Answer to Question #926 Submitted to "Ask the Experts"

Q

I am traveling to the Ukraine with a 6100 Artisan Technology dosimeter. What levels are safe in mR? What is too much exposure in any given period? Any other guidelines? This is a charitable mission so your help is greatly appreciated.

A

As might be expected from an accident that resulted in emissions over a significant period of time, the deposition levels in the surrounding region (Ukraine, Belarus, and parts of Russia) are very heterogeneous. It is perhaps not well appreciated how lucky the situation was in that none of the major cities received major contamination.

In general you need not be concerned about radiation dose, unless you venture out into the countryside where contamination was significant. There are a variety of published maps that indicate these areas. One of the more accessible maps is published by Likhtarev and others as figure 1 in Health Physics on page 343 of the October 2000 issue.

The highest levels of contamination that one might expect to encounter would be about 1,500 kBq of ¹³⁷Cs per sq. m. In an as-yet-unpublished article that has been submitted to Health Physics Likhtarev and collaborators indicate that the currently expected level of dose rate in air from a distributed source of ¹³⁷Cs is 0.21 pGy per s per kBq (1991 levels) per sq. m. In the old system of units (that correspond to your question) this would be equivalent to an exposure rate of 0.085 micro R per h per kBq per sq. m. For the level of 1,500 kBq per sq. m, this would give an indicted exposure rate of 130 micro-R per h.

I am personally not familiar with the 6100 Artisan Technology dosimeter, but an important consideration is whether that instrument is sensitive enough to see even this highest expected level. A normally occurring level of exposure rate from background is 15 to 25 micro R per h, but there can be local variations lower and higher than that range.

If you might be in an area of contamination, there are two exposure pathways of possible concern: external exposure (as discussed above) and the consumption of contaminated food. Unless you are seeking "wild" food, like mushrooms or game, you would be very unlikely to encounter contaminated food, so the main concern is external exposure.

If you are concerned about areas within the contaminated region shown on the map, you should have a meter with a sensitivity of a few micro R per h. The second question of what level in mR is safe is a more difficult and controversial question. In general, radiation is regulated under the assumption that there is no "safe" level, but that risk increases linearly with dose. Most organizations follow the recommendations of the International Commission on Radiation Protection (ICRP) in that the effective dose to the public should be limited to no more than 1 mSv per year above background. In terms of the units discussed above, this would be equal to 19 micro R per h above background averaged over the year. Of course, it is not likely that you would be in an exposed situation for an entire year. If your visit to Ukraine might be for 10 days, then the 1 mSv per y limit would be achieved only if the exposure rate would be 680 micro R per h. It is highly improbable that such a rate could be experienced anywhere in Ukraine in an uncontrolled environment. As a rule of thumb, I would recommend against spending prolonged time in areas where the indicated total exposure rate is more than 100 micro R per h.

The other provision of the ICRP regulatory posture is that exposure to radiation should be kept to a minimum unless there is some benefit of that exposure. Benefit in your case is pretty much in the eye of the beholder. I trust the above has not discouraged you.

I have been to Ukraine many times, including several trips inside the 30-km zone and to the Chernobyl Power Plant. I would have no qualms about going again.

Lynn Anspaugh, Ph.D.