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24MI11A
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© 2011 Collegium Basilea & AMSI
doi: 10.4024/24MI11A.jbpc.11.04
Journal of Biological Physics and Chemistry 11 (2011) 132–145
Received 11 October 2011; accepted 30 December 2011
132
*E-mail: susan@susanmichaelis.com
1. INTRODUCTION
This paper sets out to answer the question: Is there linkage
between oil fume events and flight safety and adverse
health effects? Note that the term “fume” describes both
visible and invisible fumes or other emanations that may
or may not be detectable using the bodily senses. In
particular, the term does not apply to smoke only.
“Contaminated air in aircraft” refers to the air supply
within aircraft cabins contaminated by synthetic turbine
engine oils, hydraulic fluids or deicing fluids; however, the
focus in this paper is on the synthetic oils. Those
developed after World War II are made up of an ester
base stock (95% of the oil) and an additive package
consisting of engine antiwear agents, most commonly
tricresyl phosphate (TCP) (3%) together with corrosion
inhibitors and antioxidants (1–2%) [1].
2. AWARENESS
The use of engine air compressors from which
compressed air could be bled in quantities suitable to
supply cabin ventilation and conditioning was said to be a
“fortuitous circumstance” in 1946 [2]. As a consequence,
the use of synthetic oils rather than mineral oils was
required due to the higher operating and bearing
temperatures of turbine engines [3, 4]. Oil manufacturers
recognized that turbine engines with higher compression
ratios and more power had forced temperatures of oils
and bearings up, requiring better oil compatibility with
seals if seal leakage was to be minimized [5]. In 1952, the
US National Advisory Committee for Aeronautics
published a report noting that synthetic engine oils were
required to satisfy the requirements of future lubricants of
gas turbine engines [6]. There was concern about the
higher operating temperatures of the newer, more
advanced engines causing thermal degradation of the
lubricants. The report stated that there was “speculation
about probable toxicity and corrosiveness at elevated
temperatures” with these properties having not yet been
adequately studied. The Aero Medical Association in 1953
advised that pyrolysed oil “can contain irritant and
toxic aldehydes and other dangerously toxic products of
incomplete combustion ... even a small degree of bodily
impairment from toxic gases would lead to increased
pilot error and so be hazardous in aviation” [7].
A 1954 United States Air Force (USAF) study [8, 9]
investigated the toxic effects of animals exposed to mists
derived from heated components of synthetic jet engine oil
meeting the MIL-L-7808 standard. The toxicity was
found to come from the breakdown of the principal
ingredient, the base stock di-2-ethylhexyl sebacate. The
mists produced pneumonitis and degenerative changes in
the brain, liver and kidneys. In the case of the esters,
aldehydes, carbonyls, carbon monoxide (CO) and
undecomposed particulate matter were found in the
atmosphere, while in the case of the TCP, free cresols,
undecomposed TCP and CO were found. Fogs formed at
400–550 °F (204–288 °C) were “much less toxic than
those formed at 600 °F (315 °C). The products of
thermal decomposition are much more toxic than the
undecomposed material.” Fatalities were noted to
particularly increase in animals exposed to the mists
generated from the oils exposed to temperatures of 700 °F
(371 °C) compared with over those of 400 °F (204 °C).
A 1967 Esso study undertook further animal
inhalation toxicity testing on oils including Esso Turbo Oil
2380 (still in common use today) that were highly heated,
finding that the cause of death was severe irritation to the
respiratory tract [10]. In 1965 a US Navy study of
hydraulic fluids containing triaryl phosphates including
TCP and other similar compounds found that components
other than just the ortho isomers of TCP appeared to have
significant neurotoxic potential or were capable of
synergizing or potentiating the toxic effects of triaryl
phosphates [11]. A 1995 USAF study found oils containing
TCP heated to high temperatures resulted in changes to the
compounds, resulting in increased neurotoxicity [12].
The awareness of the bearing and lubricant problems
in turbine engines operating at high speeds and
temperatures was a major issue for the military in the
1950s with nontoxicity over the whole temperature range
listed as one of the six lubricant general requirements [13].
Esso reported that the contamination of cabin air with
Contaminated aircraft cabin air
Susan Michaelis*
BRC, Milestone House, 86 Hurst Road, Horsham RH12 2DT, UK
A broad overview of the subject is presented, covering all salient aspects including the
technical history, a discussion of the compounds involved in the contamination, the frequency
of occurrence, a survey of attempts to measure the contamination, safety considerations,
health considerations, and possible technical solutions to the problem of contamination.
Contaminated aircraft cabin air S. Michaelis 133
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JBPC Vol. 11 (2011)
thermally degraded oil fogs introduced the question of
toxic effects from thermal decomposition products [14].
During the 1950s, there was considerable industry
awareness about the critical operation of oil seals used
with a bleed air system. Oil leakage problems associated
with the use of engine oil bearing seals pressurized with
air that are responsive to variations in engine operating
conditions were clearly recognized [15]. Engine oil
bearing seals are used to prevent oil leakage from the
engine bearing chambers into the compressor bleed air
flow. Several different factors must be accounted for in
preventing oil leakage into the air supply.
Firstly the design of bleeding air from the engine
compressor provides a mechanism for oil leakage as a
function of system architecture. Commonly used labyrinth
seals rely on pressurized air taken from the compressor
acting on the external side of the seals to prevent oil
leakage from the bearings. The seal prevents oil leakage
by allowing the air to flow from the outside to the inside of
the bearing chamber. If there is a leakage in the air
system, or if the air system supply is inadequate, the
pressure could be less than that of the oil pressure,
allowing oil to escape and enter the air side of the
engine. The Society of Automotive Engineers (SAE)
reported that [16]:
“It is possible in some designs that lubricating oil
may leak at greater rates when an engine or APU
1 is
started and seals not yet at operational pressure and
temperature or during transient operations such as
acceleration/deceleration. Some systems rely on internal
air pressure to maintain the sealing interface. When
an engine shuts down this interface is opened, possibly
allowing some oil to exit the oil wetted side of the seal.
Upon engine startup, this oil is entrained into the air
entering the compressor of the engine. The seal interface
is again established when the engine internal air
pressure returns to operating norms.”
Secondly, operational factors further explain why it is
known that “all engines leak oil from their seals and
bearings” [17]. Labyrinth seals are known to lose
performance fast when seal wear occurs or during certain
thermal or transient conditions [18]. Carbon seals require
very high surface finish or flatness for minimizing leakage
and have a finite rate of wear [19]. Additional factors
effecting oil leakage include specific maintenance
practices and the operational setup of the airflow. Too
much airflow is a performance penalty, while too little
facilitates leakage. Maintenance failures within the engine
bleed air system and other areas of the environmental
control unit system should also be considered.
There has been wide industry recognition that
increasing compression ratios result in increasing bleed air
extraction temperatures, well above the critical
decomposition temperatures of conventional engine
lubricating oils. There was a concerted effort in early
generation bleed air aircraft to minimize the temperatures
at which the bleed air was extracted, with even failure
conditions restricting exposure to high critical
temperatures to as short a time as possible. There was,
however, recognition that the aircraft being designed in the
1960s for the future would be unlikely to pass the civilian
or military bleed air purity requirements, given the hotter
temperatures involved. At the time it was assumed that
the “rather vague” Federal Aviation Administration
(FAA) regulations (still in use today) would be revised in
the future to become more stringent. This has never
happened to the present day [20, 21].
A 1981 report noted that “some commercially
available lubricants are being stressed to the limits of
the fluid’s capabilities” [22]. The dominant industry
trend with regard to gas turbine engines since the 1970s
had been to increase fuel efficiency, which was in part
accomplished by raising engine operating temperatures,
resulting in higher heat loads on the lubricant, thereby
necessitating oils with greater thermal and oxidative
stability [22, 23]. The ongoing need to increase fuel
efficiencies in more severe operating environments,
including higher operational temperatures, will require
improved ester-based lubricants with increased upper
temperature capabilities; however, this “will require a
careful balance of ester base stocks and improved
additives” [24].
The original synthetic oils were developed using a
“type 1” base stock made from diesters, while in the early
1960s, “type 2” base stocks were developed from polyol
esters, which are more hydrolytically and thermally stable
[23, 25]. Type 2 and 3 polyol ester base stocks generally
consist of pentaerythritol (PE) or PE and trimethylpropane
(TMP) esters. Type 2 base stocks certified to specification
MIL-L-23699 were developed to possess higher load-
carrying characteristics and better oxidative and thermal
stability, therefore reducing engine deposits and foaming,
coking and elastomer swell [22, 26]. Type 2 oils include
Mobil Jet Oil II and BP 2380, commonly used today, while
current latest-generation oils include BP2197. The
temperature capabilities of typical oils such as those
specified to MIL-PRF-23699 range up to 204 °C;
however, driven by the need to develop more fuel-efficient
engines operating in more severe operational environ-
ments, improved ester-based lubricants will have increased
1Auxiliary power unit.
134 S. Michaelis Contaminated aircraft cabin air
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JBPC Vol. 11 (2011)
upper temperature capabilities of 230 °C [24]. Typical
high-stage engine compressor temperatures can range
from 300–650 °C [27] or 450–600 °C [28], or 650 °C for
the B767 at take-off power [29].
3. SUBSTANCES
Synthetic turbine engine oil substances of interest include:
1. Organophosphates (OPs). Jet turbine oils contain
tricresyl phosphate (TCP) as well as other triaryl
phosphates (TAPs) [30]. TCP, practically odourless, is a
mixture of tricresyl phosphate isomer molecules and
other structurally similar compounds. The ortho isomers
have long been known to be potent neurotoxins [30].
Until the late 1950s the toxicity of TCP preparations
had generally been considered to be related to the tri-
ortho-cresyl phosphate (TOCP) content, but it became
apparent around that time that other ortho isomers were
of equal or greater neurotoxic activity [31]. Nevertheless,
the focus for the oil and aviation industry has always been
to minimize TOCP. In 1959, the German scientist
Henschler reported that TOCP was, in fact, the least toxic
of the ortho isomers of TCP, with di-(DOCP) and mono-
(MOCP) being 5 and 10 times more toxic, respectively
[32]. Previously unreported, Mobil advised the Australian
Senate in 2000 that MOCP followed by DOCP were in
their product at far higher quantities than TOCP [33]. The
ortho isomers in the TCP mixture were listed at 0.3%
with the MOCP isomers comprising over 99% of the total
ortho isomers. In 1961 it was determined that the active
metabolite responsible for the toxicity of TOCP was
cresyl saligenin phosphate (CBDP) a very potent inhibitor
of esterases and lipases [34, 35].
While the meta and para isomers of TCP have
virtually always been assumed to be nontoxic, this has
been questioned on numerous occasions. Mobil studies of
TCP were unable to explain the low but consistently
apparent levels of neurotoxicity in “low toxicity” TCPs,
as the substance was derived from 99.1% meta and para
cresol isomers, which “were expected to be completely
inactive” [30]. The toxicity of TCP isomers other than
TOCP were again questioned by Mobil in 1993 based
on the “unexpected high neurotoxic potency of
aviation oils containing 3% TCP” with very low
levels of TOCP [36].
Preliminary studies have shown that dermal
exposure by not only the TOCP isomer, but also the non-
ortho isomers (meta and para) caused sensorimotor
deficits and neuropathological lesions in the brain [37].
Importantly, bioactivated Durad 125 (the TCP formulation
used in gas turbine engine oils) and tri-para-cresyl
phosphate (TPCP) have been found to be inhibitory to all
enzymes tested [38].
TCP with less than 0.1% TOCP has been reported as
a reproductive toxicant [39–42]. In 2009, the French oil
manufacturer NYCO, after undertaking research on 15 dif-
ferent OPs in turbine oils including TCP (which it does not
use), revised its product data sheets to include the risk
phrases R 63.G3 “Possible risk of harm to the unborn
child” and R 62.F3 “Possible risk of impaired fertility” [43].
Despite the research finding TCPs used in other oils to be
toxic, no other oil manufacturers have as yet done this.
By the 1950s and 1960s it was recognized that early
TCP/TXP (tricresyl/trixylyl) production was to be neurotoxic
and, in consequence, the ortho cresol content in the
feedstock was strictly controlled at very low levels [44].
Over 90% of the phosphate ester antiwear additives used
in lubricant manufacture globally are isopropylphenyl
phosphates (IPPP) in Europe and Asia or tertiary
butylphenyl phosphates (TBPP) in North America.
Neither IPPP nor TBPP products contain TCP [45] and
were reputed to have excellent health, safety and
environmental properties, justifying the replacement of TCP
over 40 years ago. But two sectors had TCP-containing
lubricants specified and did not wish to change—military
and aviation [45].
The levels of ortho isomers in the TCP in the engine
oil are technically classified as nonhazardous according to
the OSHA guidelines. However, the guidelines clearly
state that when dealing with possible synergistic effects or
borderline contents of selected substances (such as the
meta and para TCP isomers, which are present in jet oils)
it would be prudent to use caution and reassess the
hazards and risk phrases [1].
2. Amine antioxidants. N-phenyl-α-naphthylamine
(PAN, CAS 90-30-2) is used as an antioxidant at around
1% in lubrication oils, acting as a radical scavenger to
prevent the autooxidation of lubricants. This meets the
regulatory cutoff criteria for classification as an irritant
hazardous substance for skin, eye and mucous membrane.
Additionally, a risk phrase of R43 should be applied: “May
cause sensitization by skin contact.” Two named low-
level PAN contaminants include β-naphthylamine (BNA),
classed as a CAT 1/schedule 1 carcinogen, and phenyl-
β-naphthylamine (PBN) classed as a CAT 3 carcinogen.
PAN at 0.5% in an antirust oil was thought to be
responsible for tumours in exposed workers [46]. PAN in
jet oils meets the hazardous classification levels and
therefore the jet oils must be labelled as hazardous.
3. Base stock. As previously discussed, the base
stocks were long ago reported to be subject to thermal
degradation when exposed to very high temperatures
leading to serious toxicity and severe respiratory irritation.
The higher stage bleed air extraction point (with higher
temperature and pressures) will be used at certain phases
Contaminated aircraft cabin air S. Michaelis 135
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JBPC Vol. 11 (2011)
of engine operation, such as at low power settings when
the engine cannot meet system needs. TMP is reported to
produce respiratory and eye irritation. The base stock is
decomposed to esters and carboxylic acids upon thermal
degradation, which is associated with irritant effects and a
smell described as “dirty” or “old socks” [47, 48].
The material safety data sheet (MSDS) for a typical
jet oil states “Product may decompose at elevated
temperatures ... and give off irritating and/or harmful
(carbon monoxide) gases/vapours/fumes” [49]. As
already stated in the 1954 USAF oil study, “the thermal
decomposition of these products increased their
toxicity considerably” [8]. A growing variety of studies
have more recently confirmed a wide range of thermal
degradation products related to oil heated to high
temperatures [47, 50–53].
Apart from the early 1950s investigation of the
inhalation toxicity of heated synthetic jet oils, there is no
evidence that a thorough, objective examination has taken
place even though the early research already indicated
unacceptability of the air for inhalation. Levels of the ortho
isomers in the TCP have been greatly reduced since the
1940s and 1950s due to changes in production methods [30].
However, neurotoxicity was still identified in Mobil oil
studies in 1988 (with oils produced from 1985), despite
having considerably lower ortho TCP isomer levels. There
is no published inhalation toxicity data available from the
oil companies for the product used in its intended heated,
mixed state. ExxonMobil, when questioned about inhalation
studies with the cold or heated oils, advised that its most
recent (unpublished) oral exposure studies of animals to
the cold product demonstrated that dermal and inhalation
exposure testing was unnecessary as oral exposure could
be considered to maximize exposure and these tests
showed no neurotoxicity [54, 55].
Aviation synthetic jet engine oils are manufactured to
meet very strict military specifications [24]. Until recently
civilian oils were required to meet the same specifications
(e.g., MIL-PRF-23699 and its successors). However, now
civilian oils are required to be certified to meet a new
civilian standard SAE 5780, which is now the only
specification recognized by regulatory authorities such as
the European Aviation Safety Agency (EASA) and the
FAA. Oils developed prior to 2006 to MIL-PRF-23699
were automatically deemed to meet the new civilian standard.
MIL-PRF-23699 states: “S 3.6 Toxicity. The
lubricating oil shall have no adverse effect on the
health of personnel when used for its intended
purpose” [56], but the SAE AS 5780 specification
requires only that the substances in the oils must comply
with all “legal, environmental, toxicological and
regulatory requirements of the countries in which the
products are manufactured and sold” [57].
In 2004 ExxonMobil was issued with a $1700 citation
for inappropriate labelling and product information of its
engine oils by the US Occupational Health and Safety
Administration (OSHA) [58]. This was based on a
complaint by a large labour union representing aircrew
after the adverse neurological warnings “prolonged or
repeated breathing of oil mist or prolonged or repeated
skin contact can cause nervous system effects. Avoid
prolonged or repeated overexposure to the skin or
lungs” were removed from the oil cans and product
information [59]. However ExxonMobil contested the
citation and the penalty with OSHA, settling the case a
year later [60], and the labelling was not restored.
4. FREQUENCY
Rolls Royce has reported that one of three major causes
of engine oil loss is the “loss of liquid oil arising from
permissible leakage past certain seals ... made good
by “topping up” the system with fresh oil ...” [19]. It
has long been accepted that the majority of contaminated
air events are related to oil leaking into the cabin air
supply. [1, 61, 62] EASA reports that [63]: “the vast
majority” of fume or smoke events “are associated
with an abnormal leakage of engine or APU
lubrication fluid (aviation engine oil)”; and “Under
certain fault conditions (e.g. engine or APU oil seal or
bearing failure, engine or APU maintenance error/
irregularities, or design deficiency), engine or APU
oil, hydraulic fluid, fuel, de-icing fluid and the
corresponding pyrolysis products may contaminate the
bleed air, which then enters the cabin air supply and
can be inhaled by the aeroplane occupants.”
Underreporting of contaminated air events has been
widely accepted as occurring [1, 64–66]. EASA has
recently stated that fume events it regards as “more
minor” in nature and those viewed as a “nuisance”
mostly perceived as temporary bad smells “could
probably happen more often than the rare serious
events, and the Agency agrees it is possible that they
are underreported” [67]. The regulatory databases are
unreliable [1] and the regulators such as EASA are not
correctly interpreting the regulations, which require all
suspected contaminated air events to be reported [67–69].
If EASA does not interpret the regulations as required
under the European Directive, there is little hope that other
regulators, airlines, manufacturers or pilots will do so.
There is a variety of other reasons why contaminated
air event fail to be reported as required. The USAF
recognized the difficulty associated with getting pilots to
report matters related to adverse effects, when they have
“a profession, hobby or aircraft investment to
136 S. Michaelis Contaminated aircraft cabin air
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JBPC Vol. 11 (2011)
protect” [70]. Lack of education and awareness about
the health and flight safety implications of exposure to
contaminated air is another factor along with job security.
In Europe, there is additionally a terminology problem, with
“fumes” signifying visible smoke only, when in fact they
refer to airborne particles that may or may not be visible
(dispersed nanoparticles would be invisible), and with or
without a noticeable odour. Hence, the problem has gone
on for decades unresolved, with people affected differently.
The UK Committee of Toxicity stated that, based on
information supplied by UK airlines, contaminated air
(“fume”) events were reported by the pilots in 1% of
flights, while a confirmed engineering source reported in
0.05% of flights [71]. There is a clear contradiction how
fume events are viewed within the aviation industry.
Leaking oil at transient engine operation settings and the
issue of engine oil seal bearings providing an incomplete
seal over the whole engine operating range, provides the
basis for frequent low level oil leakage—it is part of the
engine operation process. However, varying interpretations
are provided from within the aviation industry of what
constitutes an oil seal leakage or a contaminated air event,
with many suggesting that only a major malfunction will
lead to a contaminated air event or that only “Dense
visible fumes or concentrations of toxic products
sufficient to incapacitate crew or passengers” need to
be reported [67]. Oil seal leaks apart from failure are seen
as normal and generally accepted and ignored, with bleed
air contamination being far more common than the
aviation industry acknowledges. The true extent of
contaminated air events cannot be known as there are no
detection systems in aircraft and the aviation industry is
relying upon a reporting system that is not working.
Internationally, there are a number of reporting
regulations that are required to be met. Any defect that
the captain becomes aware of must be reported in the
aircraft technical log [72]. Toxic and noxious fumes
(visible and invisible) are required to be reported to the
aviation regulator under the mandatory reporting
occurrence scheme [73]. Additionally, “Any person
involved who has knowledge of the occurrence of an
accident or serious incident shall notify without delay
the competent safety investigation authority of the
State of Occurrence” [74]. As such, EU member states
require pilots to report defects in the aircraft technical log;
contaminated air suspected to be related to oil leakage is
required to be reported to the regulator as a mandatory
reportable occurrence and noxious fumes or toxic air
requiring the use of oxygen should be reported to the BFU
as a serious incident. Underreporting is, however, a
systemic industry-wide problem [1].
5. SAFETY
Exposure to contaminated air is without doubt a flight
safety issue. Firstly, the regulations indicate this is the
case. The EASA airworthiness regulation CS 25.831
requires that each crew compartment has enough fresh air
to enable crew members to perform their duties without
undue discomfort and fatigue and that crew and
passenger compartments must be free of harmful and
hazardous concentrations of gases or vapours. Therefore,
the extensive evidence of adverse health effects
suspected to be related to contaminated air in flight
indicates a breach of the airworthiness regulations and
safety of the flight. There is, additionally, a variety of
European regulations and specifications that can be
related to aircraft contaminated air and which must be
met at the design stage and on an ongoing continuing
airworthiness basis [75, 76]. EASA CS 25 1309 requires
aircraft systems and components to be designed so that the
probability of occurrence of any major failure condition
(causing physical discomfort for flight crew) is assessed
as “remote”; that is, less than 10–5 per flight hour. EASA
CS E510 requires a safety analysis of the engine,
including the compressor bleed systems. The analysis
must show that hazardous engine effects, which include
toxic products of engine bleed air, are predicted to occur
at a rate not in excess of that defined as “extremely
remote” (i.e., less than 10–7 per engine flight hour).
Hazardous engine effects include “concentration of
toxic products in the engine bleed air sufficient to
incapacitate crew or passengers, no effective means
to prevent flow of toxic products to crew or passenger
compartments” and “degradation of oil leaking into
the compressor air flow.” Major engine effects must be
predicted to occur at a rate not in excess of that defined as
“remote” (< 10–5 per engine flight hour) and include
“concentration of toxic products in the engine bleed
air sufficient to degrade crew performance.” Similar
regulations apply to the APU under CS APU-210.
Continuing airworthiness requirements necessitate that
aircraft are maintained throughout their operating life in
the condition to which they have been certified [75, 76].
Contaminated air events are occurring at a far higher
rate than acceptable under the design standards, indicating
the regulations are not being met. EASA (unreliably)
suggests that minor contaminated air events may be
occurring at a rate of 1 in 10
000 (10–4) based on FAA
figures, as well as 1 in 10–6 (1 in 1
000
000), while the UK
COT suggested contaminated air occurrences being
reported at 1 in 100 flights (10–2) [71]. The underreporting
problem and the design and operational practice of using
bleed air, allowing smaller oil leakage to occur on a frequent
or even continuous basis, also supports the fact that the
Contaminated aircraft cabin air S. Michaelis 137
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JBPC Vol. 11 (2011)
safety analysis requirements and regulations relating to
bleed air are not being met.
In 2002 the FAA reported that “No present
airplane design fulfills the intent of 25.831 because no
airplane design incorporates an air contaminant
monitoring system to ensure that the air provided to the
occupants is free of hazardous contaminants” [77]. The
EASA and FAA airworthiness regulations CS and FAR
25.831b require that it can be demonstrated in flight that
the air is free of harmful and hazardous levels of
contaminants, yet at present there is only the human nose.
The UK Air Accidents Investigation Branch (AAIB) in
2007 and again in 2009 recommended that EASA and the
FAA consider requiring sensors be installed in the flight
deck to detect smoke or oil mist from the air conditioning
system, yet this recommendation was ignored on both
occasions [78, 79]. EASA regulation (CS 25.1309 c) requires
that “information concerning unsafe system operating
conditions must be provided to the crew to enable them to
take appropriate corrective action. A warning indication
must be provided if immediate corrective action is
required.” Therefore it is necessary for EASA and FAA
to recommend detection systems be developed and
installed in order to make the aircraft properly airworthy
and to significantly improve safety.
A review of contaminated air event records indicates
that 32% mentioned some degree of crew impairment
(ranging from minor to full incapacitation) [1], despite
crew impairment not being directly listed as reportable
under the mandatory reporting system. 20% of contaminated
air events involved impairment to at least one pilot, with
9% of events involving adverse effects in both pilots [1].
An AAIB review of the UK CAA database found that 40
of 153 (26%) reports (of which 119 resulted “probably”
from contaminated air) showed “adverse physiological
effects on one or both pilots, in some cases severe” [78].
There are a growing number of air accident bureau
investigations and other reports clearly showing that
exposure to contaminated air is a safety hazard. In 2006,
the Swiss Aircraft Accident Investigation Bureau
attributed a serious incident on an Avro RJ100 (i.e., the
BAe 146) to the cockpit filling with fumes on approach,
which “caused a toxic effect leading to a limited
capability of acting of the co-pilot.” [80]. “The fumes
were caused by an oil leak as a result of bearing
damage in engine no 1. The indicators for impending
bearing damage were not correctly interpreted before
the incident.” The smell and fumes in the cockpit
occurred even before this serious incident and the aircraft
was released for further service several times before the
event took place despite the defect not having been
rectified. The captain did not use his oxygen mask, while
medical examination of the co-pilot showed that a toxic
event had taken place. Similar events have been clearly
and frequently identified elsewhere, internationally [1].
In 2006 a German-registered Embraer 145 departed
the runway at Nuremberg after landing with visibility
reduced to 20 cm due to smoke. Subsequent engine
inspection found engine oil entered the air supply after a
significant bearing failure (BFU report 5X008-0/06).
In 1999 two pilots of a Swedish airliner suffered
temporary incapacitation apparently related to exposure to
contaminated air [81]. The investigation after the event
[52] positively identified a wide variety of contaminants,
including TCP and engine oil pyrolysis products, along with
oil leaks identified upon inspection. The air accident report
[81], however, stated that the event was due to “probable
polluted cabin air”. In the absence of sensors monitoring
contamination, a more precise result can scarcely be
expected, even though circumstantial evidence strongly
suggests a “fume event” due to an oil leak.
ICAO Annex 13 advises that a serious incident, which
must be investigated, includes the use of emergency
oxygen. Aircraft emergency checklists (in some cases)
list the use of emergency oxygen when contaminated air
events are suspected or when smoke occurs, yet not all
checklists require its use or make it the initial action. The
UK CAA recommends that airlines ensure that flight
crew are trained to immediately don oxygen masks if
smoke or other fumes are suspected [82]. However, pilots
are generally failing to use oxygen during contaminated
events, with data indicating both pilots using oxygen in 9%
of suspected contaminated air events and one pilot in only
4% of cases [1]. In the European situation, pilots will
generally fail to use oxygen as required by some of the
aircraft emergency procedures checklists as most will not
consider that there is a need to use oxygen for fumes
unless visible smoke is present, given the terminology
problem associated with the word “fumes”. Passengers are
not provided with oxygen to protect against contaminated
air exposures and do not need to be advised an event has
taken place—this is up to the airline to determine [83].
There are many admissions that exposure to
contaminated air is a flight safety issue [1]. A few of the
many recognizing this fact include: The FAA acknowledges
that exposure to oil fumes can cause impairment of flight
deck crew and impair flight safety, that it is an unsafe
condition and that oil leakage is a design flaw [84 ,85];
Rolls Royce (Germany) reported that oil leaking from an
engine entering the air supply was hazardous [86]; The
German Luftfahrt-Bundesamt (LBA) in 2003 reported
that “oil leakage ... and oil residues ... may lead to
harmful contamination of the cabin air and cause
intoxication of the flight crew” [87]. The German
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Parliament, when asked if it believed inhalation of heated
engine oil fumes was harmless to crew and passengers,
simply said: “No!” [88]. EASA CS E 690 concerns the
suitability of compressor engine bleed air for direct use in
the aircraft cabin pressurization or ventilation system and
requires “tests to determine the purity of the air
supply.” To date this has not occurred independently to an
acceptable standard. Boeing’s synthetic jet oil material
safety data sheet for MIL-PRF-23699 reports that [89]:
•This material is classified as hazardous.
•This material may cause irritation of the skin and eyes
•Inhalation: Respiratory irritant, particularly if
vapors are from heated or burning liquid.
•Signs and symptoms of exposure: exposure may
cause irritation, characterized by tears, redness and
burning sensation (eyes), redness, swelling or
cracking of the skin, or burning sensation in the
nose, throat and lungs (inhalation). Neurotoxicity
may be characterized by dizziness, headache,
confusion and intoxication.
In reality flight safety concerns include a variety of
factors including that “complete incapacitation could
result in loss of the aircraft, while even modest
impairment can reduce the crew’s ability to deal with
adverse operating conditions or high workload
phases of flight ... crew incapacitation or performance
degradation may potentially be aggravated by
chronic low dose or prior contaminant exposure
events” [16].
6. MONITORING
A range of studies reviewing air quality generally have
been undertaken within the aviation industry suggesting
that the substances found are within set government
standards or guidelines. One of the most recent is the
Cranfield University air quality study [90, 91]. Where
contaminated bleed air substances leak into the cabin air
supply, people will be exposed to the contaminants and
there is the potential for subsequent adverse effects in
flight and for health problems to arise. Evidence is
available to show this is not infrequent.
A close review was carried out of the 53 air quality
studies [1]. Of these, 33 (62%) of the studies were
undertaken specifically to look at bleed air contamination,
while 20 (38%) assessed general air quality standards only
and did not use suitable techniques to detect bleed air
contaminants such as oil. Effectively none of the studies
were undertaken during a contaminated air event. Of the
contaminated air studies no epidemiological studies were
undertaken at the time of the monitoring, with follow-up
epidemiology of a very limited nature undertaken in 15%
(5) of the studies. 27% of the specific contaminated air
studies (all with a strong industry affiliation) suggested the
air quality was acceptable. Of the general air quality
studies that were not using techniques suitable to detect
contaminated air, 60% deemed the air was acceptable,
again with all having a strong industry affiliation. Such
conclusions have repeatedly been used to suggest all air
quality is acceptable, even covering contaminating
substances not sought by the studies.
TCP was identified in 16 (48%) of the contaminated
air studies, while oil was identified as the source or part of
the problem in 21 (60%) of them. TCP was more recently
reported (in the Cranfield University study) even during
normal operations [90].
Recent Norwegian aircraft monitoring studies under
normal conditions (no contaminated air events) using
specific methods to detect TCP found low levels of TCP
in 4% of air samples, in 39% of swab samples and in all
HEPA filter samples [92]. The research revealed that
TCP detected during ground testing in an aircraft that
experienced leakage of turbine oil into the cabin and
cockpit air supply was “substantially higher” than during
normal operations, indicating organophosphate contamina-
tion is of relevance to contaminated air events. Traditional
volatile organic compound (VOC) measurements were
clearly identified as less suitable than tailored organophos-
phate (OP) measurements for oil aerosol/vapour sampling
[92, 93]. Aviation technicians and loaders were also identified
as at risk to OP exposure from oils and hydraulic fluids.
While ground-based exposure standards such as
European OSHA occupational exposure limits and the
American Conference of Governmental Industrial Hygienists
(ACGIH) threshold limit values (TLVs) are very often
used to suggest substances found in aircraft cabin air studies
are safe and below government-set levels, these standards
should not be utilized in the aircraft environment [1]. They
are not intended for use by the public, they protect
“nearly all” workers from a limited number of individual
substances only and establish a clear demarcation between
safe and dangerous exposures [94]. The standards will not
protect workers from adverse physiological effects above
5000 feet [95] and exposure standards should only be
applied in environments where the atmospheric pressure
is between 900 and 1100 mbar [96]. 900 mbar equates to
just under 3000 feet. The levels to which most commercial
aircraft cabins are pressurized is between 6000 and 8000 feet
(810–750 mbar). Exposure standards do not take into account
the unique environment of the aircraft cabin [97–99].
Environmental toxins are generally encountered in
complex mixtures but most will have been tested
individually only, ignoring additive, synergistic and other
emerging challenges to conventional toxicology [100].
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Furthermore, the type of combined action or interaction
found at toxic effect levels may not predict what will
happen at low levels [101].
TCP has not been assigned an occupational exposure
limit. In 1958 Henschler reported that the toxicity of the
total TCP mixture was far greater than TOCP alone with
meta and para isomers implying the presence of the more
toxic mono- and di-ortho isomers, which were respectively
10 and 5 times more toxic than TOCP [32]. It was
therefore no longer permissible to relate an analysed
proportion of ortho-cresol to tri-ortho-cresol phosphate,
with the “old method of calculation” being “invalid”
and “the term TOCP poisoning should no longer be
used.” Mobil advised that the adequacy of the TOCP
exposure standard ought to be questioned, given the
greater toxicity of the mixed isomers of TCP, recognizing
that OSHA may have incorrectly used the TOCP standard
to cover TCP as a whole [102]. The Australian Defence
Forces have suggested that an exposure standard for TCP
as a whole should be 100 times less than the present
standard for TOCP [103]. However, this still does not take
into account that exposure standards should not be applied
in aircraft cabins and that they do not take account of the
heated synergistic effects of the combined exposures.
Back in 1966 Esso recognized (correctly) that the mineral
oil exposure standard of 5 mg/m3 did not apply to synthetic
jet oils, for which no standard had been set; however, as
there was no standard the mineral oil standard could be
used [104]. In other words, the adoption of the mineral oil
exposure standard for polyol ester-based synthetic oils
was deemed (incorrectly) to be acceptable given that no
synthetic oil exposure standard was ever adopted [105].
The misuse of the mineral oil standard is commonplace
within the aviation industry.
The monitoring studies undertaken cannot be used to
suggest that air quality is acceptable and therefore
unrelated to adverse health effects on aircraft occupants.
The analysis and interpretation of substances found
relying upon ground-based industrial exposure limits cannot
be appropriated to suggest all levels found are safe. There
is a growing body of data available indicating exposure to
contaminated air can have adverse health effects.
7. ADVERSE HEALTH EFFECTS
Exposure to turbine engine oils including TCP have
repeatedly been connected with a range of short term
effects including skin, eye and respiratory irritation along
with neurotoxicity [89] and with a range of “dangerously
toxic” substances from pyrolysed oil [7]. The oils contain
irritants, sensitizers and neurotoxins [1, 106]. Irritation to
the mucous membranes and chemical pneumonitis was
reported by the USAF in 1954 along with degenerative
changes of the brain [8]. According to a 2004 UK Civil
Aviation Authority (CAA) study, the “symptoms of
irritation could be induced by short chain organic acids
formed during pyrolysis of aircraft lubricants” [47].
Decomposition products of the base stock were listed as
causing “severe irritation of eyes and throat and can
cause eye and lung injury. Cannot be tolerated even at
low concentrations” [47, 107].
TCP is listed as “toxic by inhalation, ingestion or
by absorption through the skin, with symptoms of
exposure including: irritation of the skin and eyes,
flaccid paralysis without anesthesia, motor activity
changes and muscle weakness. It may cause respiratory
tract and mucous membrane irritation. It may also
cause serious damage of the nervous and digestive
systems and muscular pain. Other symptoms include
gastrointestinal upset, discomfort in distal portions of
the arms and legs, soreness, aching, numbness,
headache, vertigo, loss of appetite, parethesias and
decrease of strength in the arms and legs. It may cause
vomiting, diarrhea and abdominal pain ... exposure
may also lead to tingling sensations of the hands and
feet and cramps” [108].
Inhalation of PAN is reported to cause short-term
effects including: blue lips, skin or fingernails, confusion,
convulsions, dizziness, headache, nausea and unconscious-
ness. Repeated or prolonged contact may cause skin
sensitization [109].
Three case study surveys were undertaken over 10
years by Michaelis [1]. An extensive 4 year case study was
undertaken for BAe 146 pilots in the UK on a non-self-
selected basis. Of the 274 past and present pilots in the
survey, 238 consisted of working pilots with the remainder
no longer retaining medical certification. Identifiable
trends of pilots being unwilling to talk about contaminated
air were evident; health effects are effectively denied by
the airline industry and indicate operation contrary to
aviation legislation—despite the high degree of awareness
of exposure to contaminated air in the workplace,
acknowledged as predominantly originating from oil by
aircraft manufacturers, the CAA and others. 63% of the
pilots advised they had experienced adverse effects
consistent with occupational factors (the work environment).
32% reported medium- to long-term effects and 44%
reported immediate or short-term effects. 13% of those
surveyed were no longer able to maintain pilot medical
certification, were retired with a consistent pattern of
long-term ill health or deceased for reasons considered
relevant to the study.
There was a clear pattern of adverse effects,
including a range of neuropsychological, neurological,
respiratory, cardiovascular and gastrointestinal irritancy
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JBPC Vol. 11 (2011)
and general symptoms reported in the immediate and
short-term aftermath of putative exposure with a clear
development into the medium or longer term for a
considerable number of those reporting specific
symptoms. A key chronic ill health pattern was also
identified in a smaller subset.
For example, the main immediate or short-term
symptoms were upper airway irritation and breathing
problems (17%) and eye irritation and vision problems
(10%); neuropsychological symptoms reported include
performance decrement (13%), intense headaches (11%),
memory impairment (10%), dizziness (10%), confusion
(8%), fatigue and exhaustion (15%) and nausea (11%).
These represent a considerable risk to flight safety.
In the longer term, the main symptoms reported were:
upper airway and respiratory symptoms (17%);
cardiovascular symptoms (10%) such as palpitations,
altered heart rate and chest pain; skin irritation, rash or
blisters (8%); memory impairment (14%); performance
decrement (11%); intense headaches (8%); tingling in the
extremities and other peripheral nerve problems (8%);
exhaustion and fatigue (9%); chronic fatigue (10%); and
others, including the development of chemical sensitivity.
Of the 13% (36) of pilots no longer able to fly due to
chronic ill health the symptoms reported (substantiated
with diagnosis) included: neuropsychological (64%);
neurological (53%); general (53%); respiratory (39%);
and cardiovascular (25%). The rate of permanent ill
health or loss of flying ability or both found in this study
ranged between 37% and 433% higher than the published
rate of loss of pilot medical certification within the civil and
military aviation industry for all reasons [1].
The majority of affected pilots associated their
symptoms with exposure to contaminated air, while all
pilots surveyed are acknowledged to be operating in a
contaminated air environment by the aircraft manufacturer.
There was sufficient commonality between the
symptoms seen in the surveys and similar patterns noted
internationally to support a symptom basis for aerotoxic
syndrome, a distinctive occupational syndrome. The close
temporal relationship between exposure and ill health was
supported by an extensive exposure history, industry
documentation and, in the medium- to longer-term cases,
medical records with all three case studies supported by
other published studies. Features of this syndrome are that
it is associated with aircrew exposure at altitude to
atmospheric contaminants from engine oil or other aircraft
fluids, temporarily juxtaposed with the development of a
consistent symptomology of irritancy, sensitivity and
neurotoxicity. These symptoms may be reversible
following brief exposures; however, following repeat
exposures a longer-term irreversible pattern develops,
consisting of neuropsychological, neurological, respiratory/
cardiovascular effects along with immune system effects,
chemical sensitivity and chronic fatigue. Passengers are
exposed to the same air, with adverse effects of a similar
nature reported in a number of cases [1].
A recently published case study describes 87 smoke/
fume events over two years with one US airline that likely
or definitely involved cabin occupant exposure to oil or
hydraulic fluid fumes [110], reported on 47 aircraft and on
every aircraft type in the fleet. Most events were
characterized only by an unusual odour (most commonly
described as “dirty socks”) without any visible smoke or
haze. Still, after 27 of the flights, one or more crew
members had symptoms serious enough to require
emergency medical care, after 43 flights one or more
crew members required additional medical care, and after
37 flights one or more crew members lost work time due
to prolonged illness. Although the odours were reported
prior to take-off on 44 of the flights, only 20 of those flights
were either cancelled or delayed, while the rest flew to
their planned destinations, many with crew health and
potential flight safety consequences. Mechanical records
confirmed that oil would have contaminated the air supply
on 41 of the 87 flights. After 30 flights, no mechanical
cause was identified but oil was suspected as the cause
based on the event characteristics. Some of the pilots and
cabin crew in this dataset developed chronic neurological
symptoms post-flight, including two pilots who lost their
FAA medical licence to fly because of ongoing
neurological symptoms after in-flight exposure to oil
fumes that they did not smell and had not been trained to
associate with their acute symptoms.
Contamination of the cabin air sufficient to cause
symptoms of irritation, fatigue, toxicity or discomfort
indicates that the aviation airworthiness ventilation
legislation FAR/CS 25.831 a/b is not being met. Therefore
the aircraft is not airworthy. In 2000, BAe Systems
advised the Australian Senate that [17]:
“There is absolutely no doubt in our minds that
there is a general health issue here. The number of
people who have symptoms indicates that there is a
general issue ... it is very clear that there is an issue
here ... it is a health and safety issue, it is not a
safety issue. With the weight of human evidence
and suffering, which is quite clear, there must be
something there.”
In 2000, a year-long Australian Senate inquiry into
cabin air contamination found that contaminated bleed air
was occurring with no real possibility to eradicate it totally
while air was drawn in via the engines [65]. The problem
was occurring on aircraft types other than just the BAe
146; health problems were occurring that appeared linked
Contaminated aircraft cabin air S. Michaelis 141
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to the leaking oil and there was serious underreporting. Oil
leakage into the air supply clearly contravened civil
aviation regulations, rendering the aircraft unfit to fly until
the defects were fixed, with a clear link between crew
health and flight safety—given that almost any
impairment can have an impact on the crew’s capacity to
operate an aircraft. In 2010 the Australian High Court
upheld an earlier decision that stated, with regard to a
flight attendant claiming adverse effects from exposure to
leaking jet engine oils, that: “Smoke from pyrolysed oil
can be hazardous to the eyes, mucous membranes and
lungs” [111]. In 2008, an FAA-funded medical document
was published providing a protocol for health care
providers on how to deal with exposure to bleed air
contaminants for aircrew and passengers [112].
The health effects being reported should be reviewed
with some urgency given some recent findings, including:
exposure to jet oil fumes being linked to organophosphorus-
induced chronic neurotoxicity (OPICN) [37, 113]; TOCP
detected in airline passengers not exposed to fumes and
with no immediate symptoms but slow neurodegeneration
[114, 115]; and techniques being developed for exploiting
biomarkers for exposure to jet engine oil triaryl phosphates
[116]. Note that less toxic triaryl phosphates have recently
been identified [38, 43].
8. PROBLEM AREAS
Contaminated air is an airworthiness and flight safety
issue. The very extensive amount of data available [1]
indicates that the aviation industry has, in effect, viewed
air free of oil contamination not as a mandatory
requirement, although the industry has known for decades
that air was being contaminated from leaking heated
turbine engine oil. There was considerable awareness in
the 1950s and 1960s that this was a serious problem with
significant toxicity hazards and flight safety at stake;
however, this was pushed aside in the 1970s as fuel
efficiency and engine/aircraft performance took priority.
From the 1970s until the late 1990s there was almost total
denial that contaminated air posed a problem; there is now
an almost industry-wide concerted effort to marginalize
and control how the contaminated air issue is addressed.
In 2000 a UK House of Lords inquiry, reviewing
cabin air contamination, called for research to “refute”
the “common allegations” and inspire public confidence
regarding aircraft air quality [117]. This resulted in a wide
variety of government- and industry-funded projects that
have, in fact, dealt not at all or inappropriately with the
issue. The European Commission (EU) and the European
aviation industry have spent almost 60 million euros
(shared roughly equally) since 2001 on these projects
[118]. As an example, the EU CabinAir study, which ran
from 2001 to 2004, failed to use techniques to monitor
contaminated cabin air, nor indeed did it intend to, however
the findings were used to suggest cabin air quality was
suitable for human inhalation. The study was used as the
basis of the European cabin air quality standard EN 4618,
which fails to address cabin air contaminated by oils and
hydraulic fluids [119]. A further joint EU-industry-funded
study, the Ideal Cabin Environment (ICE), formed the
basis of a further air quality standard (prEN 4666), still in
draft stage [120, 121], which relies on EN 4618 and in
effect totally ignores oil contamination.
Both EASA and the FAA, and other regulators, have
thus far avoided responding to the call for contaminated air
detection systems to be fitted to aircraft and the common-
sense need for bleed air filtration to be developed and
introduced, despite a growing number of authoritive bodies
calling for this, including the American Society of Heating,
Refrigerating, and Air Conditioning Engineers (ASHRAE),
the National Research Council (NRC), AAIB, the Australian
and US Senates, the SAE and Defence Force Australia.
Both the FAA and EASA have stated they are waiting for
the outcomes of industry and Government research, such
as the Cranfield University study [90], the FAA-funded
Airliner Cabin Environment Research (ACER) and the
Occupational Health Research Consortium in Aviation
(OHRCA) and ASHRAE work. This is despite the fact
that contaminated air is an airworthiness issue that must
be addressed for an aircraft to be deemed airworthy, a
fact ignored by the FAA and EASA. Industry has moved
from a fragmented approach to a powerful coalition that
ignores or manipulates external data and works towards a
solution agreeable to its partners, in a manner previously
seen with the tobacco and asbestos industries. Effectively
all data that implies there is a problem has been brushed
aside in favour of commissioning yet more research that
has reached the stage of going around in circles, and
ignoring the fact that the toxicity of heated jet engine oil
was already recognized in 1954.
9. SOLUTIONS
Some of the practical solutions include: inhalation
toxicity research into heated synthetic turbine oils,
including triaryl phosphate additives, and modern research
into the thermal degradation (pyrolysis) of ester base
stocks; biomonitoring techniques for assessing human
exposure to contaminated air; better designed engine and
APU oil seals; bleed air systems that do not allow oil to
leak; introduction of real time monitoring systems and
filtration technology for bleed air; research into health
effects related to contaminated cabin air; less toxic oils;
correct labelling of the oil product information; better
engineering practices, especially concerning engine
142 S. Michaelis Contaminated aircraft cabin air
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JBPC Vol. 11 (2011)
maintenance; adherence to existing aviation and
occupational health and safety (OH&S) legislation
(including education on the recognition of, and requirement
to report, all suspected contaminated air exposures); the
requirement for pilots to use oxygen and emergency
procedures when crew suspect contaminated air;
appropriate medical procedures implemented for crew
and passengers exposed to contaminated air; all future
aircraft to be designed to no longer use bleed air.
Furthermore, a full scale epidemiological study into
contaminated air exposure would be very timely.
Every worker has the right to working conditions
which respect his or her health, safety and dignity. It is not
acceptable to knowingly expose crew and passengers to
contaminated bleed air, to risk their health and safety and
to allow such exposures to degrade a person’s right to live
with dignity, respect and freedom, which ill health and
jeopardized flight safety can take away. It is a human right
to breathe clean air.
There is a large volume of clear and convincing
evidence that there is a link between cabin air
contamination by leaking synthetic turbine oils and
subsequent adverse health and compromised flight safety.
To ignore this evidence with assertions that there is none or
no link is highly reprehensible. The failure of the industry
to react appropriately to this volume of evidence is
indicative of “manufacturing uncertainty” to delay
regulation [122].
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