is aircraft cabin air toxic?

TheConversation: Following the death of commercial airline pilot Richard Westgate, an inquest was launched into the cause of his death. Westgate had previously claimed that his health problems were caused by exposure to on board toxic chemicals and a subsequent coroner’s report raised concerns with the pilot’s employers, British Airways, and the Civil Aviation Authority (CAA), regarding the health effect of aircraft cabin air on aircraft occupants. Both organisations have responded to the report and stress that they take the matter of cabin air quality very seriously but that scientific evidence had not established a risk to ill-health. The investigation is ongoing and the inquest has not yet been heard.

This hasn’t stopped at least 17 former and serving cabin crew from seeking legal action against various British airlines for ill health they allege was caused by pollutants in cabin air.
Whether their claims are successful remains to be seen, but at least one precedent has been set: in 2010 a former flight attendant in Australia was awarded compensation for respiratory damage sustained as a result of exposure to on-board toxic chemicals.

What are these toxic chemicals?

Airline crew have been reporting ill-health following exposure to contaminated air for many years. Most commonly reported symptoms are: irritation to the eyes, nose and throat; headaches, light-headedness and dizziness; fatigue, weakness; generally feeling unwell; confusion and difficulties in concentration.
While passengers have occasionally complained of similar symptoms, this is a much rarer occurrence (unsurprising perhaps because of the relative infrequency of travel compared to aircrew). Symptoms may be the result of exposure to the organophosphate known as tricresyl phosphate (TCP); a flame retardant additive in jet engine oil and hydraulic fluids. As well as acting as an irritant, these substances are a type of neurotoxic compound, and can interfere with nervous system functions, resulting in cognitive, emotional and behavioural problems.
Since their use as nerve gas agents in World War II, it is known that organophosphates can cause ill health and death in high doses. However, controversy still surrounds whether low levels are actually harmful. Establishing a link between exposure and chronic ill-health lies in the difficulty we have with establishing an accurate estimation of exposure. The absence of routine air quality monitoring on commercial aircraft make it impossible to determine what chemicals enter the cabin and in what quantities.
In addition, relying on self-reported measures of exposure is notoriously problematic as they depend on memory and a capacity to detect noxious substances (both of which vary enormously in reliability). So, before a causal relationship can be determined, our understanding of how exposure might occur, and the level of this exposure, needs to be improved.

How would exposure occur?

Air is supplied throughout the aircraft to allow crew and passengers to breathe. The human body is used to breathing in air of around 15°C, at a pressure of 14.7 pounds per square inch, or psi, (at sea level). However, at an altitude of 35,000 feet the air pressure is only 3.46 psi with temperatures lower than -50°C, so fresh air is pumped into the plane from outside the aircraft but only after it is warmed and pressurised to a safely breathable level.
As part of the propulsion process, aeroplane engines heat and compress air before fuel is added and combusted. On most aircraft this air is then “bled off” and pumped into the aircraft, unfiltered. Ordinarily this process is relatively safe. But occasionally faulty seals can result in contamination by allowing heated and broken down engine oil fumes to escape into the airflow.
The incidence of these “fume events” is difficult to quantify, as commercial aircraft are generally not fitted with equipment for monitoring on-board air quality. There is also significant under-reporting of exposure: for example, the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment estimated that fume events occur on about 0.05% of flights.
However, in 2003, a survey of pilots belonging to the British Airline Pilots Association suggested that up to 96% of contaminated air events may go unreported. Collectively, the pilots in the survey claimed to have experienced more than 1,674 smoke or fume events but only 61 instances were formally reported to the CAA, possibly due to lack of awareness, commercial pressure and the perception that exposure to such contaminants is normal and part of their everyday job.

Oxygen masks for all? 'mikkahoo/flickr', CC BY-SA

To better establish the incidence of fume events, the UK Department for Transport commissioned Cranfield University to carry out an air monitoring study of affected aircraft types. They monitored 100 flights, measuring the levels of several chemical compounds that were present in the cabin during different stages of flight. A number of chemicals were detected over the course of this study, including TCP and carbon monoxide.
All levels were reported to be within safe limits – regardless of an absence of aircraft safety standards regarding TCP. While reassuring for routine flight safety, no fume events were observed in this small sample size due to the relative rarity of cabin air contamination. The study reinforced the lack of clarity around possible exposure that aircrew and passengers may face during a fume event.

 

So what evidence is there?

Without accurate measures of exposure, it is very difficult to reliably determine whether there is a relationship between ill-health and exposure to fume events, though there is some supporting evidence. For example, symptom surveys from 2003, and case studies of exposed passengers and crew (where subsequent mechanical inspections of the aircraft confirmed oil leaks had occurred) have demonstrated signs of ill-health consistent with organophosphate exposure.
In 2013, biological markers of possible neurotoxicity were found in flight crew with these symptom profiles. And in 2012, a neuropsychological study found that a group of airline pilots had a specific pattern of cognitive impairments similar to that seen in organophosphate-exposed farmers. While these studies may provide evidence consistent with exposure, it is still very difficult to claim causation, mainly because of small sample sizes.
It seems there is still a large amount of scientific uncertainty regarding the long-term effects of inhaling pyrolysed engine oil on human health. However, with the growing pressure from lobbyists (such as the Aerotoxic Association), potential legal suits and inquests, we may soon have a more definitive answer to this question.
The European Aviation Safety Agency has launched a preliminary cabin air quality measurement campaign, which should allow for the development of instruments that may be able to monitor air quality in real time. In addition, biomarkers for exposure to TCP are being developed by researchers from the universities of Washington and Nebraska.
Given these advancements, we may not be far away from establishing a valid and reliable measurement of exposure. And with that a better answer on whether there is a clear link between ill health and exposure.