Progress in understanding radon risk

​Radon has been recognised as a human lung carcinogen in 1988 by the World Health Organisation. The main source of information on risks of radon-induced lung cancer has been epidemiological studies of underground miners. More recently, several studies were developed to analyse lung cancer risk associated to residential radon exposures in the general population.

Recent results from miner epidemiological studies provided precise estimates of lung cancer risk associated to radon cumulated exposure. Risk coefficients provided by these low-level exposure studies are generally higher than those previously published. The resulting exposure-lung cancer risk coefficient in a European combined cohort was ERR per 100 WLM = 2.60 (95%CI= 1.83–3.36), to compare with the UNSCEAR 2006 figure of 0.59 (95%CI = 0.35-1.0) based on a comprehensive review of all available epidemiological results. The differences in the estimated ERR/WLM between whole cohorts and period-restricted or levelrestricted subsets could reflect the effects of several concomitant factors: better quality of exposure assessment in the later periods, lower exposure rates and shorter time since exposure.

Recent results from miner epidemiological studies also allowed a better quantification of modifying factors of the exposure-risk relationship and of the interaction with smoking. Results confirm the major effect of time since exposure (decrease of the ERR with increasing delay since exposure) and of age at exposure (higher ERR for exposures received at young age).

The relationship between radon exposure and lung cancer risk appears to be only slightly modified after controlling for smoking, confirming that the lung carcinogenic effect of radon persists even when smoking is adjusted for. Additional analyses provided arguments in favour of a sub-multiplicative interaction between radon and smoking, even if a multiplicative effect may be possible at low levels of radon exposure.

Consideration of these recent results from miner studies led to an increase of the estimated lifetime excess absolute risk of lung cancer death from radon and radon progeny compared to previous ICRP estimates (5 instead of 2.8 10-4 per WLM).

There is compelling evidence from cohort studies of underground miners and from  casecontrol studies of residential radon exposures that radon and its progeny can cause lung cancer. Comparisons of relative risk estimates between miners and residential models yielded very coherent estimates.

Collaborative works between epidemiologists and dosimetrists allowed calculation of lung dose-risk relationships (instead of exposure-risk models previously). Results confirmed the major contribution of exposure to radon decay products to the organ dose and there is currently a close agreement between dosimetric and epidemiological approaches.

Finally, it appears that the currently available results do not allow quantifying the health effects of radon in water. Nevertheless, it seems that cancer risk posed by radon in household public water supply is small and can mainly be attributed to the transfer of radon into air and the subsequent inhalation of radon decay products, rather than to ingestion of water. The risk could however be higher for people using private wells for water supply where radon levels could be high and variable.

For solid tumours other than lung cancer, and also for leukaemia, there is currently no consistent evidence of any excess associated with radon and radon progeny exposures.

Perspectives for the future include (i) continuing international epidemiological research by “pooling” of data from different countries and continents, (ii) continuing dosimetry studies in order to reduce uncertainties of organ doses (iii) getting more information to quantify the effects of exposures received during childhood (iv) identifying radon biomarkers for better classification of individuals regarding exposures, or for providing early indicators of diseases or to discriminate between standard and sensitive individuals. Finally several results published during the last years raised the issue of risks other than lung cancer potentially associated to radon. Answer to this issue requests very long follow-up periods and good control for potential confounders. The extension of the follow-up of current miner cohorts should provide new elements regarding this issue in the next 10 years.