Answer to Question #11967 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
It is not clear to me in your position statement "Radiation Risk in Perspective" what the time period is over which the 100 millisievert (mSv) dose is delivered. You state, ". . . below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero." Is it 100 mSv per year? 100 mSv per lifetime? Or something per hour?
The short answer is that it refers to 100 mSv over a lifetime. Radiation effects in people are not statistically different from zero below lifetime radiation doses of 100 mSv (above background). Because of this, the Health Physics Society (HPS) advises "against estimating health risks to people from exposures to ionizing radiation that are near or less than natural background levels because statistical uncertainties at these low levels are great."
As noted in the position statement, "Epidemiological studies have not consistently demonstrated adverse health effects in persons exposed to small (less than 100 mSv) doses protracted over a period of many years." In addition, the position statement specifically notes that "references to 100 mSv . . . should not be construed as implying that health effects are well established for doses exceeding 100 mSv." Thus, while 100 mSv represents the lower bound on our knowledge with respect to lifetime exposure, it does not represent a clear threshold above which potential harm is well understood.
In part, the large uncertainties associated with low-dose and low-dose-rate health effects of radiation exposure are specifically tied to not knowing the role background radiation exposure plays in low-dose studies. As the position statement points out, "A person [in the United States] might accumulate an equivalent dose from natural background radiation of . . . about 250 mSv during an average 80-year lifetime." So, while the average lifetime background radiation exposure is about 250 mSv in an 80-year lifetime, it can vary from approximately 100 mSv to 1,000 mSv, depending on where you live. This is where the problem lies because a lifetime exposure of 100 mSv (above what is received as background radiation) represents the same level of exposure as the lower estimate of lifetime natural background radiation. So not understanding the role and the risk of lifetime natural background exposure causes great uncertainty in our understanding of low-dose and low-dose-rate health effects of radiation.
In summary, the 100 mSv above natural background radiation referenced in the position statement refers to lifetime exposure, with the understanding that exposures above 100 mSv (and above natural background radiation) over a lifetime do not definitively result in harm.
Barbara Hamrick, CHP, JD
Past Chair HPS Scientific & Public Issues Committee
HPS Past President