) or thousands of a sievert (millisieverts mSv).

Now for some comparisons. According to the data painstakingly compiled on the chart:

-  Living for a year within 50 miles of a nuclear power plant for a year (0.09 μSv) involves just under twice the exposure you get from sleeping next to someone (0.05 μSv), and slightly less than you get from eating a banana (0.1 μSv)

-  If you live within 50 miles of a coal fired power station for a year you will absorb 0.3 μSv, more than three times the exposure  of someone who lives within 50 miles of a nuclear power station for a year (0.09 μSv)

-  That in turn is less than a third of what you would receive from an arm X-ray or using a CRT monitor for a year (1 μSv)

-  These in turn are only a tenth of the dose from background radiation received by an average person on a normal day (10 μSv)

-  Which is only a quarter of what you would receive taking a single flight from New York to Los Angeles (40 μSv)

- Living in a stone, brick or concrete building for a year involves higher exposure again (70 μSv)

-  All of which pale into insignificance alongside a single mammogram (3 mSv, i.e., 3000 μSv), which is roughly comparable to the normal yearly background dose (about 3.65 mSv)

The fact is that just about everything is radioactive to some degree.

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