Answer to Question #11131 Submitted to "Ask the Experts"

A patient called me with questions about radiation dose from CT (computerized tomography) after reading information on the Health Physics Society Radiation Answers website.

She asked what is considered "a short period of time." I could not find a definition. Is there any definitive answer?

The document you cite refers to possible health effects of various radiation doses delivered over possibly different time intervals. The specification of what is a "short" or "long" time period depends somewhat on what the dose magnitudes are.

For doses to the whole body of at least up to 50 mSv, and likely up to 100 mSv, one would expect to be able to discern no deleterious effects to the dose recipients, regardless of how short the time interval was—a fraction of a second or spread out over a year or more. At higher doses the time intervals become more important.

From about 100 to 500 mSv, doses delivered within seconds or less, or over a longer time, would not be expected to produce any negative effects that could be overtly observed in the individual affected, although there may be some increase in the likelihood of the individual contracting cancer over his/her lifetime. Additionally, at doses on the order of 200 mSv and more one could also observe some changes in certain blood characteristics, particularly chromosomal changes in circulating white blood cells.

Doses around 1 Sv, delivered over a period of perhaps a few hours or less, may well produce some observable effects such as possible nausea, headache, and general ill feeling; the same doses delivered over days, weeks, or longer would not likely produce any overtly observable effect to the individual, but some chromosomal changes would be likely, and increased risk of cancer would be a likelihood that would extend over many years following termination of the exposure.

At still higher doses there would likely be adverse direct effects for doses delivered within a few days that could extend from health-threatening effects such as reduction in red and white blood cells to gastro intestinal tract damage, which would be a concern as doses approached perhaps 6 Sv, and central nervous system effects would be possible as tens of Sv doses accrued. Long-term increased risk of cancer for these short-term exposures would also be a concern. Longer-term exposures at these higher doses would all produce likely increased cancer risks and, depending on the exact doses and times, other direct effects could be observed.

I hope this adequately addresses your question.

George Chabot, PhD