Air travel has become such a convenient and accessible mode of transport that an estimated one billion of us take to the skies every year (Chest, 2006; 130: 575-7). Yet, there's growing concern over the effects of air travel on our health, particularly for 'frequent flyers' and those who travel over long distances-a group that now make up a substantial chunk of the travelling public.
Besides jet lag and deep vein thrombosis (DVT), a number of less well-known health risks are linked to air travel, including cancer.
The cancer connection
Several studies have found a link between air travel and cancer. These studies, however, involved people who fly for a living. Alarmingly, the findings consistently show increased risks of breast cancer among female flight attendants, and skin cancer among both pilots and flight staff (Cancer Invest, 2004; 22: 743-61).
In one study from Japan, female flight attendants had a 40-per-cent increase in breast cancer compared with the general population (J Travel Med, 2006; 13: 127-32). Other studies from Finland, Iceland, Norway, Denmark and the US also found higher breast-cancer rates in female cabin crew, up to five times higher in those with five years or more on the job (Occup Environ Med, 2003; 60: 807-9).
As for skin cancer (malignant melanoma), incidence rates for cabin crew are two to three times the expected rate (Occup Environ Med, 2003; 60: 810-4; J Travel Med, 2006; 13: 127-32), while commercial airline pilots may be 10 times more likely to develop melanomas compared with the gen-eral population. Some pilots, such as those on international routes, appear to have an even greater risk (Occup Environ Med, 2000; 57: 175-9).
Researchers have also uncovered an excess of other cancers among pilots, although the studies are less consistent. One found a significantly higher risk of acute myeloid leukae-mia (AML), while others showed higher rates of cancers of the colon, brain, prostate and rectum, and Hodgkin's disease (Lancet, 1999; 354: 2029-31; Occup Environ Med, 2000; 57: 175-9).
So what's the reason behind this occupational hazard? According to some, it is not the job per se that puts pilots and cabin staff at risk, but the lifestyle factors that are asso-ciated with it. Increased rates of breast cancer might be down to reproductive factors (stewardesses often have no children, or have them at a later age than average, both of which are risk factors for breast cancer) and higher skin-cancer rates could be due to more sun exposure (air crew may be spending more time in sunshine destinations than others) (Occup Environ Med, 2002; 59: 428-33).
However, several studies have taken these factors into account and concluded that they are unlikely to explain all of the increased risk of cancer among flight crew (Occup Environ Med, 2003; 60: 815-20; 807-9).
A more sinister explanation has to do with what's in the sky. Air crews are subjected to 'cosmic radiation', a form of ionizing radiation that comes from the sun and other stars, every time they fly. Given the high altitudes that are the domain of commercial airliners, their exposure rates are hundreds of times greater than for those of us at ground level (J Am Board Fam Pract, 1999; 12: 195-9).
Indeed, a UK National Radiological Protection Board survey carried out in 1999 showed that nuclear workers received a lower annual average dose of radiation than did air crews (Occup Environ Med, 2002; 59: 428-33).
Cosmic radiation is an important concern because the combination of particles and electromagnetic waves are energetic enough to disrupt the cellular structure of the body and potentially have an impact on the exposed person. In fact, a recent study found an increased rate of chromosomal translocations-an event commonly seen in cancer and indicative of cumulative ionizing-radiation exposure-in airline pilots with long-term flying experience. For each one-year increase in flight years, the likelihood of such a translocation rose by 6 per cent (Occup Environ Med, 2009; 66: 56-62).
The same researchers who found the link between pilots and AML (see above) also discovered evidence that cosmic radiation may well be the culprit. They found chromosomal damage in 57 per cent of flight staff with AML or myelodysplasia (a 'pre-leukaemia' bone-marrow disorder) compared with only 11 per cent in non-flight staff. The researchers concluded that the chromosomal abnormalities seen in myelodysplasia and AML could indicate "previous exposure to ionizing radiation" (Lancet, 2000; 356: 2158).
A number of other studies support the idea that cosmic radiation is responsible for the increased risks of cancer among flight crew. A Norwegian study found a statistically significant exposure-response rela-tionship between the cumulative radiation dose and malignant melanoma (Scand J Work Environ Health, 2000; 26: 106-11). What's more, the International Agency for Research on Cancer (IARC) has concluded that there is sufficient evidence that neutrons-which constitute 30 to60 per cent of cosmic radiation-are carcinogenic to humans (Occup Environ Med, 2003; 60: 815-20).
Nevertheless, the research so far is not conclusive. As some scientists point out, the irregular working hours of flight crew and frequent disturbance of the circadian rhythm (jet lag) may also play a role in increasing cancer risk (Occup Environ Med, 2003; 60: 805-6).
Other effects of flying
Besides cancer, cosmic radiation has been associated with eye damage in airline pilots. A recent University of Iceland study found that commercial pilots were three times more likely than usual to develop cataracts. They also noted that the increased risk could not be explained by other factors such as ultraviolet exposure and smoking, known to increase the risk of cataracts, and concluded that "cosmic radiation may be a causative factor in nuclear cataracts among commercial airline pilots" (Arch Ophthalmol, 2005; 123: 1102-5). Such an increased risk of cataracts has also been reported by the US National Cancer Institute in radiological staff exposed to ionizing radiation (Am J Epidemiol, 2008; 168: 620-31)
Far more disturbing, however, is the evidence that cosmic radiation is dangerous to the fetuses of pregnant flight staff. According to the US Air Force Inspection Agency, "Animal studies of low-dose ionizing radiation exposures, and human studies follow-ing high-dose exposures, suggest that pregnant aviators may inadver-tently subject their fetuses to a risk of decreased cognitive capacity or frank mental retardation, as well as childhood leukaemia" (Aviat Space Environ Med, 1998; 69: 1061-4).
For this reason, flight staff are advised to limit air travel during pregnancy, but may still unknowingly exceed the recommended fetal dose limit of 1 milliSievert (mSv) over the entire pregnancy (www.who.int/ionizing_radiation/env/cosmic/WHO_Info_Sheet_ Cosmic_Radiation.pdf). A recent study found that the dose to the fetus can exceed 1 mSv after 10 round trips on commercial flights between Toronto, Canada, and Frankfurt, Germany (Health Phys, 2008; 95: 407-12).
The risk to passengers
Clearly, cosmic radiation is a concern for those who fly for a living, but what about the general public?
According to radiological physicist Dr Robert Barish, passengers are exposed to cosmic radiation in the same way as the crew members with whom they travel. While this is not a problem for the occasional traveller, radiation exposure may be some-thing that the frequent flyer should consider, particularly if pregnant.
"If this radiation originated at a regulated industrial or medical facility," says Barish, "many frequent flyers would receive annual exposures in excess of the present legal limit applicable to members of the public" (J Am Board Fam Pract, 1999; 12: 195-9).
However, according to Barish's calculations, the risk of actual harm from cosmic radiation is small. He estimates that the maximum permissible annual radiation dose of 1 mSv-achieved by flying around 75,000 miles-has a four in 100,000 chance of leading to a fatal cancer and a three in 10,000 chance of causing a serious health problem in the unborn child. However, these estimates do not take into account solar flares.
As Barish explains, "Solar flares are caused by instabilities in the nuclear processes, which drive the fusion reactions that produce the sun's energy. Although as many as 10,000 solar flares occur each year, a relatively small number (usually fewer than 20) emit enough extra radiation to affect the airliner environment. When they do, however, these charged-particle events can produce exposure rates that are 10 to 20 times their usual values. In other words, if the radiation-producing flare lasts as long as the flight, a single one-way trip could give as much radiation exposure to a passenger or crew member as they would ordinarily receive in 10 to 20 trips" (J Am Board Fam Pract, 1999; 12: 195-9).
Given these complexities coupled with the lack of data on passenger dosages, it's currently impossible to tell whether cosmic radiation poses a genuine threat to the air-travelling public. Yet, considering that more and more people are flying at higher altitudes-many long-haul flights travel over the poles, where cosmic radiation is more pronounced-the risk is certainly one that deserves our attention.
Other flight risks
- Jet lag. A disturbed body clock is known to cause problems with sleep, and mental and physical performance. Animal studies are also showing that chronic jet lag can lead to more serious consequences such as heart and kidney disease, although the results may not apply to humans (Am J Physiol Regul Integr Comp Physiol, 2008; 294: R1675-83).
- DVT. The risk of deep vein thrombosis-or 'economy-class syndrome'-increases by two- to fourfold after air travel (PLoS Med, 2007; 4: e290).
- Hypoxia (oxygen deprivation). Oxygen levels on airplanes are usually 20- to 25-per-cent lower than on the ground, which can cause problems for those with respiratory, cardiac or postsurgical conditions. For this reason, these people should always see a doctor before flying (Am Fam Physician, 1999; 60: 801-8, 810).
The World Health Organization (WHO) advises frequent flyers who are concerned about cosmic radiation to:
- keep informed about the health effects of cosmic radiation;
- limit air travel during pregnancy.
For frequent flyers whose air time is similar to that of air crew, they are advised to:
- record their personal cumulative radiation doses on a regular and permanent basis;
- consider radiation exposures when selecting flight schedules.
An estimate of the radiation dose for a specific flight can be obtained from:
- www.helmholtz-muenchen.de/epcard/eng_fluginput. php;
- www.who.int/ionizing_radiation/env/cosmic/en/ index1.html