Oct. 21, 2003 -- Researchers know that airline pilots and flight attendants have a higher rate of certain types of cancer. What's less concrete is exactly why.
But what has eluded scientists is the exact cause and effect: Is it the higher altitudes that boost risk? Irregular work schedules, which can disrupt their circadian rhythms? The fact that flight crews spend more time sunbathing on beaches than the average Joe?
Three new studies in the November issue of Occupational and Environmental Medicine by researchers in Iceland and Sweden provide more clues to this baffling association.
The Longer the Career, the Higher the Risk
"We found that those who have been flying for five or more years have double the risk of breast cancer compared to those flying shorter periods," lead researcher Vilhjalmur Rafnsson, MD, PhD, of the University of Iceland, tells WebMD. "In previous studies we conducted, we found that flight attendants had an overall 50% increased risk compared to women in the general population."
However, among flight attendants who worked until 1971, he finds those with five years or more on the job had five times the rate of breast cancer than their less-tenured peers.
This increased rate seen among flight attendants remained even after Rafnsson adjusted for the women's reproductive history. "We found that flight attendants who never had children were more likely to develop breast cancer than those who gave birth," he says. "While this is known in the general population, it has never been studied specifically in flight attendants."
But a Swedish study shows the risk of breast cancer wasn't much higher than the general population. And it shows no link between length of employment and breast-cancer risk, and they question whether reproductive history is really a factor.
Ruling Out the Sunbathing Effect
Rafnsson's team also published another study examining rates of malignant melanoma, a sometimes-fatal form of skin cancer, among 1,000 airline workers and 2,000 Icelanders who didn't work on planes.
He found a surprise: While airline workers take more holidays to sunny climes and report more sunbathing than other Icelanders, "these factors alone do not account for the increased risk in skin cancer," he tells WebMD.
Meanwhile, the study by Swedish researchers finds that both male and female airline crew members from Sweden face a higher risk of melanoma, and men have more non-melanoma skin cancers compared with the general population.
"The message of our study is that cabin-crew members have an increased risk of skin melanoma and possibly also of breast cancer," epidemiologist and lead researcher Anette Linnersjö, MSc, of the Stockholm Center of Public Health, tells WebMD. "But these increased risks may be due to exposure at work, or outside of work, and need to be further investigated."
Still, these three new studies are helping to narrow the list of suspected causes. And one leading factor may be higher levels of radiation that result from flying specific routes -- and not necessarily at a higher altitude. Most of the studies thus far have involved flight crews in Scandinavian countries, including Finland and Norway. Other studies have involved flight crews in the U.S. and Canada.
"On routes across the north Atlantic Ocean, as often done by our flight crews, you are exposed to higher levels of ionizing radiation than other routes, such as those flying inside continents," Rafnsson tells WebMD. "As you get closer to the north and south poles, the magnetism of the earth is such that it attracts more cosmic radiation.
"So possibly, it's the radiation on these routes that is increasing their risk of both types of cancer. We need more studies to verify, but with our new report, we have more or less ruled out the sunbathing effect for increasing skin cancer rates."
In an accompanying editorial, Elizabeth Whelan, ScD, MPH, of the CDC, notes that ionizing radiation exposure has increased for today's flight crews, since newer planes fly for longer periods. But she writes that other factors, such as irregular working hours that disrupt circadian rhythms, or sleep/wake cycles, could also predispose flight crews to these diseases.
"The next step is to study these disease rates in frequent fliers and not just flight crews to determine if they also have an increased risk," says Rafnsson.
In the meantime, he recommends that anyone who works in airplanes -- or uses them frequently -- take the necessary precautions to reduce risk of breast and skin cancers. "That means getting a mammogram at least once every two years and wearing sunblock whenever exposed to sunlight."
SOURCES: Whelan, E, Occupational and Environmental Medicine, November 2003, vol 60, pp 805-806. Rafnsson, V, Occupational and Environmental Medicine, November 2003, vol 60, pp 807-809. Linnersjö, A., Occupational and Environmental Medicine, November 2003, vol 60, pp 810-814. Rafnsson, V, Occupational and Environmental Medicine, November 2003, vol 60, pp 815-820. Vilhjalmur Rafnsson, MD, PhD, professor of preventive medicine, the University of Iceland, Reykyavic, Iceland. Anette Linnersjö, MSc, statistician, department of Epidemiology, Stockholm Center of Public Health, Stockholm, Sweden.