Our study has several important findings. First, we found that melanoma occurred more frequently among indoor tanners compared to persons who never engaged in this activity. Second, we found a strong dose response relationship between melanoma risk measured by total hours, sessions or years. Furthermore, this dose-response was also seen for melanomas arising on the trunk, not only in men but also in women, who would not ordinarily expose this site to ultraviolet radiation except when tanning or sunbathing. Third, we found an increased risk of melanoma with use of each type of tanning device as well as with each period of tanning use, suggesting that no device can be considered “safe”. In addition, burns from indoor tanning appeared to be fairly common and conferred a similar risk of melanoma to sunburns. These associations remained significant even after adjusting for the potential confounding effects of known risk factors for melanoma.
We did not confirm the IARC report's emphasis on an increased risk of melanoma with first exposure to indoor tanning “in youth”, defined as use before the age of 36 (5
). Except for one cohort and two case-control studies that examined indoor tanning during adolescence in relation to melanoma (30
), all other reports considered use prior to ages 25 to 30 (11
), or restricted the analysis to cases diagnosed before the age of 36 (22
). This restriction, however, could have resulted in the exclusion of older cases and controls who may have been exposed at a younger age. An elevated risk of melanoma associated with first use at younger ages has been consistently observed across these studies, but this is also the case for indoor tanning used at older ages in some reports reviewed by IARC (11
). Our study was designed to specifically evaluate indoor tanning use initiated at any age. And by simultaneously accounting for duration of use among indoor tanners, our analysis indicates that early age exposure is most likely a marker for cumulative exposure, the reason for an excess risk of melanoma, not that younger individuals are at increased susceptibility to the effects of ultraviolet radiation. Although no other study has analyzed data in the same manner as we did, three reports provide further support for our observation. One recent report found total hours of sun bed exposure to be much higher (34 vs 9 hours) among persons who first tanned indoors before compared to after age 15 years (32
). And in two studies that stratified frequency of indoor tanning use by age of cases, elevated risks for melanoma were observed for those with 10 or more sessions, regardless of age (22
), or for those with regular use up to the age of 60 (28
With our carefully designed questionnaire to elicit use of specific devices that emit differing amounts of UVB and UVA, we observed considerably stronger odds ratios for melanoma among users of high speed or high pressure devices than among users of conventional devices. We still cannot be certain, however, these results reflect higher exposure to UVB from high speed devices or higher exposure to UVA from high pressure devices. First, the proportion of subjects reporting use of these devices was quite low. Second, studies have shown that the percent of UVB and UVA emitted depends on the type of lamp, the quality of maintenance and the level of degradation, information that cannot be collected through retrospective recall (50
). And recently, inspections of tanning devices in European tanning salons have revealed poor compliance with regulations for the allowable distribution of UVB versus UVA, with a concomitant increase in the proportion of UVB beyond permissible limits over time (54
). If UVA is carcinogenic in humans, as stated in the IARC report, our findings are biologically plausible. However, it is also possible that the devices we assessed, regardless of our classification scheme, emitted sufficient UVB for that component of ultraviolet radiation to be the reason for the observed associations. Similar to our experience, other studies that collected information about device types have not been able to single out any one type as being higher risk than another (21
). Nor have most studies, ours included, found higher risks of melanoma associated with indoor tanning exposure in a specific period, despite changes in emission of UV components over time (21
). While disentangling which wavelength is responsible for melanoma development may not be possible in epidemiologic studies, the evidence also indicates that all indoor tanning devices are harmful.
We did not find lifetime routine sun exposure or sun exposure via recreational outdoor activities or occupations to be associated with melanoma risk, nor were these results changed by a detailed examination of sun exposure according to season, decade age, type of outdoor activity, indoor tanning status or tumor site. Indeed, published studies reveal that the relationship between sun exposure and melanoma is complex, and depends on whether the exposure is intermittent or chronic; inconsistencies in its measurement further complicates an understanding of these relationships. A meta-analysis (58
) and a pooled analysis (59
), of 57 and 15 studies, respectively, each reported fairly weak associations between total sun exposure and melanoma, no relationship to chronic exposure (based on outdoor occupations), moderately strong associations with intermittent exposure (usually defined as sunbathing, time spent during sunny vacations, or outdoor recreational activities) and strong associations with sunburn. Thus, our results are in agreement with these reports for chronic exposure and sunburns. To the extent that sunburns are a marker of intermittent sun exposure, then our results adequately represent the independent effect of indoor tanning use on the risk of melanoma. Differential under-reporting of sun exposure by cases seems to be a less likely explanation of these trends in our study; had it been operative, we might have expected the same to occur for cases' report of artificial solar exposure. While our findings could reflect less variation in sun exposure among a relatively homogenous population residing in Minnesota, or the younger age of our study sample in contrast to most case-control studies of melanoma, we cannot exclude the possibility that non-differential misclassification obscured a relationship between sun exposure and melanoma.
Although the prevalence of indoor tanning among participating controls (51.1%) is high compared to most other reports, we do not think this is due to differential selection of indoor tanners into the study. In a 2002 Minnesota statewide survey of adults, age 18 and older (37
), we found that overall 36.3% of respondents reported indoor tanning use; prevalence was higher (42%) in the sample with the same age range as the current study. More importantly, the frequency of indoor tanning use was very similar when we compared participating and non-participating cases and controls and crude odds ratios for the association between indoor tanning use and melanoma were identical for participants and non-participants. We were also concerned that cases who had discussed the study with their physician may have reported higher frequency of indoor tanning use than cases who did not. We attempted to address this potential bias by querying both cases and controls in the latter part of the study. The fact that several controls (whose physicians were not contacted) reported discussions with their physician about the study prior to participating is also interesting. As the prevalence of over-reporting was similar for both cases and controls in this group, and the adjusted odds ratio among cases and controls who did not speak with a physician was similar to what we reported for the entire sample, recall bias seems less likely to explain our results. This conclusion is further supported by a recent nested case-control study, which reported no consistent pattern of recall bias for indoor tanning or other melanoma risk factors (60
In summary, our study provides strong evidence that indoor tanning is a risk factor for melanoma. Due to the strength of the association, the dose-response, the results by tumor site (especially the trunk), and the ability to account for known confounders, our results address several limitations of previous studies. Our results also indicate that the number of times an individual is exposed to indoor tanning is more important than exposure to indoor tanning at an early age. Our ancillary studies on bias, while limited in scope, suggest that our results are not explained by selection or recall bias. In conclusion, our results add considerable weight to the IARC report that indoor tanning is carcinogenic in humans and should be avoided to reduce the risk of melanoma.