Almost every commercial flight begins with a member of the cabin crew delivering a spiel to passengers about inflight safety. And yet, for the attendants themselves, the job is particularly dangerous when considering the cancer risks.

A new study published in the journal Environmental Health measured cancer rates among US airline-cabin crews relative to the overall American population and found a greater prevalence of cancer among flight attendants. Rates among flight attendants were especially high for breast, uterine, cervical, gastrointestinal, and thyroid cancer. There are a multitude of factors, and not one single reason for this cancer gap, according to the study.

The Harvard University scientists behind the work came to their conclusion by examining data from more than 5,300 US-based flight attendants who filled out an online survey between December 2014 and June 2015 as part of the larger “Harvard Flight Attendant Health Study.” Those data were then compared to surveys filled out by 2,700 other Americans with similar levels of education and income, but working in other sectors. Some of the differences are particularly striking. Even when flight attendants reported having stereotypically good health, diet, and exercise regimens, the likelihood that they would be stricken with certain cancers was still higher than the other survey respondents.

Among female flight attendants, the rate of breast cancer was 50% higher when compared to the general population. Melanoma rates were just over 200% higher and non-melanoma skin cancer rates were about 300% higher. Male flight attendants, meanwhile, were about 50% more likely to suffer from melanoma and 10% more likely to have non-melanoma skin cancer.

So what gives?

There’s been very little research on the health risks faced by flight attendants. In general, though, airplane cabin crews are exposed more regularly to ultra-violet light and cosmic radiation than the average person—at higher altitudes, cosmic radiation goes through less atmospheric filtering. That might help explain some of the higher rates in skin cancers.

Jet lag might also be an attributing factor, write the authors of the study, as fluctuations in immune-system strength can change cell metabolism and increase the likelihood of tumor growth. Then there are the flight attendants who’ve been in the business for several decades, and worked aboard planes before 1988, when smoking during flights was banned. Other concerns include the myriad substances cabin crews are exposed to because of engine leaks and flame retardants, both of which can contain suspected carcinogens.

Already the US flight attendants union released a statement on the study, saying it will “use the results to encourage airlines, airline manufacturers, and regulators to prevent exposures and change working conditions to reduce risk.” The association also noted that the US federal government currently does not require airlines to educate cabin crews about onboard radiation exposure, or to offer additional protections from radiation—including for pregnant flight attendants.

“That is unacceptable and we won’t stop working to fix it,” the group vowed.