Author's response to reviews Title:The self-reported health of U.S. flight attendants compared to the general population Authors: Eileen McNeely (emcneely@hsph.harvard.edu) Version:4Date:30 January 2014 Author's response to reviews: see over
Referee #1: Major Compulsory Revisions. 1) In the introduction there is discussion of longer tenure being a proxy for longer exposure. This is an important point on which a key aim of the study rests therefore it is important to cite other occupational studies that have demonstrated this association, simply to demonstrate that there is evidence to show that job tenure serves as an adequate proxy variable for exposures to job hazards. The concern is that workers change their exposure contexts over time and it may be inaccurate to assume steady state exposures. If time specific exposure histories are not available, it is important to show that tenure can provide a global but relatively accurate proxy for exposure. Thanks for this comment. We added the following text to the third paragraph of the introduction-- When exposure data are not available for study subjects in an occupational cohort, employment tenure or job duration can serve as a surrogate for cumulative exposure to occupational hazards with the assumption of a uniform exposure intensity, which does not change over time or across study participants (ES Johnson, 1986 BMJ). Several aviation studies use tenure as a proxy for exposure (Pinkerton, 2012), and tenure often correlates with radiation exposure (Waters, 2009; Hammer, 2000). We also added this as a limitation in the last paragraph of the Discussion Furthermore, another limitation to this work is the use of tenure as a proxy for exposure; similar to most occupational settings, the flight attendants in this study likely do not have uniform exposures over time and between participants. 2) More information is needed on the survey itself including how it was constructed, how the questions were arrived at (e.g., was item wording from other surveys used?), and whether the survey was pilot tested and revised before full implementation. If any reliability of validity of the questions were determined from previous studies, this can be reported here. Treatment of duplicates is mentioned and it can be explained how duplicate surveys would come about in the first place. Specific wording about the fatigue and depression questions is provided in the discussion this may be better placed in the methods and more general description is needed on the structure of the items for the other conditions as well. Sure, in the first paragraph of the Methods section we updated the text to read, In the few cases of duplicate surveys, such as when flight attendants completed a mailed survey and another survey from the on-site airport distribution, we accepted only the mailed survey returned by the flight attendant. In the final study sample, participants selected at
random outnumbered the participants we added at the airports 2:1. The Harvard School of Public Health Institutional Review Board approved all protocols for human subjects. In the second paragraph of the Methods section we added, The survey was constructed using standardized questions from other surveys (Job Content Questionnaire (Karasek et al. 1998), Centers for Disease Control National Center for Health Statistics (CDC- NCHS), National Health and Nutrition Examination Survey (NHANES) (CDC-NCHS 2005-2008)) and feedback generated from focus groups and a pilot study sample. With respect to a general description of the NHANES variables, we added Supplemental Table 2 and the following language in the Methods section-- Most of the questions between the FA survey and NHANES questionnaire aligned to binary answer choices for prevalence (yes/no), but the fatigue and depression variables contained slightly different time interval answer choices. To estimate the prevalence for fatigue and depression, we used a conservative approach and coded symptoms over the past week that occurred every day (7 days) in the flight attendants as a yes for prevalence, and only the category of nearly every day over the past 2 weeks in NHANES as a yes for prevalence. We weighted the NHANES data by their four-year sample weights, primary sampling units, and strata according to the NHANES analytic guidelines (http://www.cdc.gov/nchs/data/series/sr_02/sr02_161.pdf). 3) More information is needed on the survey administration protocol including what the flight attendants were told about the survey, how many times they were contacted and asked to complete the survey, and what if anything is known about the people who did not complete the survey and why they opted not to complete it. Similarly, it will be informative to know how the random sampling was carried out and more about how the convenience sampling was conducted. If this is information that is published elsewhere, then it can be summarized and the reference to the other pub(s) can be provided. All of this information will be important if other researchers seek to replicate this study in the future. We added the following paragraph to the Methods section To each flight attendant selected at random from union membership lists, we mailed two surveys and 2 reminder postcards Also, researchers visited the five target airport hubs across the country and met some of those flight attendants already contacted via mail and reminded them to mail back or return the surveys directly to the research team in the field. In addition, new participants outside of the survey mailing lists were recruited to participate at the time of our on-site reminder/recruitment campaign. 4) A citation of the methods for the NHANES sample weights will be informative to document that these are standard and replicable methods. Similarly, citation for the SPR to illustrate it is a standard measure of association is needed. More on how current employment was categorized in NHANES would be useful. In description of the covariates, explanation as to how they were treated e.g. continuous or categorical in
the model building is important. In the results it would be informative to see the demographic characteristics of the NHANES sample to which the flight attendants are being compared. We added in a citation for the NHANES analytic guidelines (http://www.cdc.gov/nchs/data/series/sr_02/sr02_161.pdf) and a citation for standardization (Miettinen, 1972). We added Supplemental Table 1 to display the NHANES variables used in the SPR analysis. However, current employment was categorized with the OCD150 variable in the NHANES occupation data file for 2005-2006 (ocq_d.xpt) and 2007-2008 (ocq_e.xpt). The question asks Which of the following were you doing last week... The answers are (1) working at a job or business, (2) with a job or business but not at work, (3) looking for work, or (4) not working at a job or business? (7) Refused, (8) Don t Know, (.) Missing. We used categories 1 and 2 to classify NHANES respondents as a binary currently employed variable. The NHANES sample we used in our analysis was set based on demographic characteristics that included age, education, employment, and poverty, so a table of these results would not be an accurate description of the NHANES demographic characteristics for 2005-2008. Rather, it would be a manipulated description of the dataset. We added in a sentence for the reader to reference NHANES summary statistics for 2005-2006 and 2007-2008 by visiting the continuous NHANES selected bibliography. The sentence appears in the last paragraph of the results section before Table 4: The relationship between... The sentence states For the full results of NHANES from 2005-2006 and 2007-2008, please see the continuous NHANES selected bibliography (http://wwwn.cdc.gov/nchs/nhanes/bibliography/). 5) It is discussed that health conditions that were diagnosed by a health provider received closer attention to minimize the bias of self-report. The manuscript can talk about whether or not self-report is a concern in this study (this could be included in a weaknesses followed by strengths section prior to the conclusion) and also address whether opportunities to corroborate self-reported illnesses exist in the future or if such methods are recommended for conducting large scale health research on flight attendants in the future. Thanks, we added this to our limitations at the end of the last paragraph above the conclusions. The sentence reads An additional limitation to this work is that both the FA survey and selected NHANES questions rely on self-reported health conditions; these data were not corroborated with medical records due to the cost and scope of the work. At present, we do not have funding to cross reference our survey results with medical records.
6) Minor essential revisions: The appearance of the tables can be improved by removing the excess row and column lines and repeated % symbols in Tables 2,3 and Supplement 2. Thank you for this observation. We removed the extra % symbols and excess rows in Tables 2, 3, and Supplement 2. Referee #2: Minor Essential Revisions 7) The first paragraph is confusing. The authors do not differentiate the different trends in occupational exposures between long haul and short haul flying. Short haul flight attendant employment is characterized by flying more sectors, and therefore more in flight service episodes, and short turnarounds. Much of the ultra long haul flying in international flights is in A340 and B777 aircraft that are not new jumbo sized planes. The A380 is a different phenomenon due to its very high passenger numbers making boarding and deplaning a more protracted and arduous process. As the authors say, the nature of work as a flight attendant is changing, with less direct baggage handling, and less bending, twisting and reaching than in the B747. Thanks for this comment. We added the following sentences to the Results section under the text for Table 1 U.S. flight attendant jobs demand moderate flexibility in terms of routes, schedules, and seating capacity and layout according to different types of aircraft. In our cohort, crew seemed to equally share work on both long and short haul flights and multiple aircraft types. While there are differences in occupational exposures according to long haul and short haul flights and type of aircraft, our survey could not separate flight attendants into neat categories. Rather, half of the flight attendants in the survey answered that they worked a combination of long and short segments; 33% stated that they worked single segment, long haul flights 12% worked multiple segments in one duty period; and flight attendants reported working in multiple types of aircraft over the past 12 months. 8) In the second paragraph, the authors should acknowledge the limitations of using overseas data on health outcomes in flight attendant health, as the employment patterns of US flight attendants differs significantly from those of flight attendants working in many other countries, where flight attendants are on short term contracts, and often work in a range of settings and for a range of employers; older flight attendants are much more characteristic of US airlines that the majority of airlines worldwide. The SPR may give misleading impressions of the risk of morbidity and mortality in this group, as the SIR and SMR for most conditions in flight attendant studies are less than 100.
The SMR and SPR are forms of indirect standardization and thus, cannot usually be compared across different studies. They are only comparable across studies when stratum specific ratios are nearly constant across strata or when stratum specific population sizes are the same for the study and reference population. We added this text to our limitations in the last paragraph of the discussion Our results describe a U.S. flight attendant population and may not reflect the same experience for crew of foreign airline carriers. The SPR is an indirect standardization measure, and like the SMR, should not be compared across studies unless stratum specific ratios are nearly constant across strata or when stratum specific population sizes are the same for the study and reference population. General Comments 9) The methodology was appropriate, although it may not have achieved the results the researchers had hoped for. The flight attendant occupational group is notoriously difficult to access for study, and the 48% response is to be expected. A low response rate and the use of convenience sampling limits the degree to which inferences can be made about health outcomes in flight attendants. The choice of the employed, general population from the NHANES data as the control groups may not have been ideal, as many studies have used teachers as the control group due to the similarities in the nature of their work. We added the following text to the second paragraph of the discussion In a review of studies on flight attendant health, researchers found that most studies are not random samples, were conducted many years ago, rely on self-reported questionnaire data, and suffer from low response rates. While our response rate was 48%, it was higher than we expected and higher than the most recent large, random sample conducted by Ebbert in 2007, which yielded a response rate of 14%. We inserted the following language to the first paragraph of the discussion Using the NHANES population as a reference, allowed us to 1) compare the health of the general population of the US to domestic flight attendants, 2) control for important SES characteristics (education, poverty, employment status) and 3) measure similar survey questions without limiting the data to a comparison of one particular occupation. 10) The findings of a higher prevalence of self-reported cardiac disease is surprising, given that not only are this group less likely to have risk factors for coronary artery disease, but the healthy worker effect from ongoing medical surveillance would have been stronger given that flight attendants with significant cardiac disease are usually unfit to work in a safety-critical role. One wonders if earlier diagnosis of relatively minor disease unlikely to interfere with their abilities to assist in emergency evacuation be relevant here. The delayed effects of ETS exposure during the period when smoking on
board aircraft was permitted is striking, and would explain the excess risk of chronic bronchitis, whereas the risk of cardiac disease due to ETS exposure may be exacerbated by circadian dysrhythmia, especially in long haul crews. Yes, that was surprising, because they have low risk (BMI, smoking, hypertension) and healthy worker effect as we wrote in the discussion. We also do discuss the idea of a chronic inflammatory process. 11) It would be helpful to have self-reported inserted before health in the title and stressed in the conclusion that self-reports may not accurately correlate with ill-health. Yes, we agree. We inserted self-reported into title and the following language to our limitations-- An additional limitation to this work is that both the FA survey and selected NHANES questions rely on self-reported health conditions; these data were not corroborated with medical records due to the cost and scope of the work.