Answer to Question #10031 Submitted to "Ask the Experts"
Category: Radiation Workers
The following question was answered by an expert in the appropriate field:
It seems like it is a common statement that doses under 50 mSv in a year or 100 mSv over your lifetime are safe or have a minimal risk. I also have read that the occupational dose is 50 mSv each year. My question is, for a radiation worker or people who get routine medical procedures of 30 mSv each year, their yearly dose would be under 50 mSv, but their lifetime dose would add up quickly. Would that still be considered safe to get these yearly small doses? I do not understand why you would say 100 mSv in your lifetime; the radiation does not store up in your body, so you would think it would be safe to get yearly amounts as long as you did not exceed the safe yearly limit.
Your question refers to several points that warrant some discussion. The value of 50 mSv in a year that you quote is a number recommended by the Health Physics Society (HPS) in a position paper in 2010. It was stipulated as a value below which the HPS recommended that we should not attempt to quantify health risks. A similar recommendation applied to the dose of 100 mSv accumulated over a lifetime. Both of these doses were intended to represent doses in excess of natural background radiation doses. There is considerable uncertainty associated with the estimation of risk from relatively low doses, and this accounts for this caution by the HPS and other groups (this is discussed a bit more later in this discussion). This does not mean, however, that there is no risk, but rather that there may not be any risk and, if there is, we are not certain about how to quantify it. Regarding the 50 mSv per year value that represents the occupational annual effective dose limit for most radiation workers in the United States, this was established in consideration of the belief that it represented a sufficiently safe level such that workers so exposed would not be experiencing appreciable risk of serious health consequences beyond what accrues to most other workers employed in other safe work environments.
It is important to recognize, however, that all facilities licensed to conduct activities involving radioactive materials and radiation sources, in addition to having to abide by dose limits, are required to institute a program for maintaining workers' doses at levels as low as reasonably achievable (ALARA). This philosophy results in occupational doses among typical radiation workers that are considerably below the allowed annual limits. The average annual effective dose for all occupational workers in this country is less than 10 percent of the 50 mSv limit, and most radiation workers receive less than 10 mSv per year. Many other countries have adopted the International Commission on Radiological Protection (ICRP) recommendations of a 20 mSv per year occupational effective dose limit with allowances to go as high as 50 mSv per year so long as the average annual dose over five years does not exceed 20 mSv .
While the annual dose limits represent the maximum allowed annual doses for routine operations within a licensed facility, most workers, as we noted, actually receive much less. The National Council on Radiation Protection and Measurements (NCRP) has recommended in its Report 116, Limitation of Exposure to Ionizing Radiation, that an individual's cumulative occupational effective dose not exceed the worker's age multiplied by 10 mSv. Thus, for a 65-year-old worker, the NCRP would recommend a cumulative occupational effective dose of no more than 650 mSv. This recommendation has not yet become a legal requirement for facilities licensed by the U.S. Nuclear Regulatory Commission and Agreement States, which represent the major licensing and enforcement groups in the United States. Similarly, the ICRP has stated that a worker's lifetime cumulative dose should not exceed about 1 Sv.
If we consider your hypothetical worker who receives 30 mSv per year and assume he/she receives this same annual dose throughout his/her career, from perhaps age 18 through age 65, his/her cumulative dose would be 1.41 Sv. This would be more than twice the NCRP recommended value and about 40 percent more than the ICRP recommendation, but still within legal limits, although possibly not within ALARA limits, depending on the exposure conditions. Using ICRP risk estimates, such an occupational dose would result in about a 5 percent probability that the radiation received would cause the worker's death and would be associated with a reduction in life span of about six months. Other professional and recommending organizations, such as the ICRP, have concluded that doses as high as 100 mSv delivered over any time interval up to a year would produce no significant measurable effect.
You are correct in saying that the radiation itself is not stored up by the body, but the rationale used in radiation protection for restricting cumulative doses has been that the probability of inducing certain serious effects, such as cancer, is proportional to the dose. This leads to a conclusion that, although the radiation does not accumulate in the body, the risk is cumulative, and provides grounds for limiting lifetime doses as well as annual doses. Most of the actual risk data that we have are based on high doses (greater than 100 mSv) received over a very short time. We project risk from these data to lower dose levels of the magnitude that might be expected, for instance, for occupational workers whose doses are generally low and received at low dose rates. The fact that we have assumed this extrapolation (usually referred to as the linear no-threshold [LNT] extrapolation) to be valid has been the cause of much controversy in the field of radiation protection. There is considerable laboratory evidence that low doses of radiation, even levels similar to and exceeding the annual dose limits that now prevail, have a radioprotective effect, and many health physicists and radiobiologists do not feel that we should be applying the LNT approach to estimate risk at occupational dose levels or at even lower levels encountered by members of the general public.
In your question you also raise the possible concern about someone undergoing routine medical procedures who might be accruing several mSv per year. It is true that such an individual would be assumed to experience the same or possibly greater health risks from the radiation exposure as would an occupational worker who was receiving the same doses. The major difference between the two is that the individual who might be receiving relatively significant doses from medical procedures is suffering from a possible medical malady for which the risks from the radiation exposure may be small compared to the risks from the undiagnosed or untreated physical condition. It is important, however, if we believe that all radiation exposure is potentially harmful, that the medical community evaluate the appropriateness of procedures that are ordered for the treatment or diagnosis of patients to ensure that the benefit of the procedures exceeds potential risk from the procedures.
I realize that this topic of dose limits and dose effects still engenders considerable uncertainty, and I regret that I cannot provide unassailable answers to your questions, but I hope the discussion provides some clarification for you.
George Chabot, PhD