It's the tanning salons! No, it's the cigarettes!

American and Swedish researchers can't seem to agree on what's causing squamous cell cancers

By Chris Williams

There's been something of a turnaround in our thinking about sunlight in the last couple of years. No longer do we consider all sun exposure a bad thing. In fact, it's become clear that while obsessional sun avoidance may help prevent skin cancers, the resulting shortage of vitamin D can potentially increase the risk of far deadlier internal cancers, as well as other disorders like multiple sclerosis.

Last summer, the world's media reported that tanning salons may be even more dangerous than natural sunlight, based on a Swedish population study which found a dramatic increase of squamous cell cancers on parts of the skin that are unlikely to see the light of day. Their titanic retrospective study collected data on 10.2 million people from the Swedish Cancer Registry from 1961 to 1998. "In summary, the data shows a large increase in reported cases of squamous cell cancer, of which in situ cases have increased drastically," the authors wrote last July in Archives of Dermatology. "Among invasive cases, the increase has been largest among covered sites, which also showed the largest cohort effect, suggesting contribution by intentional tanning."

That would seem a logical enough conclusion, but the data suggested that the increased use of tanning salons in Sweden is too recent to have already produced so many extra squamous cell cancers, according to Dr Grant. "And there were other discrepancies as well," he added. "What really raised a red flag for me was that most of these extra cancers were appearing in men, yet it was women who were doing most of the artificial tanning." In fact, squamous cell cancer on covered skin is twice as common in Swedish males as in females. Yet women use sun beds twice as frequently as men do in Sweden. "A much more likely explanation can be found in the effects of smoking," noted Dr Grant.

"Smoking is a well-known risk factor for squamous and basal cell carcinoma, even for covered skin areas. Smoking deaths in Sweden increased significantly over the study period, and the proportion of women rose dramatically. The 1995 sex ratio of smoking deaths in Sweden is very similar to the sex ratio reported for squamous cell cancer in covered skin. Also, the adverse effects of smoking generally happen in older people, as is the case for the squamous cell cancer data reported."

The study authors Dr Hemminki and colleagues haven't taken Dr Grant's critique lying down. Their reply has also been published: "We do not want to consider whether tobacco smoke is causing squamous cell cancer of the skin, because the Ad Hoc Working Group of the International Agency for Research on Cancer refrained from making such a call in 2002. Yet, the trends of smoking prevalence among Swedish men, peaking in the 1960s, and their lung cancer incidence, peaking around 1980, show no match to the continuously increasing incidence of squamous cell cancer of the skin."