Retorts

Radiation Produced Free Radicals

Dr Steven Gieseg in his article "Reducing Free Radicals" [July 1999] notes that reactive free radicals are produced by ionising radiation. He then comments, "Thankfully, exposure to gamma rays is not an everyday event for most of us".

On the contrary, it is.

All of the world's population is exposed in varying amounts to the natural radiation sources of terrestrial radioactivity (naturally occurring radionuclides in soil, rocks and building materials), cosmic rays, radionuclides incorporated in our bodies, and radon gas. Radiation exposures are measured in terms of the quantity effective dose in units of sieverts (Sv) and its submultiple, millisievert (mSv). Natural exposures range from less than 1 mSv per year to more than 30 mSv per year in different locations, with a world average of about 2.5 mSv. An effective dose of 30 mSv per year over a 70-year lifetime accumulates a dose of about 2 Sv, which is approaching the mean lethal dose of 3-4 Sv if received as a single acute exposure.

Direct action and free radical-induced damage causing double strand lesions in DNA is considered to be the lesion type of greatest importance in the induction by radiation of cell lethal events, chromosomal abnormalities and gene mutations. A topic of considerable interest in the radiobiology and radiation protection community is whether, at the very low rates of environmental exposure to radiation, some beneficial as well as detrimental effects might also occur. These could conceivably arise from stimulation of repair mechanisms and immune response, the benefits from which might outweigh detrimental effects at very low dose rates.

The issue is fundamental to the debate on the validity of the "linear no-threshold" (LNT) hypothesis, which postulates that stochastic effects and dose are linearly related without threshold (that is, there is some probablity of an adverse effect even at very low doses). The LNT assumption underlies the system of protection recommended by the International Commission on Radiological Protection, in particular the principle that radiation doses should be as low as reasonably achievable. Some consider this to be unduly precautionary where annual doses are only a fraction of those incurred naturally.

Andrew McEwan, National Radiation Laboratory