Cabin air quality: a response


We read with interest the article ‘Cabin Air Quality – the killer inside?‘ which appeared in the September 2017 issue of the journal (1). Unfortunately, this article makes no useful contribution to the ongoing debate as to whether aircraft cabin air contamination or fume events, related to the leakage of pyrolyzed engine oil into the air-conditioning system, causes ill health in aircrew and passengers. An argument similar to that laid out in the article appeared recently in the WHO Public Health Panorama Journal, authored by members of the Aerospace Medical Association (2) in response to our publication ‘Aerotoxic Syndrome – a New Occupational Disease?‘ in the same journal (3). We responded to their letter with a line by line rebuttal, which does not seem to have been mentioned in Robert Coppinger’s publication. You can see our response in the link cited in reference 2. The continued negativity by the airlines, their medical staff and the regulators and their lack of a more sensible, scientific and open-minded approach to this issue is disappointing. Surely, they are aware, or ought to be aware, that the leakage of pyrolyzed engine oil into the aircraft air-conditioning systems, with the consequent risk of ill health among those on board, has been well documented since the mid-1950s (4-7). The consequences of aircraft cabin contamination in the above outlined circumstances and their health consequences currently called Aerotoxic Syndrome is clearly an occupational and public health and safety issue that must be addressed.

It is pertinent to set out here, in brief, the conclusions of our studies that were published in the Panorama WHO Journal (3). The aim of this report was to record the results of an in-depth investigation of aircrew involved in suspected aircraft air contaminated events to determine whether the reported symptoms and diagnoses were consistent with exposure to pyrolyzed jet engine oil/aircraft fluids or to other factors. Two studies were reported, the first being a BAe146 pilot health survey which was conducted to ask the question ‘What health effects are being reported in UK Bae146 pilots exposed to contaminated bleed air?’ The second study was a review of well-documented fume or contamination incidents to determine if the observed health effects were consistent with the toxic effects expected after the inhalation of jet engine oils and hydraulic or de-icing fluids and their pyrolyzed products, or other factors.

The experience of ill-health and, in some cases, serious clinical outcomes reported in our two studies, (3) lead to the conclusion that there is a reasonable link between aircraft air supplies contaminated by engine oil and fluids and the development of acute and chronic ill-health and the possible long-term impairment in some of those affected. These findings suggest a cause and effect relationship. When the Bradford Hill causation criteria (8) are applied to our studies, it is clear that eight of the nine criteria are met, with only the dose response criterion not being met (9). The types of exposures being seen here is about his chronic exposure to low-doses of a complex heated mixture, with acute events on top.

We understand that others have a different opinion and, in reaching their views, have formed conclusions based on information published elsewhere. For example, the European Aviation Safety Agency has published studies looking at cabin air contamination measurement, oil pyrolysis and toxicity (10, 11). It is pertinent to point out that the monitoring studies included only ’69 measurement flights’.

The air was measured at various stages of flight with the findings reported as follows:

‘Total volatile organic compounds (VOC) concentrations ranged from 0.024 – 2.1 mg/m3 (main study) and 0.012-0.489 mg/m3 (B787 study). In this study low amounts of formaldehyde (range 0.03-48 μg/m3 (main study) and 0.02 – 17 μg/m3 (B787 study), acetaldehyde (range 0.02-42 μg/m3 (main study) 0.01- 15 μg/m3 (B787 study)) and other aldehydes mostly at trace levels were detected. Organophosphates were analyzed in all samples (n = 516). In the group of tricresyl phosphates (TCP) only traces of meta- and para- isomer were detected (mean 0.009 (main study) and 0.020 μg/m3 (B787 study), max 1.515 (main study) and 0.403 μg/m3 (B787 study). No ortho isomers were detected. The most prominent airborne organophosphorous compounds (OPC) in this study were tri-n-butyl phosphate (TBP) which amounted in the main study from EASA – Preliminary Cabin Air Quality Measurement Campaign Page 9 of 128 0.037 to 2.484 μg/m3 (mean 0.430 μg/m3); and in the B787 from 0.037 to 1.482 μg/m3 (mean of 0.237 μg/m3), and tris(chloro- isopropyl)phosphate, a typical flame retardant, which amounted in the main study from 0.023 to 9.977 μg/m3 (mean 0.506 μg/m3), and in the B787 study from 0.041 to 2.633 μg/m3 (mean of 0.502 μg/m3). Other OPC were detected in trace amounts in most of the samples’.

Additionally it was reported that the TCP meta & para isomers were detected in nearly all the samples, TCP related to oil leakage was recoded in over 50 % of the samples, with permanent low level leakage confirmed at below the monitoring equipment limits of detection. The oil pyrolysis studies identified 127 differing compounds with over 634 chemical peaks of which only 27% could be identified. (11)

Thus, it is clear that the EASA studies did detect a large number of toxic substances in aircraft cabin air. However, the report goes on to state ‘The observed frequency, pattern and concentration levels were similar to findings of other indoor environments’. These findings lead to the conclusion that EASA accepts that contamination of aircraft cabin air does occur. However, the latter comment, drawing a similarity to other indoor environments, implies that EASA is of the view that the volatile organic hydrocarbons contaminants, in the measured concentrations, do not pose a threat to health and safety. This conclusion is incorrect, for those clinicians with a wide experience of medicine and occupational health in the general community and in other industries are well aware that illness may be caused by very low concentrations of toxic or sensitising substances, such as seen in some cases or occupational asthma and in the Sick Building Syndrome. Perhaps the authors of the EASA report are not aware that industry accepted environmental concentration standards are set to protect most (not all) of those exposed. To protect all relies on a complete absence of the substance in question from the environment. It is well recognized by clinicians with a broad experience of occupational health, that some individuals will develop disease at environmental concentrations well below these standards (12). Other major limitations with the EASA/European Commission studies are clearly set out elsewhere. (13-16) Based on reviewing all the evidence, the results of these studies, and EASA’s conclusions, can only be viewed with scepticism.

The Airline Medical Directors Association (AMDA) is reported, in Robert Coppinger’s article (1), to have stated ‘We [AMDA] have serious concerns about the science behind the studies reported’ claiming that our paper (3) ‘makes the bold claim that it has established a causative link between cabin air and long-term illness described by a group of individuals’. This is incorrect. Our published conclusion was ‘Aircraft air supplies contaminated by pyrolyzed engine oil and other aircraft fluids can reasonably be linked to acute and chronic symptoms, findings and diagnoses, thus establishing causation’. Our view is that the weight of evidence, that is the number of aircrew complaining of ill-health in similar circumstances on different aircraft and in different countries, raises the question of why this is happening. A decade or two ago, when illness following fumes events was first becoming reported, it would have been reasonable to take a sceptical approach. However, there are now hundreds, if not thousands of cases, not all reported to employer airlines. This should lead the analytical and open-minded clinician to conclude, that there is likely to be a cause and effect. History is peppered with similar experiences – cigarette smoking and lung cancer, asbestos and lung disease and the observation by astute clinicians in San Francisco in the late 1970s and early 1980s of unusual infections and malignancies (eg Kaposi’s sarcoma) among the gay community which led to the identification of HIV-AIDS. The fact that a medical condition, such as Aerotoxic Syndrome, does not appear in the medical textbooks, Index Medicus or the International Classification of Diseases does not mean that it does not exist. Numerous conditions have been ‘discovered’ in the last twenty to thirty years or more, including HIV-AIDS, obstructive sleep apnoea, several arthritic and haemorrhagic conditions, and many new causes of occupational asthma. Thus, history tells us that the knee jerk reaction of negativity, in this context, is taken with significant risk.

Whilst the airline industry and its regulators dismiss the Aerotoxic Syndrome as a non-entity, they have suggested reasons, other than chemical toxicity, as causative, such as stress, hypoxia, nocebo and hyperventilation as a mechanism for the observed and reported symptoms and illness (17). Although hyperventilation may occur in stressful situations, to argue that fume-affected aircrew suffer from an hyperventilation syndrome, simply because their symptoms resemble those seen in an hyperventilating person, is to ignore the fact that over breathing also occurs in individuals with cardiac, lung and neuromuscular disorders. Furthermore, if symptoms are, in fact, due to stress-related hyperventilation, this would call into question the industry’s selection process for aircrew. Additionally, this rationale ignores the strong correlation between observed symptoms and the hazard classification process and toxicity databases. In this regard, there now seems to be some light at the end of the tunnel and it is noteworthy that EasyJet has recently announced that they will be fitting air filtration systems in their aircraft in 2018 (18). This decision is a great step forward and is to be applauded.

New horizons and progress in science, medicine and health care are achieved by those who think with an open mind and fertile imagination and who apply a critical examination of the observed facts, including the occupational and medical history and the results of appropriate special investigations. It is now time that the negativity regarding the Aerotoxic Syndrome cease and to accept that there is a toxicity problem, caused by the inhalation of pyrolysed engine oils and other substances, in aviation. Progress will be made if we all move forward together to investigate this matter further, and introduce safe guards for air crew and passengers alike. Further investigation and discussion should include not only the airline industry and its regulators but also involve medical personnel from outside the aviation industry who will bring a broad experience of illness and toxicity from numerous other industries.

In conclusion, we do not feel, as a responsible media organization, you ought to be as ready to accept the views of ‘bad science’ when those suggesting this are not presenting any evidence to support such a statement. We do not agree that this issue will go on for years, if all sides will look at the plausible evidence, not simply the evidence that suits them. Technology will likely take over and provide solutions, however such exposures have remained ongoing for decades and the human effects cannot be ignored.


Susan Michaelis, PhD, MSc, ATPL School of Health Sciences, University of Stirling, United Kingdom contact:

Jonathan Burdon, MBBS, MD, FRACP Consultant Respiratory Physician, Melbourne, Australia

C. Vyvyan Howard,  MB. ChB. PhD. FRCPath Centre for Molecular Biosciences, University of Ulster, United Kingdom


  1. Coppinger Robert. Cabin Air Quality – the killer inside? Aircraft Interiors International. September 2017
  2. Martindale Valerie E., Clayton T. Cowl, Anthony Wagstatt, Jarnail Singh. Letter to the Editor.
  3. Michaelis Susan, Jonathan Burdon, C. Vyvyan Howard. Aerotoxic Syndrome: A New Occupational Disease? Panorama WHO Public Health Panorama 2017:3:198-211
  4. Loomis T, Krop S. Cabin air contamination in RB-57A aircraft. USA Chemical Corps Special Report Medical Laboratories MLSR No. 61. Maryland: Army Chemical Centre; 1955
  5. Reddall H. Elimination of engine bleed air contamination. SAE Technical Paper 550185. Warrendale: SAE International; 1955
  6. Kitzes G. Cabin air contamination problems in jet aircraft. Aviation medicine 1956;2:53-58
  7. Treon J, Cappel J, Cleveland F, Larson EE, Atchley RW, Denham RT. The toxicity of the products formed by the thermal decomposition of certain organic substances. Am Ind Hyg Assoc Q. 1955;16:187-195
  8. The Environment and Disease: Association or Causation?
  9. Michaelis S. Health and flight safety implications from exposure to contaminated air in aircraft [PhD thesis]. Sydney: University of New South Wales; 2010
  1. EASA. Research Project: CAQ Preliminary cabin air quality measurement campaign. Final report EASA_REP_RESEA_2014_4. Cologne: European Aviation Safety Agency, 2017.
  2. EASA. Research Project: AVOIL. Characterisation of the toxicity of aviation turbine engine oils after pyrolysis. Final Report EASA_REP_RESEA_2015_2. Cologne: European Aviation Safety Agency, 2017.
  3. ACGIH. TLVs and BEIs: threshold limit values for chemical substances and physical agents. Cincinnati: American Conference of Governmental Industrial Hygienists; 2015.
  4. Howard C V. Report prepared for the GCAQE- Critique of the EASA and European Commission CAQ studies. 2/11/17 (2017). Hill, Austin Bradford (1965). Proceedings of the Royal Society of Medicine. 58 (5): 295–300
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