Donald C. Cooper Ph.D., Director Medical and Data Science Ramos Law
Overview
This comprehensive review examines the scientific evidence regarding the effects of long-haul flying and irregular flight schedules on immune function, with particular focus on commercial airline pilots. Research indicates multiple mechanisms through which aviation work may compromise immune function, including circadian rhythm disruption, exposure to hypobaric hypoxic conditions, and cabin environmental factors. Studies demonstrate that these factors can lead to transient immune impairment, potentially explaining the anecdotal reports of increased susceptibility to illness among flight crew members.
Circadian Disruption and Immune Function
The relationship between circadian rhythms and immune function represents one of the most significant pathways through which long-haul flying affects health. The body’s 24-hour circadian clock controls numerous biological processes, including immune system regulation[1]. Disruptions to this system, as experienced during jet lag and irregular flight schedules, have demonstrable negative impacts on immune function.
Molecular Mechanisms
Research indicates that circadian disruption alters the expression of clock genes in multiple organs, including immune cells. A study in mice showed that chronic jet lag (simulated by shifting light schedules) altered or abolished rhythms in clock gene expression in the central clock, liver, thymus, and peritoneal macrophages[2]. This disruption was associated with dysregulation of inflammatory responses, as jet-lagged mice exhibited heightened release of pro-inflammatory cytokines when challenged with endotoxin[2].
Importantly, these effects were attributed specifically to circadian disruption rather than sleep loss or stress, as researchers found “no sleep loss, and stress measures were not altered in shifted mice”[2]. This suggests that the irregular schedules experienced by long-haul pilots may directly impact immune function independent of other stressors.
Human Studies on Circadian Disruption
Human studies confirm that circadian disruption affects immune markers. Research has demonstrated that inflammatory markers such as cytokines and chemokines—key components of the immune response—are regulated by the circadian clock[1]. These components show rhythmic 24-hour patterns under normal conditions, but this timing is disrupted during jet lag and shift work[1].
A study specifically examining immune changes in frequent flyers found “recurrent infections… indicating an impaired immune reaction after long-distance flights”[3]. The researchers observed increased concentrations of neopterin (indicating activation of cellular immunity) alongside altered differential blood counts and changes in lymphocyte proliferation rates[3]. They concluded this represented a “down-regulation of the immune system after flights,” partially mediated by stress hormones including catecholamines and cortisol[3].
Cabin Environment and Physiological Stressors
Hypobaric Hypoxic Conditions
Aircraft cabins create a unique physiological environment that may contribute to immune impairment. A controlled study examining immune function after simulated long-haul flights found “transient immune changes” including “a transient decrease in lymphocyte proliferative responses” in the days following exposure to typical cabin altitude conditions[4]. These changes normalized within a week but could explain “increased susceptibility to respiratory infections commonly seen after long-haul flights”[4].
Extreme Dehydration and Low Humidity
The aircraft cabin environment presents additional challenges to immune function. Cabin air typically has extremely low humidity—sometimes as low as 5%[5][6]—which is “drier than the Sahara desert”[6]. This severe dehydration “can cause a headache, sore throat and exacerbate the symptoms of jetlag and may weaken the immune system, making you more vulnerable to viral and bacterial infections”[5].
For each hour of flying, passengers lose approximately 8 ounces of water, which “reduces metabolism and cell function, slows blood flow, and promotes inflammation. All of these effects impair your immune system”[6].
Epidemiological Evidence in Pilots and Flight Crew
Infection Rates and Immune Markers
A comparative case-control study examining seropositivity rates for 17 common infectious agents found that airline pilots showed significantly higher rates of certain infections compared to matched office workers[7]. Specifically, the study found pilots were 2.35 times more likely to be seropositive for Herpes Simplex Virus-1 and 2.30 times more likely to be seropositive for H. pylori compared to office workers[7].
The researchers speculated these higher infection rates might be attributable to “pilots’ immune system dysregulation due to high fatigue and circadian disruption” and “factors related to local air recirculation in current airliner cabins”[7].
Research indicates that approximately 20% of air travelers develop cold symptoms within a week of flying[8]. For pilots and flight crew who fly frequently, this risk is compounded. Studies suggest that aircrew “can be at risk of exposure to infectious diseases when in contact with sick crewmembers, passengers, or their bodily fluids; by inhaling airborne pathogens; or by touching contaminated surfaces”[9].
Unique Occupational Considerations
Flight crews are also exposed to other occupational hazards that may compound immune challenges. For instance, they experience “the highest annual individual radiation dose of any occupation” due to cosmic radiation exposure at high altitudes[9][10]. While radiation exposure is primarily linked to cancer risk rather than acute immune suppression, it represents another physiological stressor specific to aviation work.
Mental Health, Fatigue, and Immune Function
Research on pilot health indicates significant connections between the demanding nature of flight schedules, mental health, and physical wellbeing. A cross-sectional study of 406 international pilots found high rates of fatigue, sleep problems, and mental health issues[11].
The study revealed that “44.8% of short-haul pilots reported severe fatigue… an additional 31.7% reported high fatigue” while “34.7% of long-haul pilots reported severe fatigue and 37.3% high fatigue”[11]. These findings are particularly concerning given that these pilots were scheduled for only “51.4–65.4% of the legally allowed duty and flight hours”[11].
Mental health issues were also prevalent, with “positive depression screenings… reported by 18.1% of short-haul and 19.3% of long-haul pilots”[11]. This is significant as psychological stress and depression are known to impact immune function.
Mitigating Strategies
Several protective measures may help mitigate the immune impacts of long-haul flying:
- Hydration: Research emphasizes the importance of maintaining hydration during flights, with recommendations to “increase your water intake two days before you fly” and “drink at least 2.5 litres of water over a 10-hour flight”[5].
- Vaccination: The CDC recommends that aircrew “keep up to date with routine vaccinations (e.g., diphtheria-tetanus-pertussis, influenza, measles-mumps-rubella)”[9].
- Hygiene practices: Frequent handwashing with soap and water or using alcohol-based hand sanitizers containing ≥60% alcohol when soap and water are not available[9].
- Immune support: Some research suggests “boosting” immune function before travel through healthy diet and exercise[5].
- Protective equipment: When appropriate, using “personal protective equipment (e.g., disposable gloves, face masks) when assisting potentially infectious travelers”[9].
Conclusion
The scientific evidence strongly supports a link between long-haul flying, especially with irregular schedules as experienced by commercial pilots, and impaired immune function. Multiple mechanisms contribute to this effect, including circadian disruption, exposure to hypobaric hypoxic conditions, extreme cabin dryness, and accumulated fatigue.
The health concerns reported by pilots—including increased susceptibility to colds, flu, and other infections despite high fitness levels—align with the documented transient immune impairment observed in controlled studies. These effects appear to be particularly pronounced for junior pilots who may face additional stressors from demanding schedules and workplace pressures
References:
- Oscar Castanon-Cervantes, Mingwei Wu, J Christopher Ehlen, Ketema Paul, Karen L Gamble, Russell L Johnson, Rachel C Besing, Michael Menaker, Andrew T Gewirtz, Alec J Davidson, Dysregulation of Inflammatory Responses by Chronic Circadian Disruption, The Journal of Immunology, Volume 185, Issue 10, November 2010, Pages 5796–5805, https://doi.org/10.4049/jimmunol.1001026
- Wilder-Smith A, Mustafa FB, Peng CM, Earnest A, Koh D, Lin G, Hossain I, MacAry PA. Transient immune impairment after a simulated long-haul flight. Aviat Space Environ Med. 2012 Apr;83(4):418-23. doi: 10.3357/asem.3162.2012. PMID: 22462370.
- Santiago Sáez A, García Martín Á, Gómez Serrano M, Liaño Riera M, Minoretti P. A Comparative Study of Seroprevalence of 17 Common Pathogens Among Airline Pilots and Office Workers. Cureus. 2023 Dec 19;15(12):e50778. doi: 10.7759/cureus.50778. PMID: 38239511; PMCID: PMC10795586.
- Ruuskanen O, Dollner H, Luoto R, Valtonen M, Heinonen OJ, Waris M. Contraction of Respiratory Viral Infection During air Travel: An Under-Recognized Health Risk for Athletes. Sports Med Open. 2024 May 22;10(1):60. doi: 10.1186/s40798-024-00725-5. PMID: 38776030; PMCID: PMC11111432.
- Ruuskanen O, Dollner H, Luoto R, Valtonen M, Heinonen OJ, Waris M. Contraction of Respiratory Viral Infection During air Travel: An Under-Recognized Health Risk for Athletes. Sports Med Open. 2024 May 22;10(1):60. doi: 10.1186/s40798-024-00725-5. PMID: 38776030; PMCID: PMC11111432.
- Venus M, grosse Holtforth M. Short and long haul pilots’ rosters, stress, sleep problems, fatigue, mental health, and well-being. Aerosp Med Hum Perform. 2021; 92(10):786–797. https://asma.kglmeridian.com/downloadpdf/view/journals/amhp/92/10/article-p786.pdf (see expanded summary below)