There are other sets of exposed populations. First, everyone is exposed to natural background radiation. There are also varying levels of exposure to indoor radon, which depends on house construction and on the region in which homes are located. The difficulty is that everyone is also exposed to many other risk factors, including natural and man-made environmental risks, diet, and heritable factors. There are other factors involved in risk determination including age and sex of the exposed person.
Risk also varies by type of cancer.
Specifically, risk factors for leukemia are calculated separately from risks of solid tumors, such as lung cancer and breast cancer. The hypothesis states that a given increment of exposure to radiation, no matter how small, will produce the same increment of cancer risk. So if a person has a certain risk of getting cancer at one rem of exposure, his cancer risk would be doubled for an exposure of two rem, and halved at 0.5 rem. Further, if ten people collectively got one rem, their collective risk would be the same as that of one person being exposed to one rem. Collective population exposure is expressed as person-rem, which is the sum of all individual exposures in a population.