Written by Alexandre Berland, Master's student in Law at St Mary's University and Intern Parliamentary Assistant in the Office of Baroness Finlay

Flying is a regular mode of transport for many people in the UK, but carbon monoxide poisoning is a threat that many are unaware of.

Carbon monoxide (CO) is an odourless, colourless and deadly gas which enters the cabin through bleed air.  If there is a malfunction, CO is produced by the incomplete combustion of hydrocarbons and constitutes an invisible threat. “Bleed air” is the air used for cabin ventilation and pressurisation which has been compressed within the engine, then bled off from the compression section of the engine.

Carbon monoxide enters the human body by the inhalation of contaminated cabin air, and results in tissue cells and organ hypoxia. In fact, CO has a strong affinity with haemoglobin molecules. This binding is approximately 240 times stronger than the oxygen binding capacity with these haemoglobin molecules. So, carboxyhaemoglobin (COHb) is formed instead of oxygen. This causes vital organs to lack oxygen leading to passengers’ headaches, dizziness, fatigue, nausea, and shortness of breath. In severe cases, it can lead to confusion, unconsciousness, or long-term neurological issues.

In civil aircraft, the pressurised air is drawn directly from the engine compressors. Inside the engine, there are seals that rely on air pressure to keep synthetic engine oil in place. However, small droplets of oil can leak into the air supply during the flight. Also, engine oil smoke or fumes from burning oil and fuel generated inside the engine, can enter the bleed air supply if a seal breaks, the oil is drained poorly into the compressor, or the oil tank has been over-filled. The hydraulic system fluid reservoir vent is connected to the bleed air system, allowing hydraulic aerosols to enter the cabin air. Bleed air can contain a wide range of toxic substances such as carbon monoxide, some thermally degraded components, nanometals, phosphate and other ultrafine particles.

Some filters do exist, for example, HEPA filters, blocking 99.97% of particles including viruses, but they are completely ineffective against gas. Therefore, CO is able to pass through completely unhindered and once in the cabin it is unmonitored.

In 1906, the first scientific paper about CO poisoning was published, proving that CO blocks the normal delivery of oxygen to the body’s tissue cells, blocking cell metabolism. The authors recognised that CO is highly toxic. In 1980, pilots and flight attendants started to report sicknesses they believed came from breathing polluted airplane cabin air. This condition was called “aerotoxic syndrome” (AS). Since then, scientific  research and aviation professionals have been campaigning to have aerotoxic syndrome recognised. This has not been recognised by the aviation and medical industry, who still disagree over the cause of the disease. The aviation industry has argued over the level of proof required over aerotoxic syndrome, slowing down its recognition. This means that instead of airlines and plane builders having to prove that the air is safe, the burden of proof falls to passengers or crew members. In France, a final court ruling by the Tribunal Judiciaire de Toulon on the 19th of December 2025, officially recognised that illness may result from chronic occupational exposure to contaminated cabin air, even in the absence of a single identified acute fume event. This decision constitutes an important reinforcement of the legal relevance of long-term, cumulative, and invisible exposure scenarios.

One way to prove that engine oil particle enters a passengers’ body during a fume event is to analyse the nano metallic engine debris. When the oil is analysed, one can frame the health of the engine and detect nano metallic “fingerprints”. Those fingerprints can then be found in the brain of victims and also in their body fat, as they are carried by engine oil during fume events, providing the strongest causality link.

A study of the European Union Aviation Safety Agency from 2027 and the reports of the UK Government Committee on Toxicity concluded that the cabin's air quality globally conforms with the standards. This conclusion was based on an analysis of the average quality assessments of cabin air in normal flights. However, a toxic incident cannot, by definition, be considered a usual event.  These reports obscure the reality of critical mechanical failure as fume events are not routine. Poisoning must be viewed as the exception.

Studies show that these toxic events affect only about 1 in 2000 flights in the UK. However, when we look at the reality of air traffic, Heathrow airport registers around 1,300 flights a day, Gatwick additionally, handles over 700. Combined, there are over 2,000 flights a day to and from these two airports alone. Therefore, it is possible that such a toxic event happens daily.

Despite this dangerous possibility, the detection of such toxic gases in a fume event currently relies on the human sense of smell. Cabin crew are expected to report a smell of “dirty socks” or “burnt oil”. Relying on the human nose to detect an odourless, colourless and deadly gas, such as CO, is ineffective and inappropriate.

In 2025, the Civil Aviation Authority (CAA) made it mandatory for certain small piston-engine light aircraft carrying passengers to carry CO detection devices. If we consider it vital to monitor the CO in smaller planes, on what ground do we refuse to offer the same protection and guarantee of safety to hundreds of passengers in a commercial airliner?

The technology already exists: Airbus is already collaborating with the Fraunhofer Institute to develop mass spectrometry systems capable of pinpointing the source of pollutants with extreme precision. The Boeing 787 aircraft uses an innovative bleed-free electrical architecture. Unfortunately, the new Boeing 787 planes only represent 1,175 planes on the approximately 30,000 commercial aircraft in service.

UK legislation must mandate every air transport service provider to install, maintain, and operate continuous active monitoring systems for CO and hydrocarbons, both in the cockpit and passenger cabins in all aircraft.

Sources:

  1. https://travelweekly.co.uk/news/uk-aviation-sector-sets-new-302m-passenger-record
  2. https://www.health.state.mn.us/communities/environment/air/toxins/index.html
  3. Burdon, J., Budnik, L. T., Baur, X., Hageman, G., Howard, C. V., Roig, J., ... & Michaelis, S. (2023). 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(1), 43.
  4. Christophe Suchy, Air Filters Do Not Work: A Scientific Analysis of HVAC Filtration and Alternative Air Purification Strategies, rapport technique, mai 2025, doi:10.13140/RG.2.2.32073.66405.
  5. Nasmith G.G., Graham D.A.L. The haematology of carbon-monoxide poisoning. J. Physiol. 1906;35:32–52. doi: 10.1113/jphysiol.1906.sp001179.
  6. Mawdsley SE. Burden of Proof: The Debate Surrounding Aerotoxic Syndrome. J Contemp Hist. 2022 Oct;57(4):959-974. doi: 10.1177/00220094221074819. Epub 2022 Jan 28. PMID: 36091490; PMCID: PMC9452852.
  7. Susan Michaelis, Jonathan Burdon, and C. Vyvyan Howard, ‘Aerotoxic Syndrome: A New Occupational Disease?’, Public Health Panorama, 3, 2 (2017), 141–356.
  8. Aerospace Medical Association to Public Health Panorama, July 10, 2017.
  9. Mawdsley SE. Burden of Proof: The Debate Surrounding Aerotoxic Syndrome. J Contemp Hist. 2022 Oct;57(4):959-974. doi: 10.1177/00220094221074819. Epub 2022 Jan 28. PMID: 36091490; PMCID: PMC9452852.
  10. Mohamed B Abou-Donia, Coen van der Goot and Michel Mulder, 'Post Mortem Study of a Pilot Exposed to Cabin Air' (2014) The Aviation Herald.
  11. COT aircraft cabin air statement
  12. Nikki Sutherland and Sarah Barber, Debate Pack: Cabin air safety and aerotoxic syndrome (House of Commons Library, CDP 2016-0068, 2016)
  13. Association of flight attendants (AFA), Air quality Campaign / Parliament of the Commonwealth of Australia, Air Safety and Cabin Air Quality in the BAe 146 Aircraft, Report by the
  14. Senate Rural and Regional Affairs and Transport References Committee
  15. caa.co.uk, Regulator mandates carbon monoxide detectors in piston engine aircraft
  16. https://www.aircraft.airbus.com/en/newsroom/news/2021-01-cabin-air-quality-key-to-a-comfortable-flight
  17. https://www.airwaysmag.com/new-post/the-boeing-787-bleedless-air-system