Lung Cancer Risk Associated with Low Chronic Radon Exposure: Results from epidemiology and animal experiments in France
D. Laurier, G. Monchaux, A. Rogel, JP Morlier, S. Billon, B. Quesne, M. Tirmarche, IRPA, 23-28 mai 2004, Madrid Espagne.
An increased lung cancer risk associated with radon exposure has been reported both in underground miners cohort studies and in animal experiments. However a large proportion of these results come from populations who received high radon exposure for a short period of time. Our aim is to analyse the effects of radon in populations exposed to low dose rates for a long period of time. In France, research was developed in parallel both in the field of epidemiology through the cohort of uranium miners, and in that of animal experiments. We present here recent results underlying the points of agreement between epidemiological and experimental approaches.
The French cohort of uranium miners includes 5098 miners employed at least one year since 1947, and were followed up to 1994. Compared to other miner studies, radon cumulative exposure is low (mean=37 Working Level Months, WLM), and protracted over a long duration (mean=12 years). The total number of deaths is 1162, from which 125 died of lung cancer. Results show a significant increase of lung cancer risk with cumulative exposure to radon. Period of exposure appears as an important modifier of the exposure-risk relationship (reflecting historical changes in radiation protection and measurement methods), but no effect of dose rate is observed. Also a nested-case control study has been implemented, with the aim to reconstruct tobacco consumption and to analyse the interaction between smoking and radon on lung cancer risk.
Experimental studies on the effects of radon are conducted in France by CEA and COGEMA since a long time. New experiments were conducted to analyse the dose rate effect and provide results at low levels of exposures (groups with cumulative exposure as low as 25 WLM). More than 4,000 rats have been exposed to various exposure rates and durations of exposure under controlled conditions, and followed-up over their life span. Histopathological analysis allowed a precise classification of the tumours. Results show that the risk increases with cumulative exposure. At cumulative exposure higher than 200 WLM, an inverse dose rate effect is observed. Such an effect is not observed at cumulative exposure lower than 50 WLM.
Epidemiological data are consistent with animal data to show an increase of risk with cumulative exposure protracted at low dose rates. Animal data are also consistent with previous studies of underground uranium miners showing an inverse dose rate effect at high cumulative exposures, but this effect disappears at low cumulative exposures.
These studies are part of a collaborative work conducted during the Fifth Framework Programme of the European Community. The project, coordinated by M Tirmarche, involved 7 teams from 5 different countries. The objective is to derive a synthesis from human and animal data, bringing together researchers involved in three different fields: epidemiology, animal experiments and mechanistic modelling. Results will provide a sound basis for assessment of risk associated with low radon exposures.