The "effective dose" is not sufficient by itself to characterize one exposure to radioactivity. Among the factors to consider, one of the most important is the dose rate that takes into account the duration of exposure.
The exposure to radiation is like the exposure to summer sunrays. Burns of a sunburn result from a short and intense exposure to ultraviolet while skin supports without damaging the same dose received over several hours.
In the case of high doses, there is good reasons to believe that spreading the exposure over time reduces risk. It is likely that cellular repair mechanisms operate at low radiation rates, but are overwhelmed with high rates. This explains the reduced toxicity of low dose rates. However, there are only littlestatistical data in humans on this reduction (see note).
Experiments on animals suggest a reduction factor of between 2 and 10. A better understanding of what happens with humans, would require studies - called epidemiological - on people so-exposed. But cases when high doses rates can be compared to low rates are rare and radioprotection experts stumble again on the impediment of a lack of data in the field.
The influence of dose rate is currently neglected by regulation and legislative texts. For example, the current legislation of radiation protection is based on a linear relationship between the number of cancers and doses. This relationship, itself based on the results of studies on survivors of Hiroshima and Nagasaki, put on the same curve a single and intense exposure whose duration was only a few seconds, as in Hiroshima and Nagasaki or a medical CT-Scan, and chronic exposures, sometimes sparingly issued days after days? This is one of the unsolved problems at the moment.
The International Commission on Radiological Protection (ICRP website) has begun to consider dose rates in its recommendations. A mitigating factor (called DDREF) is now taken into account in the case of gamma rays to reduce the effects of exposure to chronic low dose rate.
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