Answer to Question #525 Submitted to "Ask the Experts"

Q

Please can you explain the difference between the "average annual dose rate" and "collective dose."

A

Actually, these terms are somewhat related. Collective dose is a measure of the total amount of radiation exposure to everyone affected by an activity. Collective dose is usually measured in units of person-rem or person-Sieverts. For example, if there are 25 million people in the United States who smoke cigarettes and each of them receives 2 rem from smoking, the collective dose to the U.S. population is 50 million person-rem. Another example of collective dose is to say that the U.S. population receives about 81 million person-rem from natural background radiation because about 270 million people receive an average annual exposure of about 300 mrem each from natural sources. Average annual dose rate is the average radiation dose that a given population receives in a year. The average annual dose rate from natural sources in the United States and Canada is about 300 mrem per year, although people in the Rocky Mountain states receive much higher doses and Gulf Coast residents receive less. In the smoking example above, the average annual dose rate across the entire U.S. population would be about 185 mrem per year. That being said, you should also be aware that the concept of collective dose has come under attack for some misuses. The biggest example of this is in calculating the numbers of expected health effects from exposing large numbers of people to very small radiation doses. For example, you might predict that, based on the numbers given above, the population of the United States would have about 40,000 fatal cancers from background radiation alone. However, this is unlikely to be true for a number of reasons. Recently, the International Council on Radiation Protection issued a position statement saying that the use of collective dose for prediction of health effects at low exposure levels is not appropriate. The reason for this is that if the most highly exposed person receives a trivial dose, then everyone's dose will be trivial and we can't expect anyone to get cancer. Another way to look at it is that if I throw a 1-gram rock at everyone in the United States then, using the collective dose model, we could expect 270 people to be crushed to death because throwing a one-ton rock at someone will surely kill them. However, we know this is not the case because nobody will die from a 1-gram rock. The Health Physics Society also recommends not making risk estimates based on low exposure levels. Andrew Karam, CHP University of Rochester