The study is based on data from the Swiss National Cohort, a national research platform linking census data with mortality data. Analyses include data from more than five million individuals who were at least 20 years old at the time of study begin. In the study period, from December 2000 to December 2008, 1,900 deaths from malignant melanoma were registered according to death certificates.

Residential radon exposure was modelled for each person's address at study begin. The model used had been developed and validated taking nearly 45,000 measurements into account and considers building characteristics and geological aspects.

Individual UV exposure was also modelled with respect to the address at study begin. The model considers UV-Index measurements and several environmental characteristics.

The relative risk of dying from malignant melanoma increases by 16 % (95 % confidence interval: 4 % - 29 %) per 100 Bq/m³ increase in radon exposure for the 60-year-olds. For individuals in this age group who lived at the same address in 1990 and 2000, the increase in risk was somewhat higher (20 % per 100 Bq/m³ increase in radon exposure; 95 % confidence interval: 5 % - 38 %). The observed increase in risk due to radon tended to decrease with age: The relative risk of dying from malignant melanoma was more than twice as high at age 30 years (41 % per 100 Bq/m³ increase in radon exposure; 95 % confidence interval: 9 % - 80 %) compared to age 60 years.

Civil status, level of education, social status of the neighbourhood, gender, mother tongue and outdoor work with potential UV exposure were additionally considered in the analyses. These factors had practically no effect on radon risk estimates. However, UV exposure was only associated with melanoma risk when these factors were included in the analyses.

In addition to the risk for malignant melanoma, Vienneau et al. also investigated the risk of death from non-melanoma skin cancer. As non-melanoma skin cancer (e. g. basal cell carcinoma, squamous cell carcinoma) has a much lower case fatality, only few cases were observed during the study period. No statistically significant association was observed between non-melanoma skin cancer and radon exposure for the 60-year-old. Risks for other age groups have not been reported. For the 60-year-olds with the same address in 1990 and 2000, the risk even decreased – though not statistically significant – by 16 %. Between UV exposure and mortality due to non-melanoma skin cancer there was no statistically significant association in the study either.

This is to our knowledge the first epidemiological study that finds a statistically significant association between residential radon exposure and the risk of dying from malignant melanoma.

A small – statistically non-significant – increase in melanoma risk with radon exposure has been found in two earlier studies. One of them, a Danish cohort study (Bräuner et al. 2015) investigated the incidence risk of more than 50,000 individuals and took prior addresses and the duration of residence into account when modelling individual radon exposure. Additionally, individual social status and information on skin type and leisure activities were considered in adjusted analyses. When these factors were not included in the analyses, the increase in risk with radon exposure was higher and statistically significant. A clear exposure dependence over radon exposure quartiles was observed neither in the adjusted nor in the unadjusted analyses.

Statistically significant associations between different sub-types of non-melanoma skin cancer and radon exposure have been found in some incidence studies. Most of these studies were so-called ecological studies which used aggregated data, e. g. regional cancer rates and regionally averaged radon concentrations. Therefore, they are only of limited explanatory power. Remarkably, the above mentioned Danish cohort study also found a statistically significant association between individually modelled radon concentration and incidence for basal cell carcinoma. However, the association was stronger for persons living in apartments at enrolment. As radon concentrations and their variations are presumably smaller in apartments than in single detached homes, there is some doubt that the relationship is causal.

Generally, radon concentrations in underground uranium mines are much higher than in residential buildings. In historical uranium miner cohorts a striking number of lung cancer cases was found. Regarding skin cancer, there was no such observation. Among the 30,000 deceased miners in the German uranium miner cohort 3,942 lung cancer deaths were observed, and 69 deaths from malignant melanoma. An additional 18 miners in the cohort have died from non-melanoma skin cancer. In none of the uranium miner cohorts there is a statistically significant increase in melanoma mortality compared to the general population (Darby et al. 1995, Kreuzer et al. 2008, Tomasek et al. 1993). Only in the time window 15 to 24 years after first exposure melanoma mortality was increased significantly in the Czech cohort. However, there have been only six cases, and there was no evidence for an exposure-effect-relationship. Another Czech study (Kulich et al. 2011) found some evidence of an increase in malignant melanoma incidence with radon exposure.

In summary, there seems to be only rather weak evidence for an association between radon exposure and skin cancer. Studies investigating the association between indoor radon exposure and skin cancer lack individual radon measurements as well as individual information on other risk factors, particularly UV exposure. Results from miner studies suggest that the skin cancer mortality risk per unit radon exposure is at least much smaller than lung cancer mortality risk.

Strengths of the study of Vienneau et al. (2017) are the large range of radon and UV exposures in Switzerland und the enormous sample size, allowing in principle to detect even relatively small increases in risk. Additionally, individual information on radon exposure and cause of death is available.

The extent to which modelled radon concentrations at participants' addresses at the beginning of the study represent long-term radon exposure is, however, questionable. Furthermore, skin cancer risk depends strongly on intermittent UV exposure, e. g. resulting from journeys with high UV exposure, number of nevi and number of sunburns (S3-Leitlinie Prävention von Hautkrebs 2014). It is known from several studies that skin cancer risk is also associated with social status. Some of these aspects have not been considered in Vienneau’s study, for others only rough information is available. If one or more of these factors is also associated with radon exposure as it is modelled in the study, a spurious association between radon exposure and skin cancer risk might result or at least the size of the association might be overestimated. In the Danish cohort study (Bräuner et al. 2015) the strength of the association between melanoma risk and radon exposure decreased considerably after such factors had been taken into account and was no longer statistically significant.

A more precise description of the statistical methods and results would facilitate evaluation of the true radon risk. Notably, risk estimates and numbers of cases for different categories of radon exposure would be helpful. As there is a strong interaction between age and radon exposure in the study, risk estimates for the different age categories would be good to know.

Overall, it is unclear whether the observed increase in melanoma risk is really related to long-term radon exposure. Nevertheless the study provides some evidence for a possible association between radon exposure and melanoma risk.

The hypothesis of an association between residential radon exposure and non-melanoma skin cancer is not supported by the study. There was no statistically significant increase in mortality risk for non-melanoma skin cancer with radon exposure, and for persons having the same address in 1990 and 2000 the risk even decreased. As non-melanoma skin cancer is only very rarely fatal, data on causes of death are not well-suited for investigating this association.

Bräuner EV, Loft S, Sorensen M, Jensen A, Andersen CE, Ulbak K, Hertel O, Pedersen C, Tjonneland A, Kruger Kjaer S, Raaschou-Nielsen O; Residential Radon Exposure and Skin Cancer Incidence in a Prospective Danish Cohort, PLoS One 2015; 10(8): e0135642.

Darby SC, Whitley E, Howe GR, Hutchings SJ, Kusiak RA, Lubin JH, Morrison HI, Tirmarche M, Tomasek L, Radford EP et al. Radon and cancers other than lung cancer in underground miners: a collaborative analysis of 11 studies. J Natl Cancer Inst 1995; 87(5): 378-384.

Kreuzer M, Walsh L, Schnelzer M, Tschense A, B. Grosche B; Radon and risk of extrapulmonary cancers: results of the German uranium miners' cohort study, 1960-2003. Br J Cancer 2008; 99(11): 1946-1953.

Kulich M, Rericha V, Rericha R, Shore DL Sandler DP; Incidence of non-lung solid cancers in Czech uranium miners: a case-cohort study. Environ Res 2011; 111(3): 400-405.

Vienneau D, Hoog de K, Hauri D, Vicedo-Cabera A, Schindler C, Huss A, Röösli M; Effects of Radon and UV Exposure on Skin Cancer Mortality. Environmental Health Perspectives 2017; 125(6): epub 2017/07/08.

Tomasek L, Darby SC, Swerdlow AJ, Placek V Kunz E; Radon exposure and cancers other than lung cancer among uranium miners in West Bohemia. Lancet 1993; 341(8850): 919-923.

S3-Leitline Prävention von Hautkrebs; Version 1.0; Januar 2014; AWMF-Registriernummer: 032/052OL.