Flight crew are occupationally exposed to several potentially carcinogenic hazards; however, previous investigations have been hampered by lack of information on lifestyle exposures. The authors identified, through the United Kingdom Civil Aviation Authority medical records, a cohort of 16,329 flight crew and 3,165 air traffic control officers (ATCOs) and assembled data on their occupational and lifestyle exposures. Standardised incidence ratios (SIRs) were estimated to compare cancer incidence in each occupation to that of the general population; internal analyses were conducted by fitting Cox regression models. All-cancer incidence was 20–29% lower in each occupation than in the general population, mainly due to a lower incidence of smoking-related cancers [SIR (95% CI) = 0.33 (0.27–0.38) and 0.42 (0.28–0.60) for flight crew and ATCOs, respectively], consistent with their much lower prevalence of smoking. Skin melanoma rates were increased in both flight crew (SIR = 1.87; 95% CI = 1.45–2.38) and ATCOs (2.66; 1.55–4.25), with rates among the former increasing with increasing number of flight hours (p-trend = 0.02). However, internal analyses revealed no differences in skin melanoma rates between flight crew and ATCOs (hazard ratio: 0.78, 95% CI = 0.37–1.66) and identified skin that burns easily when exposed to sunlight (p = 0.001) and sunbathing to get a tan (p = 0.07) as the strongest risk predictors of skin melanoma in both occupations. The similar site-specific cancer risks between the two occupational groups argue against risks among flight crew being driven by occupation-specific exposures. The skin melanoma excess reflects sun-related behaviour rather than cosmic radiation exposure.
There are concerns that professional flight crew may have raised cancer risks because of their exposure to several occupational hazards known, or suspected, to be carcinogenic. In particular, professional flight and cabin crew are classified as occupationally exposed to ionising radiation1, 2 because they sustain an annual ionising radiation dose of 2–6 mSv,3 which is in addition to the background radiation of the general population. This dose is regarded as low and within the limits for occupational exposure to radiation for non-pregnant adults; however, there are concerns about the health effect of high-energy radiation from neutrons.2
Previous studies of flight and cabin crew have reported increased risks of melanoma and non-melanoma skin cancers,4–6 acute myeloid leukaemia5, 7 and cancers of the breast in women8 and prostate in males7; however, these incidence studies tended to be relatively small. A large pooled analysis of mortality data from several European countries confirmed the increased risks for skin melanoma, but did not observe increases for any other cancer sites.9–11 Flight and cabin crew members have a complicated exposure history because their occupation leads to specific lifestyle characteristics, and these may act as possible confounders to the health effects of occupational hazards. Such lifestyle characteristics include recreational sunlight exposure during rest periods in hot places overseas or leisure activities. Exposure of flight crew to solar ultraviolet (UV) radiation entering through the cockpit windows is unlikely to explain their increased skin cancer risk.12
We examined cancer risks in a large United Kingdom population-based cohort of flight crew and attempted to disentangle for the first time the effects of occupational exposures from those of lifestyle characteristics. This study benefits from several unique features. First, to minimise the impact of the healthy worker effect and other health-related selection bias, we included a cohort of air traffic control officers (ATCOs) to act as a comparison group. ATCOs share a similar socioeconomic background and undergo regular medical surveillance as do professional flight crew. Second, we had access to data on occupational exposures as well as lifestyle characteristics, including smoking habits and UV-related exposures, from medical records and postal questionnaires. Third, cancer incidence data were obtained through linkage to the UK national cancer registries. Many previous studies have relied on mortality data, but because of high survival rates for many cancers, mortality studies are much less informative.
ATPL: airline transport pilot; BMI: body mass index; ATCOs: air traffic control officers; CAA: Civil Aviation Authority; CI: confidence interval; HR: hazard ratio; ICD: International Classification of Diseases; MRS: Medical Records System; OR: odds ratio; SIR: standardised incidence ratio; UV: ultraviolet radiation
Material and Methods
The study design and data sources have been described in detail elsewhere.13–15 In summary, the study population was identified from the Medical Records System (MRS) of the UK Civil Aviation Authority (CAA), which holds data gathered from routine surveillance examinations of holders of UK licences, that is, holders of a UK professional flight crew licence or ATCO licence. Flight crew licences comprise licenses for airline transport pilot (ATPL), senior commercial pilot, commercial pilot, basic commercial pilot, flight engineer and flight navigator. CAA medical examinations are undertaken every 6 or 12 months by flight crew and every year or alternate year by ATCOs (both depending on age). Flight crew and ATCOs who held a valid professional licence at any time during the period from January 1, 1989 (when the MRS was computerised) to December 31, 1999 were eligible for entry into the study. During 2000–2001, all eligible individuals were mailed a letter of invitation to complete a postal questionnaire and consent for researchers to access their CAA medical records and follow them up passively through UK health population registers. All eligible UK resident subjects were mailed twice (the second time using registered post) to ensure that they were given the opportunity to refuse participation. The CAA MRS is completed at the time of each medical examination when a standardised form is administered to aircrews and ATCOs. This form collects data on a limited number of demographic, occupational (e.g., type of licences) and lifestyle variables (e.g., smoking habits) as well as data on selected physical characteristics (e.g., current height and weight, hair and eye colour). The postal questionnaire obtained more detailed information on demographic (e.g., country of birth and residence) and lifestyle (e.g., reproductive history and UV-related exposures) variables as well as a full occupational history.
Study subjects were followed up to the end of 2008 through the National Health Service Central Registers (recently re-named as NHS Information Centre) if resident in England, Scotland and Wales or through the Central Services Agency if resident in Northern Ireland. These are virtually complete population registers for their respective countries. Follow-up information included details of cancer registrations, death certifications and migration between and outside the UK countries. Cancer sites and underlying causes of death were coded in accordance with the International Classification of Diseases (ICD), revisions 9 and 10.16, 17 Grouping by main cancer sites, according to the ICD-10 main chapters, was used in the analyses.
Ethical approval was obtained from the Defence Medical Services Clinical Research Ethics Committee, the London School of Hygiene and Tropical Medicine Ethics Committee and the National Health Service South East Multi-Centre Research Ethics Committee. The study was conducted according to Section 23 (Disclosure of Information) of the UK Civil Aviation Act, as advised by the Queen's Counsel.
Follow-up time was defined from date of entry (January 1, 1989 or the date of issue of the first professional licence, if later) to the date of exit (date of cancer registration, date of death, date of emigration, 90th birthday, or December 31, 2008, whichever occurred first). All-cancer (excluding non-melanoma skin cancer because of its acknowledged under-ascertainment and under-registration; see below) and site-specific cancer incidence rates among flight crew and ATCOs were compared to those of the UK general population by calculating standardised incidence ratios (SIRs), that is, the ratio of observed to expected numbers of cancers for a set of individuals with a given length of follow-up, where expectations were calculated according to the age-, sex-, calendar year- and country-specific incidence rates of the reference population using Poisson regression models.18 We used respectively cancer incidence rates of England and Wales for the English and Welsh participants, of Scotland for the Scottish participants and of the United Kingdom for the Northern Irish ones (no reliable estimates were available for Northern Ireland).
Initial comparisons of cancer incidence between the two occupational groups were based on the ratio of their SIRs. SIRs were also stratified by occupational and lifestyle exposures of the cohort members using Poisson regression,18 separately by occupational group, to identify potential explanatory factors for the observed reduced (or raised) SIRs. Internal analyses were also conducted by fitting Cox regression models (with the time scale defined by age) to estimate hazard ratios (HR) as a measure of the association of occupational and lifestyle variables with all sites, site specific and cancer rates, while adjusting for sex, calendar period and (implicitly) age.19 Heterogeneity and linear trends were assessed using the Wald test.19
Analyses of cancer risks among flight crew and ATCOs relative to the UK general population excluded non-melanoma skin cancers because of concerns of differential under-diagnosis and under-registration as flight crew and ATCOs are from a higher socioeconomic group and undergo more intensive medical surveillance than the general population. However, to exploit all available information, both melanoma and non-melanoma skin cancers were included in internal analyses as the degree of under-diagnosis/registration for the latter is likely to have been comparable for the two occupational groups.
When appropriate, further analyses were conducted among flight crew by type of their licence (categorised as ATPL/not-ATPL, as ATPL holders tend to fly large planes over World routes); cumulative flying hours at baseline and cumulatively up to year 2000-1 (both categorised using the tertiles of their respective distributions); route flown (World/Europe/UK) and type of aircraft (large/medium/single pilot). Similarly, further analyses were conducted among ATCOs by cumulative number of hours of radar duties and cumulative number of night shifts (both categorised using the tertiles of their respective distributions). All P-values are two sided.
Characteristics of the study subjects
A total of 27,392 flight crew and ATCOs satisfied the criteria for entry into the study. Of these, 7,903 were excluded for reasons given in Figure 1. Thus, the analyses were based on 19,494 participants, 50% of whom completed a postal questionnaire.
Flight crew were slightly older at entry than ATCOs [median age (in years) of males: 37 for flight crew and 35 for ATCOs; of females: 29 and 25]. They were also less likely to be current smokers than ATCOs at entry, but not at the time of the questionnaire (∼10 years later) when the prevalence in both groups was much lower (Table 1). Among those who completed the questionnaire, and within each gender, a slightly higher percentage of flight crew than ATCOs reported drinking alcohol regularly with about 90% in each group reporting exercising regularly at least once a week (Table 1). On average, female flight crew had a lower BMI at entry than female ATCOs [median (inter-quartile range): 22.0 (20.6–23.7) and 23.0 (21.5–25.1) kg/m2], whereas males had similar values [24.4 (22.7–26.3) and 24.2 (22.3–26.3) kg/m2]. The skin type distribution and the proportion of those reporting having ever been sunburnt were similar between the two occupational groups and sexes; however, females consistently reported a higher use of sunscreens and sunbeds (Table 1).
Table 1. Selected lifestyle, host and occupational characteristics of flight crew and ATCOs
At entry, 69% flight crew had an ATPL, about one-third had accumulated more than 5,500 flight hours (Table 1), and 23% had flown world routes (data not shown). As expected, the cumulative number of hours flown were substantially higher in 2000–2001 (Table 1), when the questionnaire was administered, than when the study commenced. Among ATCOs who completed the questionnaire, the large majority were radar qualified and worked night shifts (Table 1). There was no difference in the distribution of baseline variables between participants who completed the questionnaire and those who did not (Supporting Information Table S1).
Cancer incidence in each occupational group relative to the general population
A total of 773 incident neoplasms (excluding non-melanoma skin cancers) occurred among flight crew and 151 among ATCOs during 285,259 and 54,045 person-years at risk (median follow-up time: 19 years for each group). Flight crew and ATCOs had a 29% (SIR = 0.71, 95% CI = 0.66–0.76) and 20% (SIR = 0.80, 95% CI = 0.68–0.94], respectively, lower all-cancer incidence than the UK general population. Rates were lower among flight crew and ATCOs than among the general population for most cancer sites, the only exceptions being the similar rates for cancers of the prostate and female breast and the much higher rates for malignant melanoma of the skin, in each occupational group (Fig. 2; Supporting Information Table S2).
The low all-neoplasm incidence in each occupational group was largely accounted for by a very low incidence of smoking-related cancers, with flight crew having only 33% and ATCOs only 42%, of the rates found in the general population (Table 2). There was a clear trend in the relative incidence of smoking-related cancers with smoking status categories, with current smokers at entry experiencing similar rates to the general population (especially if ATCOs). In contrast, there was no clear variation in the incidence of smoking-related or non-smoking-related cancers across categories of BMI, height, regular alcohol consumption and regular physical exercise (Table 2), and no obvious trend with amount of alcohol intake [p for linear trend (p-trend) = 0.83 in flight crew and p-trend = 0.26 in ATCOs] or amount of regular exercise (p-trend = 0.76 and p-trend = 0.95). (Data not shown for these two analyses).
Table 2. Incidence of smoking related and non-smoking-related cancers among flight crew and ATCOs relative to the UK general population by lifestyle characteristics
Site-specific cancer incidence in relation to occupational exposures
There were no clear differences in rates of any cancer site between the two occupational groups, before (Fig. 2) or after further adjustment for smoking habits and BMI at entry (Supporting Information Table S2). The incidence of skin melanoma among flight crew increased with increasing cumulative number of hours flown, as recorded at entry into the study (p-trend = 0.02) or at the time of questionnaire administration (p-trend = 0.07; Table 3); however, there were no clear associations in the risk of this cancer with licence type (Table 3), route flown or type of aircraft (data not shown).
Table 3. Incidence of all-cancers, and of selected cancer sites, among flight crew, by their occupational exposures
The incidence of digestive cancers was higher among flight crew who held licences other than ATPL (p-trend = 0.02); however, there was no association with cumulative number of hours flown (Table 3). The incidence of male genital cancers was higher among flight crew who held ATPL licences (p-trend = 0.07), with rates increasing with cumulative number of flight hours at entry (p-trend = 0.04) (Table 3). Similar associations with licence type were present when the analyses were restricted to colorectal cancer (p = 0.05), which accounted for 63% of all digestive cancers, and prostatic cancer (p = 0.09), which accounted for 90% of all male genital cancers, with a positive trend in rates with flight hours at entry also present for the latter (p-trend = 0.06). There were no associations between cumulative number of hours of radar duties, or cumulative number of night shifts, and site-specific cancer rates among ATCOs (Supporting Information Table S3).
Skin cancer risks in relation to host characteristics and occupational and sun-related exposures
Internal comparisons by occupational group focussed on skin cancer rates because of the raised incidence of this cancer relative to the general population. Both melanoma and non-melanoma skin cancers were included in these analyses to exploit all available information (Table 4).
Table 4. Minimally and mutually adjusted hazard ratios for skin melanomas and non-melanomas among flight crew and ATCOs
Skin melanoma and non-melanoma rates were similar between flight crew and ATCOs [HR of flight crew versus ATCOs adjusted for age, sex and calendar period: 0.72 (95% CI = 0.42–1.24) and 0.59 (95% CI = 0.27–1.29)]. Host characteristics and exposure to recreational UV-related radiation were therefore examined controlling for occupational group. Occupational exposure to cosmic radiation was examined in terms of cumulative flying hours, with analyses conducted initially on all participants and subsequently on the subset with complete data to allow assessment of whether results from the latter were affected by selection bias.
Minimally adjusted analyses (i.e., adjusted for age, sex, calendar period and occupational group) showed statistically significant raised melanoma rates for participants who reported their skin burning easily when exposed to hot sunlight without using sunscreens (p = 0.001) and among those who reported having blonde/fair/ginger hair (p = 0.03), as well as non-significant raised rates among those who reported using sunscreens regularly and having ever been sunburnt or having ever sunbathed to get a tan (Table 4), but no trend in risk with days/year spent sunbathing (p-trend = 0.88). Similarly, non-melanoma rates were significantly raised among participants who reported having blonde/fair/ginger hair and non-significantly raised among those who reported their skin burning easily when exposed to hot sunlight, using sunscreens regularly, and having ever sunbathed to get a tan (Table 4), but no trend in risk with days/year spent sunbathing (p-trend = 0.81). The effect of these host and recreational sun exposures on skin cancer did not differ between flight crew and ATCOs, except for borderline evidence that an association with sunscreen use was stronger among flight crew (p for effect modification = 0.05). There was an indication of a linear trend in the rates of both skin melanomas and non-melanomas with increasing flight hours (p-trend = 0.07 and p-trend = 0.14; Table 4) in line with the observed trend in SIRs for melanomas (Table 3). When analyses were repeated distinguishing between melanomas in normally covered (i.e., trunk and lower limbs) from those in uncovered areas of the body (i.e., head and neck and upper limbs), we found that the trend was stronger—albeit not significant—in the latter (Table 4).
Mutual adjustment for occupational and lifestyle exposures—restricted to subjects with complete data—revealed that skin that burns easily when exposed to sunlight (p = 0.001) and sunbathing to get a tan (p = 0.07) were the strongest predictors of melanoma risk, whereas blonde/fair/ginger hair (p = 0.01) was the strongest predictor of non-melanoma risk (Table 4). Notably, there was no evidence of differential melanoma and non-melanoma rates between ATCOs and flight crew after adjustment for these host and recreational exposures (flight crew versus ATCOs, melanomas: HR = 0.78, 95% CI = 0.37–1.66; non-melanomas: HR = 0.66, 95% CI = 0.25, 1.77). Furthermore, there was no evidence that cumulative flying hours had a significant effect when adjusted for environmental exposures (p-trend = 0.39 for melanomas; p-trend = 0.33 for non-melanomas; Table 4).
To maximise power, the analyses reported above were carried out treating the questionnaire data as valid retrospectively. However, prospective analyses from the date of the questionnaire administration (based on 21 melanomas and 22 non-melanomas diagnosed subsequently) showed a similar pattern [e.g., mutually adjusted analyses identified skin that burned easily (p = 0.02) and blonde/fair/ginger skin (p = 0.02) as the main predictors of risk for melanomas and non-melanomas, respectively].
By necessity of their profession, flight crew are healthier individuals than the general population. They also undergo strict medical surveillance, leading to even greater health advantages. Indeed, we found that UK professional flight crew had a markedly lower cancer incidence than the general population. However, such comparison may hide increases in specific site-specific cancer risks experienced by this occupational group. To be able to identify such increases, if present, we took UK ATCOs, which have a similar socioeconomic background and type of medical surveillance as flight crew,13 as a comparison group. We found that flight crew had similar cancer risks to ATCOs, arguing against their risks being driven mainly by occupation-specific exposures.
The reduced all-cancer incidence in both occupational groups relative to the general population was mainly due to a markedly low incidence rate of smoking-related cancers, in line with their much lower prevalence of smoking.13 We have previously shown that the prevalence of current smoking at the time of questionnaire administration was much lower in the two occupational groups than in the general population (e.g., 7% vs. 27%, respectively, for males),13 with “ever smokers” having started to smoke later than the general population.13 The slightly lower incidence of smoking-related cancers among flight crew than ATCOs is also consistent with the observation that flight crew were less likely to be current smokers at entry (see Results section).
Consistently with previous reports,4, 5, 9, 11 our study found that professional flight crew had a marked excess of skin melanoma relative to the general population. The reasons for this excess had not been properly investigated in the past. The increases in risk relative to the general population with increasing cumulative number of flight hours observed in this study would be consistent with a putative effect of exposure to cosmic radiation on the flight deck. However, no significant trend in the risk of skin melanoma or non-melanoma with flight hours was observed in internal analyses for which differential distributions in host characteristics and recreational sun exposures were adjusted for. In fact, there was no indication that rates were higher among flight crew than ATCOs, even in the highest flight hours category [≥5,500 cumulative flying hours versus ATCO, HR 0.91 (95% CI = 0.40–2.10) for melanomas and 0.69 (95% CI = 0.24–1.94) for non-melanomas]. Circadian disruptions, leading to melatonin disturbances, have also been proposed as putative risk factors for the excess skin cancer among flight crew.4 We did not have direct information on night shifts for flight crew; however, interestingly, night shift work was not associated with skin cancer in ATCOs. Furthermore, a recent study found a protective effect of night shift work on skin cancer.20 Alternatively, it has been postulated that the high incidence of skin melanoma among flight crew may be due to recreational sun exposure, including exposure during stopovers in sunny places. The findings from this study are consistent with this interpretation. The fact that the trend in risk with flight hours appeared to be stronger for cancers in areas of the body that are usually covered (i.e., the trunk and lower limbs) suggests that number of flight hours is possibly a correlate of intermittent and intense sunbathing during stopovers in sunny places. The high skin melanoma rates in ATCOs are also likely to be related to their easy access to holidays in sunny places, as ATCOs in the United Kingdom have historically had access for discounted air travel.
We found a positive association between risk of skin melanoma and having ever sunbathed to get a tan, albeit of borderline statistical significance, but no association of risk with days spent sunbathing. However, accurate recall of past sun exposures is notoriously difficult. Furthermore, UV exposure is likely to be affected by an individual's host characteristics as those who tend to burn rather than tan are more likely to avoid high exposure to sunlight and use sunscreens regularly. Thus, unfavourable host characteristics may have acted as negative confounders on the association of UV-related exposures with skin cancer.
In contrast to previous reports,5, 7 we found no evidence that flight crew had an increased risk of leukaemias or lymphomas; however, the number of cases was too small to be conclusive. Furthermore, chronic lymphocytic leukaemia is not related to ionising radiation, and they represented a larger proportion of the all-leukaemia cases (7 of 13 among flight crew and 1 of 3 among ATCOs). Some previous studies reported excesses in prostatic cancer among flight crews.7 The overall incidence of prostatic cancer among flight crew in the our study was similar to those in the general population and among ATCOs; however, there was a trend in the risk of this cancer with increasing number of hours flown. We also observed an association between type of licence and cancer of digestive organs, particularly colorectal cancer. The aetiological significance of these findings is, however, unclear.
This study is the largest population-based incident study of flight crew conducted so far. Furthermore, access to a relevant occupational comparison group allows addressing the potential biases inherent in investigations of extremely healthy occupational groups. Other strengths include the availability of data on both occupational and lifestyle exposures, which allowed the examination of their independent effects on cancer risks. However, there were some limitations, namely, the lack of cosmic radiation dose estimates for flight crew, the limited power to detect small effects, the small number of female participants (which precluded examination of risks for female-specific cancers while adjusting for differences in reproductive history) and the possibility that some statistically significant results may have arisen by chance given the large number of comparisons performed.
In short, by using ATCOs as a comparison group, we were able to rule out any major cancer risks associated with occupational exposures specifically associated with being flight crew. The excess of skin melanoma relative to the general population, which was seen for both flight crew and ATCOs, is likely to be due to recreational sunlight exposure in hot places overseas rather than exposure to cosmic radiation. Organisations with an interest in the health of flight crew and ATCOs should emphasise the importance of minimising exposure to potentially harmful sunlight and the importance of early diagnosis of malignant melanoma and non-melanoma skin cancers.
This work was supported by the Medical Department of the UK Civil Aviation Authority (CAA) (to I.S.S. and B.D.S.). The authors thank Mrs. Roberta North (CAA), Mrs. Emma Forrest (CAA) and Ms. Jocelyn Hawkins (LSHTM) for their clerical assistance, and Mr. David Mayer (LSHTM), the late Mr. John Adams (CAA) and Mr. Chris Barrow (Steria CAA) for their computing and data management support.