The only dose-effect relationship available are those of miners cohorts (for much higher exposures), exposed to radon. Studies of Hiroshima and Nagasaki survivors are less informative (the exposure to radon is chronic, localized, and the ionizing radiations are alpha rays).
Although low-exposed miners experienced exposures comparable to long-term residence in high-radon dwellings, the mean cumulative exposure among miners is approximately 30-fold higher than that associated with long-term residency in a typical home. Moreover, the smoking is a significant confounding factor in all miners' studies. It can be concluded from miner studies that when the radon exposure in dwellings compares to that in mines (above 1000 Bq/m), radon is a proven health hazard; but in the 1980s very little was known on the dose-effect relationship, both theoretically and statistical.
Studies have been made since the 1980s, both on epidemiological studies and in the radiobiology field.
In the radiobiology and carcinogenesis studies, progress has been made in understanding the first steps of cancer development, but not to the point of validating a reference dose-effect model. The only certainty gained is that the process is very complex, the resulting dose-effect response being complex, and most probably not a linear one.
Biologically based models have also been proposed that could project substantially reduced carcinogenicity at low doses.
In the epidemiological field, no definite conclusion has been reached. However, from the evidence now available, a threshold exposure, that is, a level of exposure below which there is no effect of radon, cannot be excluded.
Given the radon distribution observed in dwellings, and the dose-effect relationship proposed by a given model, a theoretical number of victims can be calculated, and serve as a basis for public health policies.
With the BEIR VI model, the main health effect (nearly 75% of the death toll) is to be found at low radon concentration exposures, because most of the population (about 90%) lives in the 0–200 Bq/m range. Under this modeling, the best policy is obviously to reduce the radon levels of all homes where the radon level is above average, because this leads to a significant decrease of radon exposure on a significant fraction of the population; but this effect is predicted in the 0–200 Bq/m range, where the linear model has its maximum uncertainty. From the statistical evidence available, a threshold exposure cannot be excluded; if such a threshold exists, the real radon health effect would in fact be limited to those homes where the radon concentrations reaches that observed in mines—at most a few percent. If a radiation hormesis effect exists after all, the situation would be even worse: under that hypothesis, suppressing the natural low exposure to radon (in the 0–200 Bq/m range) would actually lead to an increase of cancer incidence, due to the suppression of this (hypothetical) protecting effect. As the low-dose response is unclear, the choice of a model is very controversial.
No conclusive statistics being available for the levels of exposure usually found in homes, the risks posed by domestic exposures is usually estimated on the basis of observed lung-cancer deaths caused by higher exposures in mines, under the assumption that the risk of developing lung-cancer increases linearly as the exposure increases. This was the basis for the model proposed by BEIR IV in the 1980s. The linear no-threshold model has since been kept in a conservative approach by the UNSCEAR report and the BEIR VI and BEIR VII publications, essentially for lack of a better choice: