Answer to Question #4099 Submitted to "Ask the Experts"

I am involved with  a charity providing aid and assistance to the country of Belarus, a former Soviet state which was badly affected by fall out from the Chernobyl disaster. I have been there twice for a period of two weeks at a time and appear to have suffered no ill effects. I would be grateful if you could explain the effects of the background radiation and how this could be measured. I have access to a new and boxed portable doserate meter (PDRM82) manufactured by Plessey Controls in the United Kingdom. This measures radiation in the air in centigray/hr. My main question is, what would be a "normal" or safe dose rate as measured by the meter? I am intending to return to Belarus next year (2005) and would like to be able to get an idea of the level of radiation still present. I understand that there are still "hotspots" in some places. Any help you could offer would be gratefully received.

The most important thing to know is where in Belarus you have visited, and where you intend to visit next.  Some parts of Belarus were fairly heavily contaminated by Chernobyl, while other parts were left virtually untouched. As one example, if you are visiting the capital of Minsk, you will find very little, if any radioactive contamination.  On the other hand, the city of Gomel received a lot of contamination, and radiation dose rates are correspondingly higher.  You can find a lot of information about the spread of radioactivity from Chernobyl in the 2000 report of the  Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000). You should click on the link for "Reports" and select the UNSCEAR 2000 report, Volume 2 (Effects).  Page 460 has a very good color-coded map of contamination deposition, and that entire section (Annex J, which begins on page 451) deals with Chernobyl.

OK - so now let's talk about your meter.

"Normal" background radiation levels in the US are about 10-20 micro-rad per hour.  This is equal to about 0.2 micro-gray per hour because 1 Gy = 100 rad.  One centiGray (cGy) is equal to 1 rad, so if you are reading a dose rate of 1 cGy/hr, you are in an area that is considerably higher than background radiation levels.  I would suspect that your Plessey PDRM82 may not be able to differentiate between normal background radiation levels and slightly elevated levels from ground contamination. You should note that 1 microGray is equal to 0.0001 cGy, and this is a few times higher than we expect to see with normal background radiation.

With respect to safety, there are a few things to keep in mind.  The most important is that the dose rate (cGy/hr) is not nearly as important as the total dose you receive (cGy or Gy).  That being said, it is still possible to make a few general statements.

In general, a radiation dose rate of less than 1 cGy/hr will not pose a health risk.  However, you may exceed regulatory or local administrative dose limits.  Regulations limit you to about 2 cGy in a year (5 cGy/yr in the US).

In general, a radiation dose rate of less than 0.1 cGy/hr will not pose either a health risk or a regulatory risk.  You would require 20 hours in this area to run afoul of European dose limits, and 50 hours to exceed US limits.

A radiation dose rate in excess of 100 cGy/hr (1 Gy/hr) poses a health risk and can be life-endangering if the total dose received is too high. At this dose rate, a 1 hour exposure may lead to radiation sickness and anyone exposed to this dose rate for longer than 4 or 5 hours may die of radiation sickness.
 
Please understand that you are very unlikely to encounter radiation levels in Belarus that will pose a health risk.  In fact, you are not likely to encounter radiation levels that would put you in danger of exceeding a regulatory limit, according to the information found in the UNSCEAR report. It is probably best to print out the contamination maps and, unless you are entering areas shown to be contaminated, to relax and enjoy yourself.  Break out your instrument if you're going to enter the contaminated areas, but don't worry unless the dose rate rises above a few hundredths of a cGy/hr.

P. Andrew Karam, PhD, CHP