Answer to Question #8325 Submitted to "Ask the Experts"

Q

I understand what a rad is; I understand what a rem is intuitively, but there is no time reference. For example, if I read a sentence that states "the dose is 1 rem, which is below the 5 rem yearly dose set by the NRC" I assume they mean, in regards to 1 rem, the 1 rem per year is the effective dose rate. But, that does not appear to be true since nowhere have I read anything that states time as part of the equation. I can explain that 1 rem of any type of radiation does approximately the same amount of biological damage, but the very next question I'm asked is "over how much time" and I can't find a reference that allows me to answer that question!  

Please can you clarify this for me?

A

To resolve at least part of your dilemma, I believe we have first to distinguish between dose and dose rate. The rem (as well as the rad) is an integral quantity that represents a particular amount of cumulative dose that accrues over either a specified or unspecified period of time. For example, in many parts of the United States, the average annual dose equivalent to an individual from background radiation and radon inhalation is about 0.3 rem. This dose accrues over the period of a year. We can estimate that the average dose equivalent rate, in rem per hour, during this one year period is the annual dose equivalent divided by the number of hours in a year—i.e., 0.3 rem/8,760 hours = 0.000034 rem per hour. In situations where individuals are dealing with larger sources of radiation, the dose rate might be much higher. For example, a radiation source that delivers a dose equivalent rate of 100 rem per hour could deliver the same 0.3 rem, which we get from natural background in a year, in just about 11 seconds.

Regulating agencies control dose by limiting annual doses to prescribed limits such as the 5 rem that you cite. While the doses in two instances may be the same, the magnitude of the doses and the dose rates at which such doses are delivered may be important in interpreting the biological impact of the doses.

There have been numerous biological experiments conducted, with nonhuman organisms, that demonstrate that the rate at which radiation dose is delivered can affect the extent of biological response. Thus a sufficiently high dose delivered over a period of a few minutes may be expected to have a greater biological impact than the same dose spread over a year. In the field of radiation protection we often apply a dose and dose rate effectiveness factor in an attempt to account for some of this difference. This applies, for instance, in the case of our attempting to use bioeffect information from the Hiroshima and Nagasaki bomb victim survivors, who sustained high doses delivered over a very short interval of time, in order to predict health effects to workers exposed to much lower levels of radiation commonly delivered over long time periods. In such instances it is common to apply a dose rate effectiveness factor of at least two to the high dose results—i.e., reduce the high, acute dose risk per unit dose factors by at least a factor of two when dealing with low, and chronic, doses. This dose and dose rate effect is taken into account in establishing standards for protection of individuals for most exposure situations.

For radiation-protection purposes, we assume that the risks associated with multiple radiation exposures or chronic exposures, occurring over extended time periods, are additive. Thus, when we are attempting to understand the potential biological impact, such as serious cancer incidence, we need to know the integral dose (rem) that has been received by an individual or a population; that dose might have been received over a relatively short time or it might represent cumulative dose over many years. For most exposure situations in which doses are relatively small and often delivered over an extended time period, we would use cancer risk factors that incorporate a dose rate effectiveness factor. If doses were large and resulted from short-duration exposures (usually such exposures are the results of serious accidents), then the dose rate effectiveness factor that reduces impact when doses are low and delivered at low rates may be eliminated in the analysis.

When you are asked whether there is a critical time period over which 1 rem of dose may have a greater biological impact than it might otherwise have, the answer is "No." One rem of dose is sufficiently low that whether it was delivered within one second or spread over a year or more, we would not expect any difference in biological effects. Because virtually all of the risk information we rely on originates from data obtained from studies of high-dose and high-dose-rate victims, there is much debate about whether it is appropriate to extrapolate such risk information to low doses and whether the risk estimates should be used at all below doses of about 10 rem. In fact, many groups concerned with radiation protection recommend against attempting to quantify biological risk when integral doses are less than about 10 rem (see the HPS Position Statement in this regard). This is a topic about which much more could be said, but more discussion would not likely lead to any more definitive conclusions.

George Chabot, PhD, CHP