(AP)
More than a third of adults in Western countries have been exposed to indoor tanning at some point, according to an analysis of past research.
Based on those exposures, the study authors calculate the number of skin cancers that can be blamed on indoor tanning each year exceeds the number of lung cancers attributed to smoking for the countries studied.
"What we knew is that indoor tanning is linked to skin cancer," Dr. Eleni Linos said. "What we wanted to find out is how common is exposure in the United States and internationally."
Linos is the study's senior author and a dermatologist at the University of California, San Francisco.
The World Health Organization (WHO), the American Medical Association and the American Academy of Dermatology have come out against indoor tanning in recent years.
In 2009, WHO labeled tanning devices as high-level carcinogens, which puts tanning on par with tobacco use as a public health threat.
In 2007, a working group affiliated with WHO found that people who used tanning beds before their 30th birthday were 75 percent more likely to develop melanoma - the deadliest form of skin cancer.
That added risk translates to about seven more women out of every 10,000 developing melanoma. For example, one past study showed that 24 out of 10,000 women who regularly used tanning beds developed melanoma compared to 17 out 10,000 women who rarely or never used them.
Previous research has attempted to estimate the number of people exposed to indoor tanning devices in individual countries, but Linos and her colleagues wanted to know estimates for people around the world.
"We basically wanted to see how big a problem this is for our population," Linos said.
For the new analysis, the researchers compiled data collected between 1986 and 2012 as part of 88 previous studies. The studies included data from almost 500,000 people in the U.S., Australia and 14 countries in Eastern and Western Europe.
Overall, they found that about 36 percent of adults and 55 percent of college students reported exposure to indoor tanning sometime in their past. The proportion was smaller for teens - about one in five.
Younger people, however, were more likely to be actively indoor tanning with about 43 percent of college students and about 18 percent of teens reporting use in the past year. That compared to about 14 percent of adults.
In all age groups, women were more likely to report indoor tanning use.
The researchers estimated that about 419,000 cases of basal and squamous cell carcinoma and almost 11,000 cases of melanoma each year are attributable to indoor tanning.
For perspective, they point out in JAMA Dermatology, about 363,000 cases of lung cancer are attributable to smoking each year in the same countries included in the study.
While people are much more likely to die from lung cancer than most skin cancers, people don't appreciate the costs of those skin cancers, said Brenda Cartmel, a cancer prevention expert from the Yale Cancer Center and the Yale School of Public Health in New Haven, Connecticut.
For example, she said the cost to Medicare may increase when the large numbers of young people who use indoor tanning become elderly.
"We really don't know what's going to happen to these people who are exposed to indoor tanning now when they get to be 60 or 80," Cartmel, who wasn't involved in the new study, said.
"(The study) is very timely in its publication as many states that don't yet have indoor tanning bans for minors are working toward getting that legislation passed," she said.
Currently, at least 33 U.S. states and the District of Columbia regulate the use of tanning facilities by minors, according to the National Conference of State Legislatures.
"I think it's important for people to realize that it's a class one carcinogen," Cartmel said. "I don't think people who indoor tan realize that."
Stay up-to-date on the biggest health and wellness news with our weekly recap.