How to express low dose risk

Most of us have read the HPS position paper on radiation risk (edited text follows).

Radiation Risk in Perspective Radiogenic Health Effects Have Not Been Observed Below 10 Rem. Radiogenic health effects (primarily cancer) are observed in humans only at doses in excess of 10 rem delivered at high dose rates. Below this dose, estimation of adverse health effect is speculative. Epidemiological studies have not demonstrated adverse health effects in individuals exposed to small doses (less than 10 rem) delivered in a period of many years. . . . the Society has concluded that estimates of risk should be limited to individuals receiving a dose of 5 rem in one year or a lifetime dose of 10 rem in addition to natural background. Below these doses, risk estimates should not be used; expressions of risk should only be qualitative emphasizing the inability to detect any increased health detriment (i.e., zero health effects is the most likely outcome).

So the HPS tells us we should not do dose calculations for an individual at doses less than 5 rem in a year or 10 rem in a lifetime. The problem is that I have a computer program (CAP88PC) that prints out on paper an individual's cancer risk. How much risk? About 7E-7. I am having a problem with telling someone that their risk of cancer was increased by about 7 in a 10 million. At what point do we simply say "there is no real risk to an individual at doses that are this low." YES, I know the LNT model is out there. I am asking about us - as HP professionals. At what point do WE tell someone that calculations of exposure to theoretical doses that are too small to measure (very small fractions of a millirem) present no actual increase in risk and that computer calculated risks for an individual at these levels do not express a "true" increase in risk?

Paul Lavely
UC Berkeley
Lavelyp@uclink4.berkeley.edu