This is the first published flight crew career exposure assessment from individual flight segments. The moderate correlation of this study’s dose data with questionnaire-edited flight experience data or years of flying (occupational or from all sources) indicates that this more detailed level of assessment from individual flight segments is likely to reduce exposure misclassification and increase etiologic clarity for flight crew studies in general.
This assessment is an improvement over self-reported flight attendant questionnaire responses, which have been found to overreport exposures (Grajewski et al., 2004
; Kojo et al., 2004
). Flights are generally logged by pilots in real time, which reduces recall bias compared to retrospective questionnaire responses. It is professional custom for pilots to log flight time for several reasons throughout their career, including student equipment rental, FAA licensure and regulatory requirements, flight time-based pay, and operational needs (e.g. fuel requirements). Preliminary comparison of pilot logs to company flight records (data not shown) suggested that despite individual variation in format, pilots logged their flights fairly accurately.
In the literature, several flight crew studies have implemented intermediate assessment approaches, including aircraft-based exposure matrices rather than relying on duration of employment. A domicile-based retrospective assessment may enhance the ongoing NIOSH retrospective mortality and cancer incidence studies of former PanAm workers (Waters et al., 2009
). Oksanen (1998)
estimated radiation incurred from 1 year of occupational flying from airline records of individual flight segments and equipment manuals to derive estimated doses for each flight. Tveten et al. (2000)
estimated annual and career doses from pilot block hours on specific aircraft and estimated 5-year dose rates for those aircraft based on airline timetable domestic and selected international flights. Hammer et al. (2000)
compared estimates based on 1 year of electronic records of individual flight segments with other estimation methods, including a job exposure matrix from flight schedules, cumulative flight hours, and duration of employment. They reported good correlation between these methods. However, our work suggests that assessment of individual flight segments over a pilot’s career may provide a more specific estimation of radiation and circadian disruption than the previously published methods. This specificity does not guarantee analytic separability between metrics of radiation and circadian disruption. To reduce circadian disruption–radiation collinearity, flight crew who work primarily North–South routes can be included in study groups (Grajewski et al., 2002
We found that military flights were the second largest source of exposure for this group of pilots. Thus, military experience, often assessed from summary hours records, was a quantifiable component of occupational exposure rather than a nuisance covariate. We noted that the correlations between time zones, block times, and dose were not as strong for military flights as for those between these same metrics in data from all combined types of flight, which we have reported in other non-military flight crew exposure assessments (Grajewski et al., 2002
; Waters et al., 2009
). Because of the diversity of types of military flight, correlations between block time, time zones, and dose would likely be lower than those observed for commercial flights.
We were able to report cumulative particle-specific dose estimates for the study pilots. As expected, neutrons were a significant component of commercial aircraft effective or absorbed dose. Our cumulative ED (median 19.9 mSv; range 5.7–48.0) for this IARC Group I carcinogen is consistent with the IARC report of aircrew lifetime average and long haul pilot lifetime maximum neutron doses of 30 and 46 mSv, respectively (IARC, 2000
). EMSs are also a significant component of absorbed dose. Although EMS relative biological effectiveness is low compared to neutrons, they may induce biological damage (NCRP, 2006
Our characterization of circadian disruption is, to our knowledge, the first comprehensive long-term assessment of its kind in this occupational group. We used non-directional time zones crossed as an exposure metric because time zones are well correlated with a state of chronic disruption as biologically measured by an increase in the overnight variability of melatonin excretion in female flight attendants (Grajewski et al., 2003
). We also assessed SSI travel because it captures the separate but related exposure of sleep disturbance rather than the desynchronization reflected in cumulative time zones crossed (Grajewski et al., 2003
). Accordingly, SSI travel appeared to be weakly correlated to other metrics, as we have noted earlier (Grajewski et al., 2003
; Waters et al., 2009
). Flight crew who work primarily on short regional flights or North–South routes can incur significant SSI travel while crossing relatively few time zones.
We were able to provide a crude estimate of the frequency that a pilot might expect to travel through significant SPEs. Our SPE assessment was descriptive and conservative for several reasons. We did not have SPE assessment for flights before 1976. But because many of our pilots began their careers during this time, over half the flights during the missed time period were in lower altitude equipment clusters, and many of these would have been excluded. We also lacked sufficient data to determine whether a flight actually passed through a region affected at aircraft altitudes by the SPE. For example, removing the 45° geomagnetic latitude criterion for flight origin and destination increased our estimates slightly (estimated SPE exposures: a median of 7 times or once every 3.2 years of work). Finally, small or minor SPEs may add to total exposure. However, the combined satellite, time, altitude, and latitude criteria we chose increased the likelihood that an identified flight passed through an SPE, which enabled us to estimate the number of times a pilot could expect to be occupationally exposed to moderate or large SPEs.
The impact of SPE exposure on cumulative galactic cosmic radiation ED has been considered by a number of investigators. Applying estimates from previous work (Beck et al., 2009
; Matthiä et al., 2009
) to our data gives a very broad range of exposure estimates. A 20% ED increase (Matthiä et al., 2009
) from SPE possibly exposed flights in our data suggested an addition to the pilots’ cumulative ED of a median 20.8 μSv or 0.06% increase (range 7–51 μSv or 0.02–0.21%). At the other extreme, adding 1 mSv to the ED (Beck et al., 2009
) for each possible SPE-exposed flight increased the pilots’ cumulative ED by a median of 17 mSv or 43.7% (range 2–48 mSv or 7.7–220%). This wide variation in estimates affirms that SPE contribution to ED is time and location dependent. While our data estimates do not allow any additional time and location precision, they also do not rule out a significant contribution of SPEs to cumulative cosmic radiation exposure.
Generally, our median results were consistent with the literature. Annual ED estimates range from 0.2 to >7 mSv year−1
for aircrew flying 600–1000 h year−1
(Friedberg et al., 1992
; EURADOS, 2004
). However, our study’s hypothetical median US airline pilot would likely have triggered radiation monitoring (i.e. recording of estimated dose) according to the European Union criterion of >1 mSv year−1
), and it would seem possible for a female pilot (and perhaps other female flight crew) to exceed the ICRP guideline for pregnant radiation workers [recommended dose limit of 1 mSv (equivalent dose) upon declaration of pregnancy for the remaining gestation period; ICRP, 2008
]. Furthermore, the ranges of these non-normally distributed metrics for radiation and circadian disruption suggest that there is a high-exposed group of airline pilots who may be at increased risk for the health effects of cosmic radiation and chronic circadian disruption compared to other pilots. Our data show that this may have been especially true in the most recent years of our study, from the 1990s to 2003. Changes in aircraft, increased polar routing, efforts to decrease fuel consumption and costs, and changes in crew contracts may each have played a role in the increases we have observed in pilots’ flight exposures to radiation and circadian disruption.
Accurate assessment of radiation and circadian disruption exposures in flight crew appears to be of increasing public health importance. Identification of high-exposed pilots would be a valuable addition to an occupational health program if counseling or intervention were offered grounded in an understanding of this occupational group, including scheduling and seniority issues.
This assessment has several limitations. The extensive records collection and processing required for individual flight records, especially from handwritten pilot logbooks, may offset the benefits of this approach, or records of this nature may be unavailable. Like many non-regulated exposures with multiple record sources, multiple assumptions were made to allow inclusion of all exposure record sources in our metrics, which could have increased misclassification. The non-occupational exposures of the comparison group are far lower than the pilots’ but may be reported with less accuracy; thus, comparisons within pilots stratified by exposure levels will be important in epidemiological analyses. When this work began, CARI was the only dose estimation software, which could process large numbers of flight segments for epidemiological studies. A Great Circle Route was assumed and SPE exposures were not estimated. We look forward to the adaptation of the real-time the National Aeronautics and Space Administration Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) model (Mertens et al., 2010
). NAIRAS will provide global, data-driven real-time radiation exposure predictions of cosmic radiation and SPEs at commercial airline altitudes.
Despite these limitations, this assessment met our objectives of developing new methods to estimate pilot workplace exposures and describing those exposures in detail for a group of US commercial pilots. The methods described are a potential improvement over other exposure assessment methods for flight crew exposures. They offer the possibility of analyses of pilot and flight crew health outcomes, which will provide a clearer understanding of the separate contributions of components of cosmic radiation and circadian disruption exposures.