Experts have highlighted the health dangers posed by engine oils and hydraulic fluids in aeroplanes. Bette Browne reports
Last month, more than 200,000 travellers were affected by delays and cancellations when a significant outage hit the UK’s air traffic control system. This outage forced the cancellation of more than 1,200 flights, causing the worst flight disruption in the UK since the Icelandic volcano in 2010.
It was a dramatic example of the potential inconveniences of airports and aeroplanes with which we are all familiar.
Now added to these issues have come serious warnings from health and science experts on the dangers posed when cabins become contaminated by engine oils and hydraulic fluids.
Fume events
The warnings were issued by a team of experts from nine countries. The International Fume Events Taskforce was formed six years ago to investigate the impact of cabin fumes. The taskforce of 17 professionals includes doctors, occupational health specialists, toxicologists, epidemiologists, and aviation experts.
Most commercial aircraft flying today produce compressed air in the engine. Excess air is redirected to systems elsewhere on the airplane, including the cabin. This feature is known as a bleed system. A fume event occurs when bleed air is contaminated by fluids such as engine oil, hydraulic fluid, de-icing fluids, and other potentially hazardous chemicals.
Exposure to the fumes is reported to cause dizziness, fatigue and impairments in both cognitive ability and short-term memory. Exposure has also been associated with cardiac, neurological, and respiratory complaints.
According to the US Federal Aviation Administration (FAA), such fume events are uncommon. However, a Los Angeles Times investigation in 2020 cited a FAA-funded study by Kansas State University which found the rate of these events occurred in one in every 5,000 flights.
The medical protocol and the review of the health threats posed for passengers and crew was published by the international taskforce on 16 May in the journal Environmental Health. It was led by global aviation health researcher and former pilot Dr Susan Michaelis (PhD), who is based in the University of Stirling, Scotland.
“There are no sensors on board transport aircraft and therefore no contaminants are collected at time of exposure,” according to the taskforce. “Cabin air monitoring studies have identified the presence of low concentrations of individual bleed air contaminants that are well below published chemical exposure limits during normal (non-incident) flights.
“However, few measurements have been undertaken during documented fume events, ground-based exposure limits were not developed for application at altitude or for complex heated mixtures, and the focus has been on individual substances rather than the complex thermally degraded mixtures.”
New guidance
The taskforce has called on the aviation industry to do more to protect passengers and crew from cabin fumes and issued new medical guidance dealing with potential exposure. “There is an urgent need for a consistent, internationally accepted medical protocol to facilitate the recognition of health effects associated with fume exposure in aircraft cabins and cockpits,” according to the taskforce.
The experts pointed out that oil and hydraulic fumes in the air supply contain ultrafine particles, numerous volatile organic hydrocarbons, and thermally degraded products. They warned that “inhalation of these potentially toxic fumes is increasingly recognised to cause acute and long-term neurological, respiratory, cardiological, and other symptoms”.
“Cumulative exposure to regular small doses of toxic fumes is potentially damaging to health and may be exacerbated by a single higher-level exposure. Assessment is complex because of the limitations of considering the toxicity of individual substances in complex heated mixtures.”
Some efforts have been made to develop guidance material for collation of medical data for the assessment of symptomatic air crew, according to the experts. However, they pointed out that this has rarely been undertaken in a “timely, systematic or comprehensive manner”.
According to the taskforce, the European Committee for Standardisation issued a technical report in November 2022 recommending medical monitoring at the commencement of air crew employment and for air crew and passengers after fume events using a best practice medical protocol.
Fume events were first described in military aircraft in the 1950s and coincided with the introduction of synthetic jet engine oils used in high performance turbine engines. There have since been a number of reports that describe air crew with acute symptoms, followed by chronic neurological, cardiological, respiratory symptoms, and other health impacts, according to the experts.
Long-term intractable cough, breathing difficulties, and central and peripheral nervous system complaints, are the common features of cases affected by a contaminated fume event, the taskforce stated. However, it cautioned that while symptoms are short-lived in many cases, for others they might take much longer to resolve.
“In some cases, particularly those who have experienced more than one fume event, symptoms can continue for months or years and, occasionally, full recovery never occurs.”
There is also the danger of misdiagnosis. This can occur as healthcare professionals may not be sufficiently aware of the complexity of the problem. “People suffering from the effects of aircraft fume events are commonly misdiagnosed as being anxious, stressed or experiencing other clinical complaints.”
The brain can also be a target organ for toxicity, with the central nervous system particularly vulnerable. “A major reason why the brain is a susceptible target organ is that nerve cells must last for a lifetime and are terminally differentiated and cannot, as with most other tissues, repair by cell proliferation. Central nervous system effects, reported after exposure to fume events include general incapacity, temporary paralysis, impaired or loss of consciousness, headache, nausea, dysarthria, balance problems, ataxia, cognitive impairment, tunnel or double vision, dilated pupils, nystagmus, and sleep problems, with early symptoms being flu-like.”
Oil and hydraulic fluids
The protocol formulated by the taskforce focuses on health problems caused by oil and hydraulic fluids rather than other types of fumes, including those from electrical devices, fan failures, exhausts, and de-icing fluids. This is because the health impacts resulting from oil and hydraulic fluids are more serious.
It is described as the first medical protocol of its kind to help treat those affected by contamination of the aircraft cabin breathing air supply and includes comprehensive data on instances of such contamination.
Air crew and passengers are exposed to what the researchers say is chronic background low-levels of engine oils and hydraulic fluids leaking into the aircraft air supply during every flight. They can also experience adverse effects from more irregular “fume events”, which mark incidents when there is a noticeable level of contaminants in the cabin.
“This has been happening for the last 70 years and reports of air crew becoming unwell continue to rise,” Dr Michaelis stated in an article on the University of Stirling’s website. She is critical of how air crew and passengers are treated and the lack of guidance for doctors and for those in the aviation industry. “Currently, when air crew or passengers become unwell, whether they are still on the plane, suffer symptoms in the days or weeks to come, or report illness in the years that follow, there’s nothing in the medical books, there’s no guidance material for the aviation industry or medical professionals and very often they get turned away or are given minimal testing.
“All of the data and evidence collected strongly suggests a causal connection between the contaminants from the oils and hydraulic fluids and people becoming unwell. This is the first comprehensive and systematic approach for documenting and gathering further epidemiological data in what is a discreet and emerging occupational health syndrome.”
The study included findings from reports of fume events and documented instances of adverse health effects, which the taskforce said have accumulated over numerous decades across countries and regions.
Prof Andrew Watterson of the University of Stirling’s Faculty of Health Sciences and Sport described the study as groundbreaking. “This is a globally important and ground-breaking study using a narrative review of a significant and complex problem for those exposed to aircraft cabin air supply fumes that result in a range of often serious adverse effects.”
A striking feature of the outcome of fume events is the difference in response observed between passengers and air crew, the taskforce noted. “While few passengers appear to suffer more than symptoms of irritation following a fume event, air crew frequently become systemically unwell and need medical attention. This differential response suggests that the pre-exposure to thousands of hours of low-dose inhalation of engine bleed air increases the vulnerability of air crew to acute higher dosage during a fume event.”
Aerotoxic syndrome
The term ‘aerotoxic syndrome’ relates to the health effects associated with breathing contaminated air in an airline cabin. The study said the syndrome is not yet widely understood by healthcare professionals. Aerotoxic syndrome is the umbrella term for a wide range of symptoms experienced by passengers and cabin crew when they have been exposed to harmful levels of contaminants in cabin air.
“The aviation industry recognises that some people experience acute symptoms following a fume event, although debate continues as to whether exposure to contaminated cabin air can cause long-term symptoms,” the taskforce said. “Our understanding of the clinical, toxicological, and pathological issues that underpin the various presentations of the aerotoxic syndrome is progressively improving, particularly over the last 20 years.”
They added: “Symptoms depend on the intensity and duration of exposure, exposure conditions, repeated exposures over time versus a single exposure, and the duration of the individual’s service in the industry. Clinical factors such as diet, smoking and alcohol use, age, co-morbidities, concurrent medication, genetically impaired enzyme detoxification, and reproductive status also play a role. The total accumulated dose over time is a key factor. Symptoms may be prompted by a single high exposure, repeated or prolonged low-level exposures.”
Initial presenting complaints are commonly and consistently described as “foggy thinking, dizziness, recognising an odour in the cabin (commonly described as a ‘dirty socks’ smell), impaired short-term memory and cognitive thinking, fatigue, headache, nausea, tremor, balance impairment, incoordination, breathing difficulties, chest pain, cough, eye, nose, sinus, and throat irritation.”
Reported neuro-behavioural and neuro-psychological health effects associated with fume events include disorientation, dizziness, confusion, lethargy, altered behaviour, personality changes, anxiety, depression, euphoria, and difficulties with problem solving, concentration, memory, and writing.
Consensus approach
In stressing the need for the new medical treatment protocol, the taskforce said a systematic and consistent process should be put in place for diagnosis and treatment of people who have been exposed to toxic fumes in aircraft cabins.
“The medical protocol presented in this paper has been written by internationally recognised experts and presents a consensus approach to the recognition, investigation, and management of persons suffering from the toxic effects of inhaling thermally degraded engine oil and other fluids contaminating the air conditioning systems in aircraft, and includes actions and investigations for in-flight, immediately post-flight, and late subsequent follow up.”
The experts said they had developed what they described as “the most reasonable protocol for observation, measurement and recording of symptoms, signs and treatment (if any), and subsequent management of affected persons and their health outcomes.”
Work on the protocol was conducted by various working groups with the expert panel, the taskforce said. The experts warned that lack of training and awareness, lack of comprehensive reporting regulations, under-reporting, and the lack of contaminated air detection systems further hinder the full understanding of the frequency of these exposures and fume events. They stated the medical protocol is “an expert-based work-in-progress” that will be updated as the field progresses.
“Ideally, the employer or airline should facilitate the investigations recommended in this protocol. However, in the interim, or if there is an inability to undertake the investigations, symptomatic air crew or passengers should be sent to the emergency room at the nearest hospital. Undertaking all the examinations and special investigations suggested may not be possible nor medically indicated in every case.
“Some investigations require specialist laboratories, and there will be practical issues of availability, timing, and cost for procedures and tests. Requests for special investigations should be based on clinical indication in each individual case.”
Despite these caveats, the taskforce strongly recommended that as much data as possible is collected. “Initial medical assessments may also be required for asymptomatic, or relatively asymptomatic persons, especially air crew who have been exposed to fumes in an aircraft cabin. In fact, air crew should not be cleared back to fly without an assessment.
“This may also be important in the conduct of clinical and epidemiological studies in order to address why some air crew react poorly to fumes in the aircraft cabin while others do not. The decision to what extent medical assessment should be undertaken after a fume event, should take into account a variety of factors, including potential exposure to hazardous substances and the duty of care, symptoms may arise at the time of the event, soon after, or on a prospective basis.”
The taskforce stressed that urgent action is needed and that there has been an awareness of the problem for a number of years. Bleed and cabin air monitoring studies have been undertaken over recent decades. In 2020 the European Cockpit Association (ECA), representing some 40,000 European pilots including those based Ireland, stressed that “cabin air contamination is a known problem that can cause serious short- and long-term health effects which may compromise flight safety”.
The Association called for “a clear standardised medical protocol”. It said there was a “need for action by operators and manufacturers based on the current knowledge and technology”. In the long-term, the ECA called for bleed-free aircraft design. It also highlighted the importance of developing further technology for detection and filtration and advocated for more studies of medical effects on crews affected by fume events. Research and development of less toxic chemicals was also stressed.
The ECA said there should be a “basic education for air crews and companies on the nature, effect, and symptoms of fume events, as well as awareness and safety management training, (along with) improved and harmonised operating procedures for smoke, fire, and fume events including post-event guidance”.
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