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Low dose radiation
ARPS Position Statement on Risks from exposure to low levels of ionizing radiation
(This was adopted at the AGM held in Melbourne on 16 November 2005)
RISKS FROM EXPOSURE TO LOW LEVELS OF IONIZING RADIATION
Controversy continues in the radiation protection literature on whether or not ionizing radiation is harmful at very low doses. There is scientific uncertainty about the dose-effect relationship below a few tens of millisieverts in a year, and in order to settle what regulatory controls, if any, should apply in this dose region an assumption has to be made relating dose to the possibility of harm or benefit. The assumption made and, more particularly, the way it is applied can have far-reaching effects not only on the scale of regulatory compliance required but also on public perception of risk and therefore on the technological choices made by society. It is important therefore that decisions reached concerning regulation of low doses of ionizing radiation have an ethical basis and derive from rational argument. It is also important that such decisions are neither portrayed nor perceived as resolving the scientific uncertainties: rather they serve merely to facilitate the implementation of appropriate safety measures.
Following a review of available information, the Australasian Radiation Protection Society has adopted the following position. Based on the features observed, the range of exposures has been divided into three broad dose groups, but it should be noted that the boundaries between them are not known with precision.
Doses above about 10 mSv in a year
There is strong epidemiological evidence that acute exposure to ionizing radiation of more than about 100 mSv carries a risk of developing fatal cancer that increases with dose, with some limited evidence supporting a risk at slightly lower doses. There are also epidemiological reports of statistically significant risk from long-term cumulative exposures that correspond to doses received at rates down to a few millisieverts in a year, but it is difficult to be confident that the observed effects can be reliably separated from possible confounding factors.
In the light of the above, for the purpose of applying regulatory controls to radiation protection when effective doses exceed a few tens of millisieverts in a year, it is reasonable to assume a generalized risk coefficient for fatal cancer of 1 in 20 per sievert for a population of all ages, as recommended by the International Commission on Radiological Protection [ICRP Publication 60]. This assumption is less reliable for exposures below 100 mSv in a year than above.
Consistent with this assumption, an effective dose limit for occupational exposure of 20 mSv per year, averaged over 5 years and no more than 50 mSv in any one year, remains appropriate, as does a requirement to optimize protection below this value. Separately, safety measures are required to avoid deterministic effects of radiation at very high doses.
Doses between about 0.1 and about 10 mSv in a year
There is insufficient epidemiological evidence to establish a dose-effect relationship for effective doses of less than a few tens of millisieverts in a year above the background level of exposure. It is possible that both an adverse effect, through causation of cancer following radiation damage to DNA, and a beneficial effect, through stimulation of repair mechanisms, may operate. It has also been speculated that such a stimulatory effect might reduce mortality from cancer caused by agents other than radiation, resulting in a net decrease in risk. Consequently, neither harmful nor beneficial effects can be ruled out.
To put doses in this range into perspective, it is worth noting that the worldwide average exposure to natural radiation sources is estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation to be 2.4 mSv in a year, with a typical range of 1 to 10 mSv in a year. There are a few areas of the world where much higher doses are received from naturally-occurring sources without causing discernible risks to health.
Taking an ethical position of caution in the face of uncertainty, the risk coefficient adopted above for higher doses may be used for the purpose of establishing control measures for exposure to radiation at lower doses. In particular, the use of an effective dose limit of 1mSv in a year for members of the public is appropriate for exposure caused by the conduct of business activities. This limit will ensure that the additional risk of harm, if any, arising from such activities is acceptably small. However, no inference may be drawn concerning the risk to health or risk of fatality of an individual from an effective dose below 10 mSv in a year. For individual doses less than some tens of millisieverts in a year, risk inferences are unreliable and carry a large uncertainty that includes the possibility of zero risk.
Doses below about 0.1 mSv in a year
The risk to an individual of doses less than a few hundredths of millisieverts in a year is so small, if it exists at all, that regulatory requirements to control exposure at this level are not warranted. Business activities causing individual effective doses of the order of 0.01 mSv in a year or less should be automatically exempted from regulatory control, provided that the activity is inherently safe: that is, there is little likelihood of accidents leading to significantly higher doses. Activities causing levels of exposure up to 0.1 mSv in a year may also be exempted if the regulatory body determines that the application of controls is not warranted, taking into account all relevant factors. In deciding whether control measures are warranted, or how stringent they should be, regulatory bodies should have in mind, inter alia, the principle that societal resources should not be wasted or freedoms inhibited through mandatory observance of unnecessary regulatory controls.
Estimates of collective dose to groups or to populations should be used with caution. In view of the uncertain association between low doses and risk, estimates of collective dose arising from individual doses that are less than some tens of millisieverts in a year should not be used to predict numbers of fatal cancers for the exposed group or population.
However, if collective doses to subgroups of an exposed population are each assigned an appropriate weight, they may play a role in making a choice between possible control measures and thus in optimizing protection. The component of collective dose arising from the summation of individual doses that are less than about 1 mSv in a year should be assigned little significance relative to components associated with subgroups receiving higher doses, and the component associated with doses less than some hundredths of millsieverts in a year may be assigned a weight of zero. Various values for this cut-off have been proposed, from 0.01 to 0.1 mSv.
What is ‘safe’?
The word ‘safe’ may be used to describe business activities that meet currently prescribed radiation safety standards. While there may be some, as yet uncertain, risk arising from such activities, it is known to be small at most and, through application of the justification principle, to be outweighed by the benefits brought by the activity. It follows that exposures of this order may be described as ‘safe’, understanding that the word is used not in an absolute sense but with the meaning of causing an acceptably small risk, if any.
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