JAMA: US scientists have pooled data from
previous studies on the incidence of melanoma in airline pilots and
flight crew, who are are exposed to higher levels of cosmic and UV
radiation than the general population. They report that flight crew have
approximately twice the incidence of melanoma compared with the general
population, and say that further research into better protection from
cosmic radiation is needed. According to a recent study, airline
pilots and cabin crew are occupationally exposed to higher levels of
cosmic and UV radiation than the general population, but their risk of
developing melanoma was not yet established.
Because
of the unique occupational risk factors of commercial pilots, cabin
crew, and flight attendants, numerous studies have investigated their
risks for various diseases. In general, mortality from cancer, as well
as cardiovascular, respiratory, and cerebrovascular diseases, is
significantly lower in pilots and cabin crew compared with the general
population.1
However, despite their overall improved health outcomes, pilots and
flight crew have been reported to have an increased incidence and/or
mortality from melanoma.2,3
The report published in JAMA Dermatology in January by Sanlorenzo et al3
assessed the risk of melanoma among airline pilots and cabin crew in a
meta-analysis that included 19 previously published studies. The studies
reflect data from 1943 to 2008 and include more than 266 000 pilots and
cabin crew. These 19 studies were selected because they reported a
standardized incidence ratio (SIR) or a standardized mortality ratio
(SMR) or had relevant data to allow for calculation of these established
measures of disease incidence and death. Pilots and cabin crew were
found to have approximately twice the incidence of melanoma when
compared with the general population (SIR, 2.21 [95% CI, 1.76-2.77]),
and mortality from melanoma among pilots was significantly increased
(SMR, 1.83 [95% CI, 1.27-2.63]). Because of the observational and
retrospective nature of the 19 studies, it was not possible to control
for certain potentially confounding factors such as aggregate flight
time, Fitzpatrick skin type, and lifestyle.3
Pilots and cabin crew are regularly exposed to higher levels of cosmic radiation during flight,4,5
and it is important to consider whether this additional radiation
exposure is associated with their increased risk of melanoma. Cosmic
(ionizing) radiation is composed of high-energy particles (neutrons and
gamma rays) that are capable of penetrating the plane’s exterior.4
Lower levels of cosmic radiation reach the earth’s surface and lead to
annual background radiation exposure of approximately 2 mSv per person
(to convert to rem, multiply by 0.1).1
Passengers and crewmembers are exposed to higher levels of radiation
depending on cruising altitude, latitude, and solar activity.4 Pilots and flight crew receive an additional annual dose of 2 to 9 mSv,5
which is significantly below the current exposure limit of 20 mSv per
year according to the International Commission on Radiological
Protection.3
For comparison, a single chest x-ray provides roughly 0.02 mSv and a
computed tomographic scan of the chest, abdomen, and pelvis provides
approximately 8 mSv. The lifetime risk of dying of cancer among US
adults is approximately 220 in 1000 and would be expected to increase
modestly to approximately 223 in 1000 after a 20-year career as an
airline crewmember (assuming 80 mSv of aggregate radiation exposure).4
In addition to the fact that this modest dose of cosmic radiation
should have little effect on cancer risk more generally, there is also
no known relationship between even relatively high doses of ionizing
(cosmic-type) radiation and melanoma.5 Thus, it seems unlikely that cosmic radiation exposure is relevant in the observed increase in melanoma in this population.
Beyond
cosmic radiation exposure, there are many established risk factors for
melanoma that could be relevant in flight crews, including skin type,
number of common and atypical nevi, family history of melanoma, history
of severe sunburn, use of tanning beds, and socioeconomic status.6,7
Many of these factors increase the risk of developing melanoma by more
than 2-fold. Notably, the relative risk of developing melanoma among
people with 101 to 120 common nevi is 6.89 (95% CI, 4.63-10.25) and with
5 atypical nevi is 10.49 (95% CI, 5.05-21.76).6
Although these risk factors significantly influence the development of
melanoma, they were not accounted for in the 19 studies of flight crews.
Therefore, the impact of these risk factors on the incidence of
melanoma observed among aircrew remains undetermined because they could
not be controlled for in the meta-analysis.
Higher
socioeconomic status, a demographic characteristic shared by pilots and
cabin crew, is associated with increased incidence of melanoma but
decreased mortality from melanoma.7
The Federal Aviation Administration has stringent health standards and
requires employees to routinely undergo medical evaluation. Regular
physical examination increases the detection of early-stage melanoma and
thus increases the reported incidence. Flight crew and air traffic
control officers undergo similar health screenings and have similar
socioeconomic status, and both have an increased risk of developing
melanoma even though air traffic control officers are not occupationally
exposed to high-altitude flight.2
On the basis of this observation, it is possible that the increased
incidence of melanoma among pilots and cabin crew could be more
associated with socioeconomic status, required health screenings, or
other melanoma risk factors rather than their greater number of flight
hours.
Episodic
UV exposure is also associated with an increased risk of developing
melanoma, whereas frequent low to moderate UV exposure is more
associated with risk for nonmelanoma skin cancers. As discussed by
Sanlorenzo et al,3
a pilot’s exposure to UV-B (280-320 nm) radiation through the
windshield is minimal because it is blocked by glass, but more than half
of UV-A (320-380 nm) radiation penetrates glass.3
The additional UV-A exposure for pilots offers a possible explanation
for their increased incidence of melanoma but does not account for the
increased risk for the cabin crew. A study with more than 3000 aircrew
members found that pilots and cabin attendants reported more
intermittent UV exposure (via sunny vacations) than the general
population but concluded that the increased incidence of melanoma
described in many studies could not be solely explained by behavioral
differences.5
Whereas many groups noted an increased melanoma risk among pilots and cabin crew, Sanlorenzo et al3
conducted the largest and most robust meta-analysis of this
association. Multiple hypotheses have been proposed, but the etiology of
the increased incidence of melanoma among aircrews remains largely
unexplained. Although this increased risk is now well established in the
literature and reinforced by this study, it will be difficult for
airlines or aircrews to take specific preventive action until the cause
is more clearly defined. It is likely that known risk factors, not
available from the original studies, substantially underlie this
association. Physicians should counsel patients to minimize skin cancer
risk by avoiding established unsafe behaviors such as sun exposure that
leads to burns and excessive aggregate UV exposure.