ROSPA-Working-Life-Nov-2011-V2

Working Life 

Breathe 

easy 

➤ Earlier this year, researchers from Cranfield University published the results 

of their DfT commissioned study which set out to examine whether cabin air on aircraft is 

contaminated with toxic fumes from the plane’s engines. But were their findings as reassuring 

as they first appear Nick Cook investigates. 

There is now over fifty years of 

concern that harmful engine 

fumes leak into the cabins 

and flight decks of jet aircraft and 

poison pilots, cabin crew and even 

passengers. In some cases the fumes 

have threatened the safety of aircraft 

by incapacitating the pilot, and many 

pilots and aircrew blame longer-term 

health effects for forcing them into 

early retirement. 

Previous articles published in this journal 

briefly outline the background to the cabin 

fume issue 1, 2 while two publications 3, 4 by 

Dr Susan Michaelis detail the health concerns 

in much greater detail. Susan herself was 

an airline pilot but claims exposure to cabin 

fumes forced her into retirement at the age 

of 35 because of ill health. 

While nobody now denies that fume leaks 

do occur, the aircraft industry has dismissed 

the evidence for health effects as largely 

anecdotal. They point out that no measurements 

have revealed exposures which breach 

legal limits. These claims are hard to refute, 

but only because of the lack of serious investigation. 

There have been no epidemiological 

studies on the link between cabin fumes 

and ill health, and few scientific attempts to 

monitor for contaminants during flight. 

But recently a study 5, 6 by Cranfield University 

has been successful in highlighting the 

potential for exposure. Flight deck measurements 

on 100 airline flights provide convincing 

evidence of engine fume contamination. 

To quote just one example, the report lists 14 

flights where triorthocresylphosphate (TOCP) 

was detected 7 . TOCP is a toxic oil additive 

component that attacks the nervous system. 

Nor is TOCP the only organophoshate to 

contaminate flight deck air. Altogether, the 

report found organophosphates on 73 of 

the 100 flights. Organophosphates have 

long been linked to neurotoxic effects, for ➤ 

The RoSPA Occupational Safety & Health Journal November 2011 9


example, following exposure during 

sheep dipping. 

Unfortunately the quantitative aspects 

of the study were less successful. Shortcomings 

include failure to measure a major 

leak (fume event) and failure to analyse for 

three of the most hazardous compounds. 

These and other failures are due to flaws in 

the design and execution of the study. This 

article will examine these flaws. But first 

it might be useful to briefly consider the 

background to the problem. 

Most jet airliners supply their flight decks 

and cabins with outside air that comes in 

through their engines. The engines heat 

and compress the air, saving the cost of a 

separate unit. But although this arrangement 

saves money, it also comes with an obvious 

risk: contamination. The air bled from the 

engines (bleed air) can contain oil, hydraulic 

fluid, antifreeze and a complex mixture of 

their pyrolysis products. 

Leaks of these contaminants into the 

bleed air are termed ‘fume events’. These 

occur when oil seals in the engine fail and/ 

or when the engine ingests hydraulic fluid 

or antifreeze. Oil seal failures can be due to 

wear and are triggered by changes in the 

engine’s power level (transients). Overfilling 

the oil reservoir during servicing is also 

believed to contribute to fume events. 

For many years now pressure has been 

increasing on governments and the airline 

Box 1. Court success for 

fume victims 

Two successful claims may well 

herald a flood of litigation: 

l On 1 April 2010 the New South Wales 

Court of Appeal awarded flight 

attendant Joanne Turner $129,000 

in damages. In making this award 

the court accepted that fume leaking 

from the auxiliary power unit during 

a 1992 flight from Sydney to Brisbane 

had caused Mrs Turner long-term 

respiratory damage. 

l On Wednesday 5 October 2011 Boeing 

agreed an undisclosed out-of-court 

settlement with former American 

Airlines flight attendant Terry Williams. 

She blamed faulty aircraft design for 

her exposure to toxic fumes. These 

fumes had resulted in long-term 

tremors, memory loss and severe 

headaches. During the course of the 

trial it emerged that Boeing had been 

concerned about the health effects of 

fume leaks since 1953. 

industry to investigate fume events. (Two 

recent landmark compensation awards 

are ominous for the industry and will add 

even more to this pressure (see Box 1). The 

floodgates of litigation may be ready to burst!) 

The Cranfield study itself was triggered by 

concerns raised by the British Airline Pilots 

Association (BALPA) in 2005. BALPA said it 

was concerned that acute and chronic illnesses 

were being induced in pilots and cabin crew 

(and passengers) through the inhalation 

of engine oil and hydraulic fluid, additives 

present in these products and pyrolysis 

products that may be emitted from the 

engines and auxiliary power unit (APU) into 

the air conditioning system of certain aircraft 

types during air contamination incidents. 

The Committee on Toxicity (CoT) reviewed 

BALPA’s concerns at the request of the 

Department for Transport (DfT) and, 

although generally sceptical about the 

health risks from contaminated cabin air, 

the committee did concede that an association 

between fume events and acute health 

symptoms was “plausible 8 ”. The DfT duly 

commissioned the Cranfield study. 

The study 

The study began in 2007 and issued Parts 

1 and 2 of its report in March and April 

2011 respectively. Flight deck monitoring 

for airborne contamination was carried out 

using the following techniques: 

l Air sampling for a specified range of 

volatile organic compounds (VOCs) and 

semi volatile organic compounds (SVOCs) 

using a TSI Model SP730 air sampling 

pump to draw air through a stainless 

steel tube containing an absorbent 

material (Tenax TA and quartz wool). 

The material trapped VOCs and SVOCs. 

Subsequent laboratory analysis enabled 

calculation of the airborne concentration 

of each specified component. This 

is of particular interest as it included 

measurement of organophosphates from 

engine oil and hydraulic fluid. 

l Continuous measurement of VOCs and 

carbon monoxide (CO) using an Ion 

Science FirstCheck+500. This instrument 

incorporates a photoionisation detector 

to measure VOCs and a gas monitor 

to measure the CO. In addition to 

monitoring this equipment was used as 

an alert for specific fume events. 

l Dust measurement using a TSI Model 

8525 P-Trak ultrafine particle counter. 

l Questionnaire: flight and cabin crew 

were requested to fill in a post-flight 

questionnaire, which included a question 

about whether they had noticed any 

Box 2. Breaking the law 

Should oil smells reported to the 

Cranfield study in 29 of the 522 

completed post-flight question naires 

have been officially reported 

Dr Susan Michaelis (GCAQE) thinks 

they should... 

“All suspected oil leakage/fumes are 

required under the European directive 

and UK CAA legislation to be reported 

under the mandatory reporting system. 

It is inappropriate to suggest fumes were 

detected and listed as air quality events 

but were non reportable according to the 

airline. This is clear proof the reporting 

system is not working and is not enforced 

by the regulator and is a breach of the 

law – an ongoing and global problem 

clearly evident with this contaminated 

air issue. Oil leakage is oil leakage and is 

hazardous and unsafe.” 

fumes (ie. odours) during their flight. 

Thirty-eight did so, twenty-nine of whom 

described the smells as “oily”. None 

of these “events” were considered by 

the crew to meet the criteria to trigger 

an official report. Was this the right 

decision Or was this failure to report 

breaking the law (see Box 2 above). 

Flaws 

The main flaws which can be identified 

with the Cranfield study are listed here. This 

list does not claim to be comprehensive: 

l Not enough flights were monitored 

Fume events are believed to represent 

worst cases of pyrolised oil contaminating 

the aircraft air supply system, so their 

measurement would provide valuable 

data. But measuring a fume event is not 

easy because they do not noticeably occur 

10 November 2011 The RoSPA Occupational Safety & Health Journal


on every flight. In fact their frequency 

has been hotly debated. Estimates 

range from one in every 100 flights to 

one in every 2,000 flights or even less. 

Obtaining a true estimate has not been 

helped by the fact that fume events are 

under-reported. However, even assuming 

a worst case of one fume event in 

every 100 flights, Cranfield would need 

to monitor at least 300 flights to be 95% 

sure of capturing a fume event 9 . Given 

that they only monitored 100 it is not 

surprisingly they failed to find any. Significantly, 

however, contaminants were 

still detected. This demonstrates that the 

crew and pass engers can be exposed 

even on flights where no noticeable fume 

event occurs. 

l The study missed three of the most 

hazardous substances 

v Monoorthocresylphosphate 

(MOCP) and diorthocresyl phosphate 

(DOCP) 

Engine oil contains approximately 

3% of tricresylphosphate (TCP), anti 

wear additives that also improve 

the stability of the oil. TCP itself is a 

mixture comprising two groups: 

• six ortho isomers (approx. 0.2% 

of TCP) and; 

• four non-ortho isomers (approx. 

99.8% of TCP). 

The sampling and analysis measured 

the non-ortho isomers as a group but 

only analysed specifically for just one of 

the ortho isomers: TOCP. 

This is a serious omission because 

the other two types of ortho isomers 

(MOCP and DOCP) have been shown to 

be much more toxic than TOCP 10,11 . They 

also are present in engine oil at higher 

concentrations. MOCP in particular is 

10 times more toxic than TOCP and is 

present in the oil at over 600,000 times 

the concentration of TOCP. For DOCP 

the ratios are five times and 1,200 times 

respectively. (Note: MOCP and DOCP are 

not single components. Each is a group 

of isomers). 

The Cranfield study’s authors justify 

their choice of sampling and monitoring 

the least toxic ortho isomer on the 

grounds that it “had a pure standard”. 

The study does not make it clear 

whether or not the results for “sum of 

TOCP and other TCPs” listed in table 

4 of the report 12 do actually include 

MOCP and DOCP. Dr Chris Walton, one 

of the authors of the Cranfield study, 

commented that, “had there been a 

large peak in the chromatogram we 

would have seen it”. But without specific 

identification and measurement of these 

two isomers we cannot be sure. 

v PAN 

N-phenyl-α-napthylamine (PAN), an 

oil antioxidant, was not analysed 

because, in the words of the authors, 

it “was not identified as a target 

chemical” and its analysis would be 

“technically challenging”. 

Technically challenging or not, the 

hazards associated with this substance 

merit its analysis. PAN is suspected 

of causing cancer and is a sensitiser. 

It is known to contain small amounts 

of impurity. These include N-phenyl- 

2-napthylamine (0.5% or less in PAN) 

and β -napthylamine (0.005% or less 

in PAN). These impurities are category 

3 and category 1 carcinogens 

respectively. β -napthylamine is a 

known and prohibited human bladder 

carcinogen. 

l Failure to measure contamination 

from the Auxiliary Power Unit (APU) 

The APU provides power and heating 

when the aircraft is still on the ground 

and is often used to start the main 

engines. Typically it is switched on first 

thing in the morning and used to keep 

the aircraft powered and warm while 

pilots carry out flight checks. During 

this time maintenance and catering 

staff prepare the aircraft for flight. 

Occasionally the APU is used while the 

aircraft is in the air, usually during takeoff 

and landing. 

The study did not specifically monitor 

for contaminants produced by the APU. 

This was unfortunate because many 

pilots consider the APU to be a major 

source of contamination. One pilot 

recalls sitting in APU fumes each 

morning while doing cockpit checks 

and having to open the windows to 

clear the air before passengers boarded 

the aircraft. This pilot is now medically 

retired due to symptoms attributed to 

organophosphate poisoning. 

l Failure to capture transients 

As mentioned previously, engine oil 

leaks have been associated with engine 

transients (ie. changes in power level). 

Surprisingly, no attempt was made to 

sample during changes in the power 

levels. Nor did the study compare the 

flight data recorder to check whether 

data samples that were taken coincided 

with any transients. 

l Full flight sampling 

Each sample was collected for just five 

minutes. This was necessary to measure 

each phase of the flight separately (Box 

3) but it meant that only two and a half 

litres of air was collected per sample. As 

a result sensitivity was reduced. Smaller 

amounts of contaminant were thus likely 

to be missed. Using a pump with a higher 

flow rate could have increased sensitivity 

for the five minute samples, and 

sensitivity for results averaged over the 

whole flight could have been increased 

by use of a second pump sampling for 

the entire duration of each flight. 

As it is, the Cranfield study tabulates 

all cases where the study failed to detect 

a contaminant as a “non-detect” (ND). 

This is very misleading as it gives the 

➤ 



Box 3. Summary of flight 

phases* 

The air was sampled for five minutes 

at each phase. Air quality evens were 

sampled as they happened. 

Sample No. 

Flight phase 

1 Immediate 

2 First engine start 

3 Taxi 

4 Take off 

5 Climb 

6 Top of climb 

7 Cruise 

8 Start of descent 

9 Pre-landing 

10 Taxi-back 

* For the full table see Aircraft cabin air 

sampling study: Part 1 of the Final Report 

(March 2011), table 2, P6 

impression that no contaminant was 

present. This is not necessarily true. 

Samples could have been present up to 

the detection limit. Equally misleading, 

for the same reason, is giving all NDs the 

value of zero when averaging the results. 

l Sampling location 

The purpose of an airborne sampling 

exercise is to determine personal 

exposure. Ideally this is achieved by 

personal monitoring, ie. getting the 

worker (the pilot) to wear the sampling 

equipment with the sampler inlet (the 

open end of the tube attached to the 

pump) as close to the breathing zone 

(nose and mouth) as possible. Where this 

is not possible the sampler inlet should 

be positioned at head height. Cranfield 

chose to position their sampler head 

20-30cm above the floor level. Samples 

taken at this level are unlikely to reflect 

breathing zone concentration. 

The sampler location was also a 

poor choice for another reason. The 

tricresylphosphates (TCPs) the study was 

attempting to analyse are semi volatile. 

TOCP for example has a boiling point of 

410°C (760mm Hg pressure) and is more 

likely to exist as a mist by the time it 

reaches the flight deck and cabin. As mist 

it is less likely to disperse as uniformly 

as a gas or vapour. Sampling just one 

position – especially at floor level – will 

not capture any variation in concentration 

across the flight deck and will not give a 

true picture. 

Sampling at floor level is inappropriate 

for another reason. TOCP and its isomers 

can settle out on surfaces and people. 

Therefore not all of it may reach the floor 

and will therefore be missed. And as TOCP 

is hazardous by ingestion and skin absorption 

as well as inhalation, airborne monitoring 

alone, even if accurate, will underestimate 

personal exposure. Swab (ie. wipe) 

samples were taken during the flights 

and these are currently being analysed by 

the Institute of Occupational Medicine in 

Edinburgh. These will tell us whether settling 

out occurred but not how much. 

l The study’s final conclusion was 

misleadingly reassuring 

It reads as follows: “With respect to 

the conditions on the flights that were 

experienced during this study, there 

was no evidence for target pollutants 

occurring in the cabin air at levels 

exceeding health and safety standards 

and guidelines.” 

This gives no reassurance: 

v because of the points already covered 

in this article, eg. failure to measure: 

• fume events; 

• MOCP, DOCP, and PAN; 

• transients; 

• contaminants in the breathing 

zone (floor level measurements 

taken instead); 

• fumes from the APU. 

v because of other points: 

• the “target pollutants” ie. the 

substances actually sampled and 

measured by the survey (Box 4) 

were actually a tiny proportion of 

the substances likely to be present 

in engine fume. These include 

oil, hydraulic fluid and antifreeze 

components as well as the pyrolysis 

products resulting from their breakdown 

in the compression chamber 

of the aircraft. Many of the pyrolysis 

products are unknown; 

• MOCP was not a target pollutant, 

but had it been present in the air 

in the same ratio to TOCP as in the 

oil it would have been present at 

very harmful levels and in at least 

one case it would have exceeded 

considerably the Workplace 

Exposure Limit (WEL) for TOCP; 

• for some target analytes measured 

concentrations were compared 

with their WELs. This is not a 

valid comparison as WELs are 

not applicable to the low-pressure 

environment of an airliner cabin; 

• nor do WELs and other exposure 

and guidance limits take synergistic 

effects into account. In a complex 

mixture such as engine fume, 

substances can act together 

such that their combined hazard 

is greater than the sum of their 

individual hazards; 

• nor is it valid to use WELs to 

specify acceptable exposure 

levels for members of the general 

public, many of whom may be 

very old, very young or even 

not yet born, and therefore 

particularly vulnerable to airborne 

contamination; and 

• even very low levels of organophosphate 

exposure are now 

believed to be harmful. This is not 

limited to the ortho isomers of 

tricresylphosphate. 

12 November 2011 The RoSPA Occupational Safety & Health Journal


Conclusion 

We still have to wait for the results of the 

swab analysis taken during the Cranfield study 

and also for the Committee on Toxicity’s 

conclusions on the overall research project. 

Cranfield meanwhile is telling us the water’s 

just fine having counted all the fish except 

the sharks and barracudas! Instead they 

counted red herrings such as “reassuring” 

comparisons of selected contaminant levels 

with those of domestic households. These 

comparisons are not reassuring because they 

do not include TCPs. 

My feeling is that what was lacking 

in this study was occupational hygiene 

expertise. As a result the study did not 

focus sufficiently on personal exposure. 

Jet engines on airliners continue to leak 

toxic fumes into a sealed tube; a confined 

space where no one can open a window 

or take a walk outside when the fumes get 

bad. Because worse possible cases, ie. fume 

events have not been measured, there is no 

guarantee that the levels of exposure are 

safe. Cases of illness and early retirement in 

aircrew strongly indicate they are not. 

Significantly, the latest aircraft from Boeing, 

the Boeing 787 “Dreamliner” does not use air 

bled from the engines to supply the cabin and 

flight deck. It uses an electrical compressor 

instead. A move towards greater efficiency as 

Box 4. Target analytes 

for pumped sampling 

and analysis 

Sample No. 

Flight phase 

Target analyte 

1 Tri-ortho cresyl 

phosphate (TOCP) 

2 Other tri-cresyl 

phosphate isomers 

(TCP) 

3 Tri-butyl phosphate 

(TBP) 

4 Toluene 

5 m & p xylene 

6 Limonene 

7 Tetrachloroethylene 

8 Undecane 

* For the full table see Aircraft cabin air 

sampling study: Part 1 of the Final Report 

(March 2011), P9 

Boeing claim Or an admission of guilt 

Perhaps the most interesting comment on 

the Cranfield study though is the fact that 

the Chair of Nanotechnology at Cranfield 

University recently held a workshop to 

scrutinise the study’s conclusions and the 

way it was carried out. And although BALPA, 

whose concerns led to the study, has 

welcomed its publication, Jim McAuslan, 

the association’s general secretary, has said: 

“Irrespective of the Committee on Toxicity’s 

conclusions there are examples of pilots who 

get ill and BALPA will continue to explore this 

as a matter of concern.” 

As for myself, looking at the evidence, 

would I want my children to become pilots 

or flight attendants Regretfully I would not. 

References 

1. Working Life: Something in the Air The 

RoSPA Occupational Safety and Health 

Journal, November 2008, P27 

2. Working Life: Air Control. The RoSPA 

Occupational Safety and Health Journal 

December 2008, P17 

3. The Aviation Contamination Air Reference 

Manual. Edited by Captain Susan Michaelis. 

2007. www.susanmichaelis.com 

4. Health and safety implications from 

exposure to contaminated air in aircraft. 

Dr Susan Michaelis PhD. Published by 

Susan Michaelis, 2010 

5. Aircraft cabin air sampling study: Part 1 

of the Final Report (issued March 2011). 

Prepared by Derrick Crump, Paul Harrison 

and Christopher Walton, Institute of 

Environmental Health (IEH), Cranfield 

University. www.cranfield.ac.uk/ 

health/researchareas/environment 

health/ieh/cabinairquality1.pdf 


of the Final Report (issued April 2011). 

Tabulated raw data, Continuous data 

recordings. Prepared by Derrick Crump, 

Paul Harrison and Christopher Walton, IEH. 

Box 5. US study uncovers 

fume events 

Between January 2009 and 

December 2010 Judith Murawski, 

industrial hygienist with the US 

Association of Flight Attendants, 

identified an alarming 87 fume 

events on 47 aircraft in a study 

carried out on one US airline*. Even 

more worryingly, in many cases, 

even when the characteristic “dirty 

socks” smell was noticed prior to 

take-off, the flight still went ahead. 

*Case study: Analysis of reported 

contaminated air events at one major 

US airline in 2009-10.” AIAA 2011-5089. 

Proceedings of Am Inst. Aero. & Astro. 

41st Intl. Conf. on Env. Sys. 17-21 July 2011 

www.cranfield.ac.uk/health/ 

researchareas/environmenthealth/ 

ieh/cabinairquality1.pdf 

7. Ibid. Table 1, P5 

8. Statement on the review of the cabin 

air environment, ill health in aircraft 

crews and the possible relationship 

to smoke/fume events in aircraft. 

Committee on Toxicity of chemicals 

in food consumer products and the 

environment (COT). Paragraph 86, 

2007. http://cot.food.gov.uk/pdfs/ 

cotstatementbalpa200706 

9. Ibid, paragraph 69. 

10. Tricresyl phosphate poisoning. Experimental 

clarification of problems of 

etiology and pathogenesis. D Henschler. 

Klinische Wochenscrifte 36: 663-674, 1958 

11. Toxicological studies of triphenyl 

phosphate, trixylenyl phosphate and 

triaryl phosphates from mixtures of 

homologous phenols. D Henschler. 

Archiv Experimental Pathologie Und 

Pharmakologie 233: 512-517, 1958 


of the Final Report (issued March 2011). 

Table 4, P12 

PDFs of Nick Cook’s previous two 

articles on this subject are available 

from: rspencer@rospa.com

ROSPA-Working-Life-Nov-2011-V2

Create successful ePaper yourself

Delete template?

Save as template?