Could poor air quality in aeroplanes pose health risks?
The air you breathe in aircrafts is routinely contaminated by small amounts of engine oils and hydraulic fluids. Could these pollutants be harmful?
Air in passenger aeroplanes can be contaminated with pollutants
- A study has identified 127 chemicals, some of which are harmful.3
- Aircraft air conditioning systems have been documented for decades to be repeatedly contaminated with engine oil and hydraulic fluid vapours.2
- It is increasingly recognised that inhaling these can cause acute and long-term neurological, respiratory, cardiological and other symptoms.2
What air do we breathe in an aircraft?
The air in a passenger aircraft usually consists of about half (± 10 per cent) recirculated air and half outside air.4 As the recirculated air in the cabin distributes the odours and microorganisms that all passengers bring with them, the air conditioning system is equipped with a HEPA filter (which filters viruses and bacteria) and optionally also with an activated carbon filter, but there are also worrying reports about the use of pesticides to disinfect the cabin air on holiday flights.5
In this article, we want to focus on the other half - the air supplied from outside. This must first be compressed, as at the typical cruising altitude there is only a fraction of the air pressure compared to sea level. Every jet has compressors on board, namely in each of its engines. These compress the air before it flows into the combustion chamber and then out again via the turbine to propel the aircraft. The outside air for the cabin is also branched off from the engine's compressor as "bleed air". This design has become widely accepted because it eliminates the need for an additional compressor for the air conditioning system (with the exception of the Boeing 787 Dreamliner, where the fresh air is supplied by electric compressors). This saves on components and energy requirements and makes the aircraft cheaper to build.3
How can engine oil get into the cabin air?
In order for the heavy rotors inside an engine to turn, their shafts have to be supported and special oils are required to lubricate and seal these bearings, which protect the engine from damage. Unfortunately these oils contain additives that are harmful to health or even highly toxic. Due to design, small oil leaks regularly occur in the engine compressor. The leaking oil pyrolises (burns) at temperatures of up to 400 °C.
As a result, highly toxic by-products of these heated engine oils are present in small quantities in the air of most aircraft. The inhalation of organophosphates (whose toxicity we know from insecticides, herbicides and pesticides, for example) is particularly relevant. They are associated with changes in gene expression, increased oxidative stress, neuroinflammation and endocrine disruption.1
The potent neurotoxin tricresyl phosphate (TCP) is also repeatedly named because it makes up around 3 per cent of engine oil. One of its ten isomers partially paralysed 30,000 people in the USA in 1930. Although this exact isomer no longer occurs in engine oils today, little is known about the use of its other five even more toxic isomers, as the engine oil manufacturers do not provide any information on this.3 During pyrolysis, less toxic TCP isomers can potentially be converted into highly toxic isomers.
In addition, oil and hydraulic vapours in the air supply also contain ultrafine particles, compounds decomposed by heat and numerous volatile organic hydrocarbons (such as the neurotoxin toluene, or formaldehyde and benzene, which are carcinogenic).2 Occasionally, carbon monoxide from defective engine exhaust systems or cabin heaters can also enter the cabin.6
High exposure can become a problem
The aviation industry claims that cabin air poses no danger. However, scientific evidence for this has not yet been provided and serious problems with cabin air have been extensively documented, wrote the Aircraft Design and Systems Group (AERO) in a report from the second international symposium on air quality in passenger aircraft.3 Unfortunately, there are no sensors on board for this and no limit values, but due to the high toxicity of some oil components, even low concentrations can cause illness over a long period of time.3 The assessment is difficult, as the toxicity of individual substances can only be inferred to a limited extent from that of complex heated mixtures.2
A study on aerotoxic syndrome, published in the World Health Organisation's journal Public Health Panorama, found a clear cause-and-effect relationship between construction-related air contamination and a distinct pattern of acute and chronic symptoms, from headaches and dizziness to respiratory and visual disorders.7,8
In a not inconsiderable number of flights, there are also cases of seal failure, so-called "smoke events" or "odour events" (in the former case, the cabin is filled with smoke, accompanied by odours; there are various videos about this on the internet). Pilots have already been so severely affected by this that they were no longer able to fly the aircraft safely.3
As a rule, the exposure of the cabin air to substances from the engine oil is low - but there is exposure and pilots and crew members have fallen ill after hundreds or thousands of hours of work or exposure even without additional visible smoke events. In studies, TCP was found in 25-100 per cent of air samples during normal flight operations, TBP (tributyl phosphate) in 73 per cent and small amounts of TBP and triphenyl phosphate metabolites in urine samples in 100 per cent.8
Conclusion
If symptoms such as dizziness, light-headedness, impaired short-term memory and cognition, nausea, tremors, fatigue, coordination and balance problems, breathing difficulties, coughing, chest pain or irritation of the eyes, nose and throat occur, aerotoxic syndrome should be considered or a smoking event should be investigated.1
Unions and pilots have been saying for years that hazardous fumes in aircraft, especially older aircraft, are a much bigger problem than anyone realises and that this is under-investigated, possibly by airlines for fear of financial and legal consequences.6,9
As with asbestos and tobacco, it may take many years of targeted studies to prove a causal link between organophosphates and serious diseases, especially those with high latency (such as cancer).
Many possible solutions have already been proposed by professional aviation organisations, such as using different oils to lubricate engines, monitoring cabin air, installing better air filters and, of course, eliminating the causal design feature: building aircraft that take their compressed air directly from the atmosphere rather than as "bleed air". This would of course mean additional costs for the industry.10
- Piccinini, M. A silent threat in the skies: the under-reported dangers of ‘aerotoxic syndrome’. Canary https://www.thecanary.co/global/2024/01/22/aerotoxic-syndrome/ (2024).
- Burdon, J. et al. Health consequences of exposure to aircraft contaminated air and fume events: a narrative review and medical protocol for the investigation of exposed aircrew and passengers. Environmental Health 22, 43 (2023).
- International Aircraft Cabin Air Conference. Die Luft in der Kabine von Passagierflugzeugen ist möglicherweise mit gesundheitlich bedenklichen Stoffen belastet. https://www.fzt.haw-hamburg.de/pers/Scholz/Aero/AERO_PR_Kabinenluft_17-09-21.pdf.
- [Only in German] Luftqualität in der Flugzeugkabine. Fraunhofer-Institut für Bauphysik IBP https://www.ibp.fraunhofer.de/de/kompetenzen/umwelt-hygiene-sensorik/luftqualitaet-im-innenraum/luftqualitaet-in-der-flugzeugkabine.html.
- Marchitelli, R. Pesticide on planes: Parents call for changes after cabin fumigated with passengers on board. CBC News (2019).
- Sainsbury, M. Jetstar’s fumes show flying high is more toxic than you think. Crikey https://www.crikey.com.au/2024/05/31/jetstar-toxic-fumes-airlines/ (2024).
- Stirling, U. of. Flights can make aircrew sick, study suggests. https://medicalxpress.com/news/2017-06-flights-aircrew-sick.html.
- Michaelis, S., Burdon, J., Howard, C. V. & Europe, W. H. O. R. O. for. Aerotoxic syndrome: a new occupational disease? Public health panorama 03, 198–211 (2017).
- Contaminated air on flights could be making you sick. The Independent https://www.independent.co.uk/travel/news-and-advice/airlines-contaminated-air-recirculated-sickness-aerotoxic-syndrome-passengers-cabin-crew-university-of-stirling-a7797321.html (2017).
- Five UK airlines sued over toxic cabin air. The Independent https://www.independent.co.uk/travel/news-and-advice/uk-airlines-toxic-cabin-air-pollution-easyjet-british-airways-thomas-cook-airlines-jet2-virgin-atlantic-a8845341.html (2019).