Slow progress in taking action to protect crews and passengers from the effects of toxic air contamination in aircraft is causing many in the aviation industry to fume.

Contaminated air events in the UK Civil Aviation Authority's database of mandatory occurrence reports from mid-1985 to July 2006 number nearly 1,800. A few were reports of smoke that may have had an electrical source, but most involved a mist or vapour containing heated engine oil constituents introduced via the engine or auxiliary power unit compressors that deliver outside air to the aircraft's air conditioning packs.

That is 85 reported events a year in one country - and then there are the unreported incidents. Many pilots say fume events are not worth reporting unless they are severe and they estimate the unreported incidents probably exceed the reported events - although the CAA says it has no evidence to support that.

The US Federal Aviation Administration, on the other hand, concluded that under-reporting of fume events is endemic after studying air-contamination events. At a conference on smoke in the cockpit in 2006, the FAA's director of flight standards Jim Ballough said: "FAA data analysis indicates numerous [smoke/fumes] events not being reported."

APU Start-Up

The vast majority of the CAA's mandatory occurrence reports involved BAe 146s or Boeing 757s, but 18 other jet or turboprop types were also included in reports, with events on Airbus A320s, Boeing 737s and Embraer ERJ-145s the most common. The BAe 146 incidents frequently occur on the ground after APU start-up, according to the reports.

DHL 757 
 © Paul Dopson

In 2002 the CAA asked the FAA to ensure Boeing action to eliminate cabin air contamination on 757s. The US manufacturer said that 757s with Rolls-Royce RB211-535C engines "appear to have a higher incidence of events than expected". These were originally owned by British Airways, and some are now flown by freight operator DHL. Boeing points out that events occur on other types also.

Former BAe 146 training captain John Hoyte says: "I flew the 146 for about 16 years. The worst time I remember for experiencing contaminated air was on the ground from the APU before the first flight of the day, when the engine was started from cold. Frequently we would have the cabin full of blue smoke as a result of the air system overcooking this meant the whole crew was affected by these visible fumes, but maybe only for 10-15min. No detecting device has ever been used to verify exactly what the blue smoke represents. Clearly the strength is there and I don't think many people would dispute the undesirability of this frequent occurrence." Hoyte, who submitted this description as part of a statement filed to the CAA and a number of other official bodies, has stopped flying because of ill health.

Exploratory Tests

The UK Department for Transport is now running exploratory tests to decide the best methods for identifying chemicals present in cabin air contamination events. The DfT tests target specific BAe 146 and 757 airframes in current airline service which have a history of reported fume events. These trials aree deemed necessary despite the fact that, in 2000, an Australian Senate inquiry concluded that toxic fumes generated by organophosphates from heated engine oil additives had not only threatened safety by disabling or incapacitating pilots and cabin crew, but had caused chronic sickness in some flight and cabin crew. Possible effects on passengers have not been studied. Mobil Jet Oil II is one lubricant identified as a source of toxic chemicals found in the cabins of the now-defunct Australian carrier Ansett, whose BAe 146s were studied by the Senate. In addition, the Royal Australian Air Force quickly acknowledged there was a problem after experiencing similar events with aircraft such as the Lockheed Martin C-130 Hercules and the General Dynamics F-111.

BAe 146
 © LGphotos

The UK DfT initially showed signs of being selective in sourcing the data on which it bases its terms of reference. It originally ignored a major reference document, "The Aviation Contaminated Air Reference Manual" (ACARM) that has been favourably reviewed by aviation medical specialist Dr Bhupi Singh, associate professor and head of research at the RAAF Institute of Aviation Medicine.

Delivering his verdict on the 815-page manual - the only single-source document that brings together referenced data on all known events, studies and inquiries on aero-toxicity - Singh says: "The events data pertain to a wide range of aircraft operators in many countries. All data is extensively researched and referenced, with its source clearly identified." Singh, one of those who acknowledged and studied the contaminated air problem in the RAAF, adds: "A notable element of the data is the widespread prevalence of denial of the existence of the problem, particularly among aircraft operators and aviation regulators."

Meanwhile, Prof Clement Furlong, research professor of genetics and medicine at the University of Washington, Seattle, who presented a paper on "Organophosphates: the effect on pilots and passengers" at the Flight International Crew Management Conference in Brussels in December, says he is close to establishing conclusively the neurological effects on the brain and nervous system of specific cocktails of organophosphates found in cabin air contamination events.

Organisations Rebuffed

Other organisations that offered evidence to the DfT investigation were also initially rebuffed. This selectiveness may be because of the vast quantity of data available, but another reason is the report on cabin air by the UK government Committee on Toxicity (COT), which alleges there is no proof that oil-contaminated air has been, or could be, the cause of chronic illness in flight and cabin crew. The Global Cabin Air Quality Executive (GCAQE), which represents pilots and cabin crew whose health has suffered from toxic air on aircraft, offered evidence to the COT, but it was refused, as was the ACARM.

Previously it had been established by a UK House of Lords inquiry that cabin air contaminants are a result of leaking engine oil seals releasing oil into the hot compressed air bled off to the cabin air conditioning system. In this process the oil, and specifically some of the synthetic anti-wear oil additives, which include a toxic organophosphate known as tri-cresyl phosphate (TCP), undergoes thermal decomposition (pyrolysis) into a range of substances such as volatile organic compounds, low molecular weight organic acids, esters, ketones and TCP isomers.

Yet the COT report insists there is no proof that TCP has been present in any of the documented cases. If the COT had checked the ACARM it would have known how many cases of TCP detection in cabins were documented, and where to find the evidence. More recently, the first tests in the DfT trials detected the presence of TCP during flights. The GCAQE says TCP is a critical component of the brain-affecting cocktail of organophosphates found in bleed air during a fumes incident, but it is still added to oil because it is an effective anti-wear additive that improves engine reliability.

French oil company Nyco makes a jet engine lubricant called Turbonycoil 600 that uses alternatives to TCP because of the latter's known toxicity. This oil is used by the US Navy and is approved for use on engines that power Airbus and Boeing aircraft.

Early Findings

Contaminated bleed air events were recorded almost immediately the test programme began, say early DfT reports. If contamination events were as rare as some allege, such early success would have been unlikely.

Chris Winder, professor in applied toxicology at the University of New South Wales, Australia, is critical of the DfT's planned sample-gathering techniques, but concedes that "even these flawed tests" have established the presence of TCP in the BAe 146 and the 757. Winder says collecting air samples for later analysis is not scientifically effective for "non-volatile mists". He says the only effective method is active, real-time analysis of the suspended chemicals and their concentration using a "direct reading machine on the aircraft during flight". But Winder concedes: "Overall, limited though these results may be, this study did identify the presence of tri-cresyl phosphate on aircraft, something that continues to be strenuously denied by sectors of the aviation industry."

Meanwhile in 2004 the RAAF's Singh pointed out that judging aviation air contamination using toxicity standards that apply in normal workplaces is invalid: "Aircrew members perform complex tasks requiring high-level cognitive skills, which may be much more sensitive to insult by hazardous contaminants in the smoke/fumes, such as tri-cresyl phosphate."

Crew Incapacitated

On 5 November 2000 a Jersey European Airways (now Flybe) BAe 146 (G-JEAK) crew only just managed to land safely at Birmingham after being badly incapacitated by a fumes event. The UK Air Accidents Investigation Branch (AAIB) report on the event says bleed air contamination was only a "possible" cause of the incapacitation, yet does not propose any alternative. In its conclusions, the report says: "The regulations JAR 25.831, JAR-APU-210, JAR-E-510 and JAR-E-690 all deal with unacceptable levels of contamination of the bleed air, but do not provide details of toxic contamination that is deemed as unacceptable."

Referring to a history of toxic air events in other aircraft types, the AAIB says: "TCP and TOCP [tri-cresyl orthophosphate] were detected in samples during research, but it was deemed that, at the measured levels, it was inconceivable that toxic effects on the occupants of aircraft would result." It adds: "There was a lack of general information available on potential contaminants of the bleed air by engine oil, and their effects on human physiology."

There are two available solutions to the bleed air contamination problem, but neither is officially under consideration: one is to use engine oil that does not contain TCP the other is to filter bleed air to remove the toxic organophosphates (see box and diagram). Most aircraft have high-efficiency particulate air filters (HEPA) that process recirculated cabin air, but the bleed air is continuously introduced into the cabin without filtration. HEPA filters take out only dust, bacteria and viruses but are not designed to eliminate organophosphates.

Considering that nobody denies contaminated air events occur, that there is a massive body of evidence they can be dangerously toxic, and that commercial off-the-shelf solutions for preventing them exist, it is remarkable that nothing is being done to protect crew and passengers while scientists identify the precise nature of the toxins and the harm they do. Meanwhile, some future aircraft will obtain outside air from systems other than engine or APU bleeds: the 787 will be the first.

Preventing Fumes Reaching The Cabin

There is, according to Capt Susan Michaelis, former BAe 146 captain, adviser to the Australian Senate Committee and the compiler of the ACARM , an alternative to industrial dissembling over the degree to which contaminated cabin air can be proven to incapacitate people: to prevent fumes getting into the cabin. This can be done by filtering the bleed air before it reaches the cabin environmental control system, or filtering it within the ECS before it mixes with recirculated air (see diagram). US-based Pall Aerospace says it can supply bleed air filters that would eliminate organophosphates from bleed air.


Aircraft Environmental Control System Schematic

Who Is Studying Toxic Cabin Air?

A large number of agencies, medical organisations, government bodies and aircraft/engine manufacturers in many countries have recorded and analysed - in some detail - multiple cabin air contamination events. The cause, in all cases cited here, involved the contamination of engine or auxiliary power unit compressor bleed air being used to provide outside air to the aircraft air conditioning and pressurisation system.

Knowledge of this pecise issue, and its source, dates to 1953 when the Blakiston, USA-based Aero Medical Association Committee of Aviation Toxicology voiced concerns about the toxicity of components in engine oils and hydraulic fluids. Since then large numbers of incident investigations and studies have confirmed the concern as valid.

Despite the passage of 55 years, there remain two areas in which some parties are demanding further scientific research: the first need is to measure and record accurately the range of chemicals released into the cabin via the bleed air the physical properties of the contamination (mist, vapour etc) the concentration and the precise cocktail of contaminants, measured in real time, to which aircraft occupants can be/have been exposed. The second requirement is to provide clinical evidence of the physiological and neurological effect of that cocktail of chemicals on humans exposed to it.

The Australian Senate, the UK House of Lords, the UK government Committee on Toxicity, and the Royal Australian Air Force have concluded the issue is real and must be investigated in detail.

Agencies involved in recently initiated research include:

  • The UK Department for Transport, which is attempting to identify the chemicals involved by collecting samples on active aircraft.
  • The Norwegian National Institute of Occupational Health, which is not only aiming to confirm the chemicals present, but also to test for their neurological effects on employees.
  • The Civil Aviation Safety Authority of Australia, which is setting up an independent committee of experts "to review potential crew and passenger health issues related to aircraft cabin air quality".
  • The Occupational Health Research Consortium in Aviation (OHRCA), a US FAA-funded group, states its objectives: "The specific objectives of this project include: to collect, review and summarise the available medical evidence provided by crewmembers that have reported exposure incident(s) in order to develop standardised medical evaluation protocols for both acute and chronic effects to develop air quality health surveillance intruments and systems for crewmembers (and eventually passengers) to report incident-related health and exposure conditions during and after flights to validate the VN sampler (the portable, cost-effective air sampling device) developed to measure semi-volatile and non-volatile chemical constituents in aircraft to design a full surveillance study of cabin crew that will establish the relationship between air quality and health effects."
  • The Air Transportation Center of Excellence for Airliner Cabin Environment works in association with OHRCA, backed by the Harvard School of Public Health and the University of California Berkeley.
  • Global Cabin Air Quality Executive, commissioned to undertake a critical review of the UK Committee on Toxicity report on cabin air contamination.  

What They Say About Contamination


Society of Automotive Engineers: "Engine compressor bearings upstream of the bleed air ports are the most likely source of lube oil entry into the engine air system and thence into the bleed system contaminating the cabin/cockpit air conditioning system."

Engine manufacturers

AlliedSignal (now Honeywell), whose LF502/LF507 engines power the BAe 146/Avro RJ series of aircraft: "Several BAe 146 aircraft are having reports of objectionable odours Very little work has been done in the aviation industry to pinpoint the chemical compounds causing such odoursthe odour appears to be coming from breakdown products of the oil."

Rolls-Royce, whose engines power the Boeing 757 versions that the airframe manufacturer says suffered the largest proportion of air contamination incidents: "In the majority of instances where cabin air contamination was a problem, it was mostly associated with small leakages of synthetic lubricant from bearing seals, and so on." (R-R discussion paper on limits for organic material in cabin air.)

Aircraft Manufacturers

BAE Systems, manufacturer of the BAe 146 and the derived Avro RJ Series of aircraft: "Every engine leaks oil from its seals and bearings" (Official record of evidence presented to the Australian Senate inquiry into cabin contamination.)

BAE service information leaflet 21-45: "Troubleshooting - source of contamination of the engine/auxiliary power unit: operator experience indicates the most likely sources are oil contamination of the engine/APU bleed air."

BAE service bulletin 21-150: "In the past, oil leaks and cabin/flight deck odours and fumes may have come to be regarded as a nuisance rather than a potential flight safety issue."


Ansett Australia: "The source of the odours has been identified as Mobil Jet Oil II leaking past oil seals in the engines and/or APU into the air conditioning system."


Despite crew incapacitation events, the Civil Aviation Safety Authority (CASA) of Australia said: "Oil fumes are more of a health problem than an aircraft technical defect as not all pilots are affected and there is no mandate [for CASA] to look at health." (CASA to the Occupational Health and Safety [OH&S] magazine in 2003.)

UK Civil Aviation Authority: "Although the exact cause of crew incapacitation is not known, the most probable source is oil leaking from the engines or APU and contaminating the air supply to the cabin and cockpit through the air conditioning system." (CAA Flight Operations department communication 21/2002, referring to incidents in which pilots and cabin crew have become incapacitated following cabin air contamination.)

Accident Investigators

Australian Transport Safety Bureau (ATSB): "The investigation found that smoke and fume contamination of cabin air is neither a new phenomenon nor a particularly rare event and that, over time, it has been experienced in many aeroplane types."

Medical Scientists

"Smoke and fumes in the cockpit is not a rare event and a clear threat to flight safety due to acute toxic effects." (Raymand and McNaughton, Aviation, Space and Environmental Medicine, 1983.)

"This compound [TCP] is is toxic by inhalation, ingestion or by absorbtion through the skin. It is also an irritant of the mucous membranes and respiratory tract. When heated to decomposition it emits toxic fumes of phosphorus oxides." (National Toxicology Programme chemical repository data for TCP.)


British Airline Pilots Association (BALPA): "Smoke and fumes in the cockpit: the first actionshould be for the flightcrew to don their oxygen masks and establish communications. If during flight it appears that both pilots are suffering from some form of incapacitation or that one pilot appears to be incapacitated for no obvious reason, then the crew should don oxygen masks without delay.

"Before seeking the opinion of specialist consultants, members should first contact BALPA's legal department so that advice can be taken from solicitors. We cannot advise members more strongly that such consultations will become part of their medical records and, as such, will be disclosed in any future legal dispute or loss of licence claim. It is therefore vitally important that caution is exercised from an early stage with regard to medical investigation to avoid prejudice to any claim at a later date."

All this data can be viewed at source, but it is also assembled with much other expert material in the Aviation Contaminated Air Reference Manual


Source: Flight International