The issue of cabin air quality on commercial aircraft, as well as the possible contamination of that air with fumes and contaminants introduced via bleed air from the engine or auxiliary power unit as part of the pressurization and heating and cooling functions, remains controversial.
Most modern, pressurized commercial aircraft use heated air drawn or “bled” from the engines or auxiliary power unit (APU) for cabin and cockpit air conditioning. As described by the U.S. Federal Aviation Administration (FAA) Civil Aerospace Medical Institute (CAMI) in a 2015 report, “air supplied to a pressurized aircraft cockpit and cabin occurs via an environmental control system (ECS). Fresh air from outside the aircraft, known as replacement or make-up air, enters the ECS in most large transport category commercial airplanes via the aircraft engines. The compressed air is then ‘bled’ through ports and is cooled before being mixed in a manifold with recirculated air, ultimately becoming distributed throughout the cockpit and cabin.”
Fume events or contaminated air quality events (CAQE) may occur as a result of the interaction of incoming make-up air and heated oils or hydraulic fluids from leaking or failed seals in the engine compartment, or from leaks around the APU.
“Factors inside the engine compartment that influence the generation of contaminants include types and amounts of oil and hydraulic fluids, temperature and humidity. Factors inside the aircraft that influence contaminant concentrations include the size of the occupied space and the number of complete air changes per hour (i.e., the volume of make-up air versus the volume of exhausted cabin air),” according to the CAMI report.
FAA and other regulatory agencies have said that fume events are rare. The CAMI report described events arising from the APU or ECS as “extremely rare.” The Australian Civil Aviation Safety Authority‘s Expert Panel on Aircraft Air Quality (EPAAQ) said, in its 2011 report, that “on the available evidence, reported fume events were quite rare, whether expressed on the basis of the number of flights or the number of hours flown.” The EPAAQ report also said, however, that “the panel was unable to find consistent or systematic reporting systems which specifically targeted aircraft incidents being used by Australian or international airlines to enable statistical incidences to be verified.”
Overall Air Quality
Results of a European Union Aviation Safety Agency (EASA) study released in 2017 said that cabin/cockpit air quality is similar to or better than what is observed in normal indoor environments, such as offices, schools and dwellings. The EASA study comprised 69 contaminant measurement flights performed between July 2015 and June 2016. No occupational exposure limits or guidelines were violated during flights conducted to assess air quality, said the study.
“To state the obvious, there is no contaminant-free indoor environment,” the report said. “The aircraft cabin is no exception. However, due to the exceptional high air-exchange rates in aircraft, the cabin air has been proven to be less polluted compared to normal indoor environments. … Volatile contaminations in the cabin are thus depleted quickly. Since the bleed air itself is suspected to be the source of hazard contaminants such as OPC [organophosphorus compounds], a special attention was paid to the detection of TCP [tricresyl phosphate] as a marker for engine oil contamination and the present physical ventilation situation in aircraft.”
In its Medical Manual, the International Air Transport Association (IATA), says “…to summarise the current body of knowledge, aircraft cabin air quality during normal operation is perfectly acceptable and often better than other well accepted indoor environments.”
Although rare fume events, or CAQEs, do occur, and potentially could pose health issues to passengers and crew and safety of flight issues if crew were to become incapacitated.
In a 2018 paper on “Cabin Fumes,” the International Federation of Air Line Pilots’ Associations (IFALPA), said, “When a fume event occurs, cabin air contamination can cause short-term physical effects which may compromise flight safety. Sufficient scientific concern exists requiring more studies in order to determine any short- and long-term effects of fume exposures.”
IATA documentation says that while in most events, there is no impact on crew or passengers, “in some events, crew and/or passengers experience symptoms. … Health effects are usually short lived and resolve over hours to days. These can include Irritation of the eyes, nose, mouth or throat, shortness-of-breath, headache, nausea and abdominal discomfort and tiredness. Some crew who have been involved in CAQEs subsequently experience ongoing ill health and may link their symptoms to the exposure, however the research to date has not established any causative link.”
On its website, the U.K. Civil Aviation Authority says that long-term ill health due to any toxic effect from cabin air is understood to be unlikely, although such a link cannot be ruled out.
IFALPA maintains that it is “unclear” whether fume events cause long-term health effects. It is acknowledged that flight crew are exposed to minimal seal leakage even in normal operations. This may explain why only some of the crew experience symptoms whilst others remain asymptomatic after a fume event. Those whose “cumulative dose” exceeds a certain threshold may experience symptoms. There is concern, as yet unproven, that this may have cumulative long-term health effects. Genetic differences in metabolism may play a role in the cumulative effects.” The Global Cabin Air Quality Executive (GCAQE) is an organization focused on addressing the issue of bleed air contamination. GCAQE, whose members include numerous pilot and flight attendant labor organisations, maintains that possible long-term health effects include numbness in fingers and limbs, memory impairment, headaches, dizziness, nausea, breathing difficulties, chest pain, skin rashes, and weakness and fatigue.
Most parties involved in the ongoing debate support further scientific research into the matter and improved, standardised reporting of the occurrence and response to fume events. There also have been calls for the installation of air quality monitoring equipment on aircraft, compliance with appropriate maintenance procedures to avoid situations such as overfilling of engine and APU oil and to maintain seals, the use of less toxic oils and other fluids, and a move away from bleed air systems.
Labour organisations encourage their members to learn to recognise fume events, and to respond to and report the presence of fumes in the aircraft air supply. Precision in reporting is considered important to helping to identify the source of fumes. These organisations also provide guidance on post-event response and actions to be taken, including coordinated reporting and medical examinations.
In March 2022, legislation known as the Cabin Air Quality Act of 2022, was introduced in the Congress. One of the bill's sponsors said at the time that the legislation is intended to protect commercial airline passengers adn crew from toxic cabin air. The bill, if passed in its original state, would require that pilots, flight attendants and aircraft maintenance technicians receive training on identifying toxic smoke and fumes.It also would require FAA to record and monitor reports of smoke or fume events, authorize FAA to conduct investigations into such events and direct carriers to install and operate onboard detectors and other air quality monitoring equipment.