Health Among Airline Pilots

Air Line Pilot, March 2001, p. 16
By Gary C. Butler, Ph.D., and Joyce S. Nicholas, Ph.D.

Airline pilots work in conditions that lead to circadian dysrhythmia, mild hypoxia, and exposure to reduced atmospheric pressure, low humidity, noise, vibration, cosmic radiation, and magnetic fields. These occupational exposures may present physiological challenges to the long-term health of airline pilots. Previous health studies among pilot groups have produced inconsistent results. The purpose of this study was to investigate self-reported disease rates among a group of more than 10,000 active and retired ALPA pilots in the United States and Canada.

We mailed a health and lifestyle survey in 1998 to 10,678 (9,276 active, 1,402 retired) ALPA pilots who were flying for two airlines selected to be included in the study. Surveys were prepared at the Medical University of South Carolina (MUSC) and mailed in bulk to the ALPA Aeromedical Office in Denver, which coded the surveys for possible follow-up and mailed them to all members at the two airlines.

Individual pilots returned their surveys to ALPA, which then mailed the surveys back in bulk to MUSC, where investigators compiled and analyzed the data but did not have access to the coding system, thus protecting pilot anonymity.

The survey was designed to collect the following data: date of birth, gender, race, cancer and non-cancer disease endpoints with year of first diagnosis, start and stop years for career as an airline pilot, general health concerns, and lifestyle factors including smoking, alcohol consumption, and night flying. The survey was constructed so that responses to disease history could be selected from a list or from yes/no questions, with the opportunity to write in additional comments. The list included common disease endpoints as well as those suggested for further study in previously published studies of airline pilots. We tested the survey in a sample group of pilots before mailing it to the entire group.

The main outcome measure for cancer endpoints among pilots was a self-reported incidence rate, age-adjusted to the 1970 U.S. population, and controlled for gender. Self-reported pilot incidence is given as the average annual number of new cases reported per 100,000 person-years of observation. A person-year is a unit of follow-up time defined for a pilot as beginning with the pilot’s first flight as an airline pilot and ending in 1998 if the pilot remains disease-free, or ending in the year of first diagnosis if the pilot develops the disease under study. Pilots were excluded from the analysis when questionnaire responses did not provide enough data for calculating person-years. Some pilots failed to supply the year in which they began airline flying, but did provide their year of birth and total number of years worked or their age when they began working. To avoid excluding these pilots from the study, the year in which they began airline flying was estimated by assuming that the minimum age at which an individual may begin flying is 18 years and the maximum age at retirement is 60 years.

To compare cancer rates among airline pilots to cancer rates among the general U.S. population, an estimated standardized incidence ratio (SIR) was constructed by comparing pilot rates to SEER rates. SEER (surveillance, epidemiology, and end results) is a National Cancer Institute program that collects data on cancer incidence and survival in the U.S. population. The SIR constructed in this way should be considered an estimate because the pilot data are self-reported and the SEER data are record-based. Also, to be included in the survey, the pilots had to be free of detectable disease at the time of beginning their careers, and they had to have survived until 1998. These constraints are not placed on the SEER population.

The main outcome measure for non-cancer endpoints among pilots was self-reported prevalence, which was calculated as the number of existing cases of a disease in specified age groups at the time of the survey (1998) divided by the total number of airline pilots in corresponding age groups responding to the survey, age-adjusted to the 1970 U.S. population. Any pilot who did not respond to the study question and/or did not give date of birth was excluded. Age-specific prevalence for male pilots was compared to age-specific prevalence for males in the 1995 U.S. population as established by self-reported data from the 1995 National Health Interview Survey.

The pilots’ ability to accurately report health histories is supported by their generally high educational levels and attention to health.

A published comparison of self-reported cancer data from the 1992 National Health Interview Survey (NHIS) to record-based cancer registry data suggests that self-reported cancer was underreported in the NHIS.

Other sources of underestimation among the pilot group include the exclusion of pilots for whom we did not have enough information to calculate person-years, the contrast of a very healthy worker group to the general population, and our inability to follow up cases in which the pilot died. Inability to follow up is particularly important in the case of motor neuron disease for which the mean survival time is 3 to 5 years. These sources of underestimation may be offset somewhat by the regular medical examinations that pilots receive. Additionally, regular medical exams may lead to early diagnosis, affecting age-specific rates.

Of the 10,678 surveys mailed, 6,609 were returned (6,533 men, 63 women). Because of the small number of women, results on women are presented separately (see "Results on Women"). Of the men, 98 percent were white, with less than 1 percent in each of the categories black, Asian, or other. The average age of the men was 54.9 years (see Table I).

The health survey included questions on smoking, alcohol consumption, and night flying. Only 4 percent of male pilots reported current smoking, while 87 percent reported that they had consumed a drink of beer, wine, or liquor in the past 30 days. Regarding night flying, 60 percent reported night flying, 8 percent reported no night flying, and 32 percent did not answer the question. Of the 60 percent who reported night flying, 82 percent reported that they stayed awake flying internationally or domestic at most 5 nights per month, 16 percent between 6 and 10 nights per month, and only 2 percent more than 10 nights per month. No statistically significant trends were found between disease rates and these lifestyle factors.

Thirteen cancer types with four or more included cases between 1970 and 1998 were compared to SEER rates for white males over the same time period (see Table II).

Of the 1,051 observed cases, 1,043 were among white males. The remaining 8 observed cases reported the following: 2 Hispanic, 1 white/Asian, 1 Canadian, 4 unspecified. The outcome measure was an estimated standardized incidence ratio (SIR) with a 95 percent confidence interval. Estimated ratios were adjusted for age and controlled for race and gender. Pilots with insufficient information for calculating person-years were excluded. The percentage of excluded cases ranged from 0 percent for kidney cancer to almost 43 percent for lung cancer.

The estimated incidence of melanoma was found to be significantly increased among airline pilots—the estimated SIR for melanoma was 3.47. Because the incidence of melanoma has increased in the general population since 1970, estimated SIRs for airline pilots were calculated separately for the time periods 1970–79, 1980–89, and 1990–98 using SEER data for comparable time periods. For 1970–79, the estimated SIR is 1.77; for 1980–89, 2.15; for 1990–98, 2.60. These estimated rates suggest that the incidence of melanoma is increasing faster among pilots than among the general population.

The suggested increase in melanoma incidence found in this study is supported by studies of pilots in other countries. Significantly increased melanoma incidence has been reported among Danish cockpit crews (SIR 2.4), pilots in England, Wales, and Sweden (SIR 2.7), pilots in Iceland (SIR 10.2), and pilots in Norway (SIR 1.8). Additionally, increased mortality rates for melanoma have been reported among British Airways pilots (Standardized Mortality Rate 3.3).

The reasons for these reported increases are unclear. The Norwegian study found a significant increasing trend for the SIR value for malignant melanoma with ionizing radiation dose. The Danish study found an increased risk of malignant melanoma among pilots flying more than 5,000 hours, regardless of aircraft type (jet, non-jet). The Icelandic study, in analyses according to number of block-hours and radiation dose, showed that malignant melanomas were found in the subgroups with highest exposure estimates. Additionally, the SIR for malignant melanoma was increased among those who had been flying through more than five time zones, suggesting that disturbance of circadian rhythm might be involved. Leisure-time sun exposure is suggested as a possible factor in all three studies.

A review of medical literature on melanoma indicates that, while nonmelanoma skin cancer is associated with cumulative exposure to sunlight (principally the ultraviolet B spectrum), the relationship of sun exposure to melanoma is less apparent. In general, risk does not correlate with cumulative sun exposure but may relate to childhood sun exposure (especially a blistering sunburn). Incidence is inversely correlated with the latitude of residence. The individuals most susceptible to developing melanoma are those with fair complexions, red or blond hair, blue eyes, freckles, and skin that sunburns easily. Other factors associated with increased risk include a family history of melanoma, the presence of clinically atypical moles, and immunosuppression. Chronic effects of sun exposure include immunologic effects. Exposure to solar radiation influences local and systemic immune responses. The ultraviolet B spectrum (UV-B) seems to be the most efficient in altering immune responses, likely related to the capacity of UV-B energy to affect antigen presentation in skin by interacting with epidermal Langerhans cells. Implications of this suppression in terms of altered susceptibility to cutaneous cancer or to infection remain to be defined.

The age-adjusted self-reported incidence of nonmelanoma skin cancer (all types) among male pilots is about 592 per 100,000. Because SEER does not collect data on basal cell and squamous cell carcinomas, we could not compare pilot and SEER incidence rates for nonmelanoma skin cancer.

The estimated incidence of most of the other cancer types was found to be significantly decreased among airline pilots. The 1,066 cancer cases reported among male pilots included prostate (76), colon (20), lymphoma (13), bladder (12), leukemia (9), testes (8), kidney (7), thyroid (7), lung (7), vocal chords (6), central nervous system (5), throat (3), sarcoma (3), squamous cell (3), rectum (2), mouth (2), and one reported case in each of the following categories: breast, vallecula, esophagus, urethra, eyelid, pancreas, armpit, nose, myeloma, cheek, and stomach.

Besides increased rates for melanoma, data from self-reports suggested increased rates of motor neuron disease and cataracts, but rates for other diseases were in general lower for pilots than those for the U.S. population.

Non-cancer disease cases that male pilots reported are motor neuron disease (21), cataracts (261), diabetes (78), heart disease (260), high blood pressure (713), high cholesterol (1,725), liver disease (45), and meningitis (31). We compared the age-specific prevalence per 1,000 male pilots against the age-specific prevalence per 1,000 males in the 1995 U.S. population for cataracts, diabetes, heart disease, and high blood pressure (see Table III, which excludes 4 pilots who did not provide information necessary to compute age).

Only about 61 percent of the cases of high blood pressure and 30 percent of the cases of high cholesterol were being treated. Data from the National Center for Health Statistics indicate that 17.5 percent (age-adjusted) of males aged 20–74 years, 1988–1994, had high serum cholesterol compared to 21.7 percent (age-adjusted) of the male pilots aged 20–74 years in this survey.

Results on motor neuron disease warrant particular attention. While the number of cases that pilots reported was considerably larger than expected, written pilot comments suggested that misreporting may have occurred.

To address potential pilot misreporting, we used the date of first diagnosis of motor neuron disease and written pilot comments to calculate an incidence rate adjusted for possible error. Of the 21 cases reported among male pilots, 17 were excluded from the incidence calculation for the following reasons: 7 pilots failed to give enough information for us to calculate person-years (year of birth or year of disease onset was missing), 3 more made written comments indicating that they had some other disease (multiple sclerosis, myasthenia gravis, symptoms subsequent to stroke), and 7 more indicated that they continued to fly following diagnosis (inconsistent with the course of the disease).

The age-adjusted annual incidence rate of motor neuron disease among male pilots calculated using the remaining cases was 5.0 per 100,000 male person-years. The mean age at onset was 58.75 years. Potentially remediable causes of motor neuron dysfunction are structural lesions, infections, physical agents (metals including lead and aluminum, drugs including strychnine and the anticonvulsant phenytoin, electric shock, and x-irradiation), immunologic mechanisms, paraneoplastic, metabolic, and hereditary biochemical disorders.

The most common form of progressive motor neuron disease is amyotrophic lateral sclerosis (ALS), often called Lou Gehrig’s disease, a progressive neuromuscular condition characterized by weakness, muscle wasting, fasciculations, and increased reflexes. The median survival time is 3 to 5 years. ALS has an average estimated annual incidence of 1.4–2.0 per 100,000 population (males and females) and a mean age of onset ranging from 55 to 65 for the general population.

Using ALS for comparison, the adjusted incidence of motor neuron disease among pilots is still higher than expected. A proportional mortality ratio (PMR) study among U.S. commercial pilots and navigators, 1984–1991, showing an approximately two-fold increase in death due to motor neuron disease (PMR 2.35) supports this increase in self-reported motor neuron disease in this study.

While ALS is predominantly a sporadic disorder, about 5 to 10 percent of cases are inherited as an autosomal dominant trait. The cause of sporadic ALS is not clearly defined; however, it has been suggested that excitotoxic neurotransmitters such as glutamate may participate in the death of motor neurons in ALS.

Magnetic field exposure may be associated with ALS. The National Institutes of Health have recently recommended further research on the potential association of neurodegenerative disease (specifically ALS and Alzheimer’s disease) and power frequency magnetic fields. A study of flight deck magnetic fields in airliners indicates that magnetic fields measured on the flight deck during normal aircraft operation are above those normally encountered at home or in the typical office environment (see "Magnetic Fields on the Flight Deck," January).

The current study indicated increased cataract formation among male pilots in each of the age groups 45–64, 65–74, and over 75 years (see Table III). Most cataracts develop slowly as a result of aging, but they may occur more rapidly in patients with a history of ocular trauma, uveitis, or diabetes mellitus. A variety of genetic diseases may cause cataracts, and radiation therapy and glucocorticoid steroid treatment can induce cataracts as a side effect. Radiation cataracts have been observed among A-bomb survivors and survivors of the Chernobyl accident. UV-B radiation produces cataracts in animals and has been associated with human cataract formation.

Given the limitations of survey methodology, our study suggested increased disease rates among pilots for melanoma, motor neuron disease, and cataracts. However, rates for other diseases were found to be generally lower than those for the U.S. population.

Flight operations can provoke sleep loss, fatigue, and circadian disruption, and these factors can result in performance decrement and alertness during airline operations.

Chronic effects of sleep loss including circadian disruption may also include immunologic and endocrine effects. That is, the wide range of body functions controlled by the internal biological clock includes body temperature, digestion, physical and mental performance, and endocrine and immune functions.

Understanding the physiological significance of chronic fatigue and potential immunologic changes will require additional research. Indeed, the critical question is whether chronic fatigue and/or chronic sleep loss compromises the health of the individual.

Given the complexity of factors that may precipitate melanoma in airline pilots, we need to undertake comprehensive research to examine potential risk factors in this group. The next phase of the ALPA/MUSC research will be to investigate the incidence of melanoma and other disease outcomes in a group of airline pilots as related to the factors of short- versus long-haul flying and overnight flying.

Further study has already begun at the two airlines initially surveyed to verify reported disease and to collect additional information on those having these diseases. Pilots of three additional airlines will receive an expanded survey that includes questions on flight histories and scheduling, fatigue, disruption of circadian rhythms, and leisure-time sun exposure.

The number of women in the study will be increased by mailing surveys to all female ALPA members.

A biomarker study (fluorescence in situ hybridization) is under way in a sample of pilots to assess exposure to ionizing radiation.

The information that will be gained from these combined studies can be used to help make changes in lifestyle and/or occupational conditions to reduce the incidence of disease.

The authors would like to thank the airline pilots who participated in this study. This study was supported in part by funds from the U.S. Department of Energy cooperative agreement DE-FC02-98CH10902 and by funds from ALPA.

This article was adapted with permission from "Health Among Commercial Airline Pilots," Aviation, Space, and Environmental Medicine, in press.