Radiation Exposure

“The release of atomic energy has not created a new problem. It has merely made more urgent the necessity of solving an existing one.”

- Albert Einstein

With nuclear meltdowns in Japan, there's a lot of talk about Radiation Exposure. And it's very confusing. While researching I came across measurements in millisieverts, mircosieverts, Grays, and millirems so just dealing with units was tricky. And then there's the question, what level of radiation is "normal" or "safe"?

Let's start with units. Radiation dosages are measured in sieverts. A Sievert represents an effective radiation dose so one Sievert "produces the same biological effect" regardless of the type or size of the radiation particle. Since Sieverts are so large you'll normally see mSv (millisieverts).
1000 mSv = 1 Sievert.
You'll also see microsieverts for hourly measurements (for example Tokyo was experiencing 1.0 to  0.6 microsieverts per hour last week).
1,000,000 micorsievert = 1000 mSv = 1 Sievert

And then there are Rems.  1 Sievert is 100 Rems.

Less confused yet? Let's look at radiation exposure. According to Forbes.com

Radiation is emitted from all sorts of sources. Cosmic rays, radon, x-rays, brazil nuts, granite. Just living in a high-elevation city like Denver gets you a higher dose because there’s less atmosphere to block radiation. The granite in Grand Central Station and the uranium-laced stone used to build the U.S. Capitol Building supposedly give off enough radiation that if they were nuclear power plants they wouldn’t pass licensing.

Here are some radiation comparisons using millisieverts, mSv:

.001 = extra hourly exposure in Tokyo last week (equal to radiation from one cigarette)
.01 = Dental x-ray
.036 = eating one banana
.04 = one chest x-ray
.4 = one Mammogram
2 = natural radiation exposure every year (.24 from comic rays)
9 = extra yearly exposure for airline crew (increased cosmic rays)
13 = yearly dose from smoking 1.5 packs a day
16 = one CT scan for heart, abdomen or pelvis
100 = yearly dose at which the increase in cancer risk is clearly evident
100 = recommended limit over 5 years for Nuclear Plant workers
400 = hourly level at Fukushima plant that forced workers to withdraw
1,000 = not fatal in a single dosage but will cause radiation sickness
6,000 = level for Chernobyl workers who died within a month
10,000 = fatal within weeks

Bottom Line

It's hard to grasp the range of values in the chart above (a factor of 10,000,000 from lowest to highest). One site compared exposure to cigarettes which are slightly radioactive (.001 mSv). Others use Banana Equivalent Dosage (BED) where one banana is (.036 mSv).  So the increased risk in Tokyo is equal to eating an extra banana every day and a half.

This helps put things into perspective.

Additional Resource

http://www.physics.isu.edu/radinf/risk.htm (Idaho State University - amazing amount of information about relative risks)

http://www.guardian.co.uk/news/datablog/2011/mar/15/radiation-exposure-levels-guide