Artificial UV radiation made its public debut in the 1940s, used for promoting vitamin D synthesis in children. Early devices were heavily weighted toward shorter-wave UVB radiation, which produces vitamin D but can easily burn skin. By the time indoor tanning became popular in the 1980s, the trend was toward longer-wave UVA rays that don’t burn skin so readily.13
IARC describes UVB as a “complete human carcinogen” because of its ability to cause direct DNA damage. UVA, on the other hand, is carcinogenic by an indirect mechanism: It’s involved in the production of DNA-damaging free radicals, such as hydrogen peroxide.13
Overstreet says most of the tanning beds used now in the United States emit a ratio of 95% UVA to 5% UVB.
Scientists started investigating potential links between artificial UV exposure and skin cancer during the late 1970s. IARC reviewed 19 such studies during a meta-analysis published in 2006.13
The results showed a 15% increased risk for melanoma, 125% increased risk for SCC, and 3% increased risk for BCC among those who had ever tanned indoors compared with those who had never done so. In a subset analysis, IARC looked at seven studies that homed in on melanoma in relation to age at first incidence of indoor tanning. That separate analysis revealed a 75% higher risk among people who had ever tanned indoors before age 35 compared with those who never tanned indoors.13
It was on that basis, combined with sufficient evidence of an increased risk of ocular melanoma associated with the use of tanning devices, that IARC classified indoor tanning as carcinogenic to humans in 2009.3
Kelly Stoddard, state vice president of health and advocacy initiatives with the American Cancer Society in Williston, Vermont, says IARC’s widely publicized move to classify indoor tanning as carcinogenic is in part what motivated her to spearhead the Vermont ban. “We have data showing that 21% of young women in Vermont use tanning beds,” Stoddard says. “And melanoma rates in the 25- to 29-year age group here are growing, which leads us to think it has something to do with UV damage during the teen years.”14
Melanocytes are skin cells that produce melanin, the pigment that gives skin its color. Short, intense waves of UVB radiation stimulate organelles known as melanosomes to produce melanin, which is absorbed by surrounding keratinocytes. Longer waves of (more ...)
But although the IARC classification was influential in terms of state health policy, the analysis on which it was founded also suffered from what many experts say is a significant shortcoming: inadequate exposure assessment. According to DeAnn Lazovich, an associate professor at the University of Minnesota School of Public Health in Minneapolis, none of the studies included in the analysis measured dose response the same way, and most of them didn’t account adequately for the confounding effects of sun exposure. What’s more, the studies were limited by the fact that early tanning devices emitted UVB radiation at levels much higher than those in use today.
More recent studies have sought to address those shortcomings, including one published by Lazovich and colleagues in 2010. For their analysis, Lazovich’s research team gave questionnaires to 1,167 patients diagnosed with invasive cutaneous melanoma in Minnesota between 2004 and 2007, and to 1,101 matched controls, in order to assess their indoor tanning habits. The team collected a broad range of data, including but not limited to years of tanning bed use, age at initial use, and types of tanning beds frequented, in order to estimate UVA/UVB ratios during exposure. The results of that study showed a 74% increase in melanoma risk among those who had ever tanned indoors versus those who had never done so. The researchers also revealed a strong dose–response relationship: Those who had tanned 10 or fewer times had a 34% higher risk of melanoma, and those who had tanned 100 times or more had a 272% higher risk, compared with those who had never tanned.15
What Lazovich’s team didn’t find, however, was evidence that melanoma risk increased with decreasing age of first tanning bed exposure. What drove the cancer risks higher, she explains, was exposure frequency. “Individuals who tan more often were at greatest risk regardless of the age they started to tan indoors,” Lazovich says. That’s important, she adds, because scientists still don’t know if younger people are biologically more sensitive to the carcinogenic effects of UV radiation, or if tanning during youth puts them at greater risk simply because they accumulate more exposures over time. It also is unclear what role burning plays in risk of melanoma; in Lazovich’s study, melanoma cases who used indoor tanning reported burning more frequently than controls who used indoor tanning.15
In a study that supplies some potential insight into some of those questions, Anne Cust, a senior lecturer at the Sydney School of Public Health in Australia, and colleagues looked at indoor tanning among 604 patients diagnosed with invasive cutaneous melanoma between the ages of 18 and 39. Compared with melanoma risk among those who did not use tanning beds, their findings showed that the risk of melanoma associated with 10 or more indoor tanning sessions was nearly 600% higher among patients diagnosed at or before the age of 29, compared with 60% higher among those diagnosed during ages 30–39.16
Those findings, Lazovich says, indicate that people diagnosed at younger ages might be genetically primed for melanoma “so that tanning triggers an illness that they might not otherwise have been diagnosed with until later in life.” But this idea, she says, needs more study.
Meanwhile, newer reports are adding to the evidence that indoor tanning boosts risks for non-melanoma skin cancers. Among them was a study in which Jiali Han, an associate professor in dermatology at Harvard University and Brigham and Women’s Hospital in Boston, and colleagues looked at cancer risk in relation to indoor tanning among 73,494 participants in the Nurses’ Health Study II. This long-term prospective study examined factors that affect women’s health, especially cancer risk. Han’s study revealed an 83% increased risk of BCC among women who used tanning beds most frequently during high school and college, and a 30% increased risk among those who used them most frequently between the ages of 25 and 35, compared with those who never used tanning beds. Significant associations were not observed for SCC or melanoma.17
A study published in December 2011 by Susan T. Mayne, a professor at the Yale School of Public Health, and colleagues backs up Han’s BCC findings. That study looked at 376 patients under age 40 who had been diagnosed with BCC—which Mayne says occurs rarely in this age group—and 390 controls. It revealed that, compared with those who did not use tanning beds, indoor tanning was associated with a 69% higher risk of BCC, with evidence of dose response for increasing sessions, years, burns, and hours of indoor tanning. Remarkably, BCC tumors showed up frequently on the trunk of the body, which is unusual given that these tumors more often occur on the face and neck in older people, Mayne says.18
Another study, this one by Portia T. Bradford and colleagues from the National Cancer Institute, detected rising rates of trunk melanoma on women under age 40. The authors cited changes in clothing patterns—namely bikinis and shirts that leave the back and front of the trunk exposed to the sun—as one potential factor in that trend but also pointed out that the use of indoor tanning beds is most common among young women.19