*Radiation Unit Conversion*
What Do the Numbers on My Dosimeter Mean?
By: MountainMedic
23 March 2011

The 2010 radiation range chart available at http://www.lowdose.energy.gov/images/ig_pics/DoseRangesJune2010.pdf is a great resource for determining what levels of radiation are harmful.

There are two versions of the chart that are the same except for the units used on the chart. One uses Sieverts and the other Rems. At the lower ranges of the chart the units are milliSieverts/year (mSV/yr) and millirem/year (mrem/yr). Unfortunately, neither of those two choices is available on the dosimeters that many of us have. The modern dosimeters usually have microSieverts/hour (µSV/hr) as a unit choice and this article will explain how to convert µSV/hr to mSv/yr and back again so you can tell if the reading on your radiation detector is dangerous.

With the current nuclear disaster in Japan, many of us are getting radiation readings in the neighborhood of 0.15 µSv/hour on our dosimeters, so let’s convert that to mSv/year. To convert our measurements to mSv/yr we need to multiply by the number of hours in a year (8,760) and divide by the number of µSv in a mSv (1,000).

When we do this for 0.15 µSv/hr we get 1.37 mSv/yr. This is lower than the average background radiation in the USA shown on the chart which is 3.1 mSv/yr. The largest reason for this difference is that our detectors do not detect all forms of radiation such as Radon.

So, how high do our meters need to read before we should start worrying?

Let’s use the DOE and NRC limit for employees of 50 mSv/year. To convert 50 mSv/year to µSV/hr take 50 and divide by the number of hours in a year (8,760) and multiply by the number of µSv in a mSv (1,000) to get 5.71 µSv/hr.

For your radiation exposure to be at the DOE's limit for workers, your dosimeter readings would have to increase from the approximately 0.15 µSv/hr it shows now to 5.71 µSv/hr AND would have to stay that high for every hour of every day for a whole year. You can be exposed to a much higher dose for a limited time without any harm.

Once I see local levels getting close to 5.71 µSv/hr, I'll start paying more attention to each family member’s exposure. But it still won’t be an emergency because it is unlikely that levels would be that high for long, and even if I was exposed to 10 times that amount for a few days, there would be the rest of the year to limit my exposure to stay under the 50 mSv/yr limit.
MountainMedic

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