Jean-Christophe Balouet, Managing Director, Environment International, 31 Rue du General Chanzy,
94130 Nogent sur Marne, France (presenter)
Chris
Winder, Head, School of Safety Science, University of New South Wales, Sydney,
NSW 2052, Australia
Introduction
Aircraft materials such
as jet-fuel, de-icing fluids, engine oil, hydraulic fluids, and so on, contain
a range of ingredients, some of which are toxic (such as tricresyl
phosphate). These materials can
contaminate airplane cabin air, and these can lead to exposure to vapours and
aerosols. The range of bleed air
contaminants and their concentrations, which may be found during in-cabin
contamination events during flight, can be extensive. Significant contaminants include: aldehydes; aromatic
hydrocarbons; aliphatic hydrocarbons; chlorinated, fluorinated, methylated,
phosphate, nitrogen compounds; esters; oxides; and combusted or pyrolised
products such as carbon monoxide, carbon dioxide, smoke particles of unknown
constitution. These may be in gas or
vapour form, or as oil mists of partially combusted particles. Gases and vapours may also be adsorbed onto
aerosol particles.
Issues that
can Impact on Exposure to Contaminants
on Airplanes
The
Impact of Altitude
The concentration of
oxygen at increasing altitude remains constant, at 20.9%. This suggests that oxygen levels are
unchanged. This is not true. Basically, as altitude increases, the
atmospheric pressure declines. While
the proportion of oxygen in air remains unchanged, the actual amount
of oxygen in air decreases.
Atmospheric pressure at
sea level is 760 mm Hg, with the corresponding partial pressure of oxygen in
air is 159 mm Hg (20.9% or 760 mm Hg).
The minimum O2 concentration for work is considered to be about 136 mm Hg
(18 kPa or 18%) O2 in air at sea level.
A minimum oxygen partial pressure of 118 mm Hg (equivalent to an
altitude of 2438 m/8000 ft) is required to prevent hypoxic cabin air in
commercial aircraft during normal operations.
This partial pressure is maintained by the cabin pressure system (a
second requirement for release of oxygen dispensing units at 4572 m/15,000 ft
is recommended).
The altitude at which
the partial pressure of 136 mm Hg is reached is also quite close to the
pressure at which airplane cabins are pressurised (118 mm Hg). There is little margin of safety in people
working at altitude, and in many cases, such workers may be beginning to become
hypoxic. This shown in the Figure
below, where the area bounded by the dashed partial pressure of Oxygen in Air
curve, and the dotted line representing the minimum physiological demand line
represents the margin of safety at which workers can be considered to have
sufficient oxygen to work safely).
Further, the position of the cabin pressurisation line shows that in
some cases, workers at altitude may not be obtaining enough oxygen for their
physiological requirements.
Figure:
Pressures and Oxygen Concentrations at Altitude
Assumptions:
Atmospheric pressure: 101 kPa (760 mm Hg) at sea level
Proportional concentration of O2 in air: 20.9% (21 kpa or 159
mm Hg) at sea level)
Aircraft Pressurisation
Pressure: Equivalent to an altitude of 2500 m (about 8000 ft).
Other problems
with lowered oxygen concentrations include changes in sensitivity to toxic
exposures (for example, the toxicity of carbon monoxide is 50% higher at 8000
ft than at sea level), and the possibility that incipient hypoxia may lead to
higher respiratory rates and therefore increased exposure.
Other factors due to the manner in which
air is circulated in planes, may also have an effect, such as humidity,
temperature, or contaminants such as carbon dioxide, carbon monoxide, ozone and
particulates.
Issues Related
to Vapours and Particulates
Airborne contaminants
are generally divided into two types: gas/vapour and particulates.
Gases/Vapours: A gas is those molecules of a chemical that exist in a gaseous
phase. Where all the molecules of a
chemical are in the gaseous phase, the chemical is considered a gas. A vapour is the gas phase of a liquid at
room temperature. Therefore, a vapour
is that amount of liquid that evaporates into air (or dissolves into air). Gases and vapours form true solutions in
air. The amount of evaporation is
dependent on the individual vapour pressure of the contaminant. Where vapour pressure is low, only a small
amount of the contaminant will evaporate.
Generally, vapour pressure increases with temperature.
Where volatile organic
chemicals (VOCs) have high vapour pressures, they will be present in air in
high concentrations, are more likely to reach toxic concentrations and are
amenable to sample collection and analysis using sorbent or gas collection
methods. Where semi-volatile or poorly
volatile chemicals have low vapour pressures, they are less likely to reach
toxic concentrations unless they are highly toxic, and sorbent or gas
collection methods are less useful for sample collection.
Particulates:
These are materials that are suspended, not dissolved, in air, and include
fumes, smoke, mists, aerosols, dusts, fibres and so on. Particulates may be in liquid phase (such as
mists), solid phase (smokes, fumes and dusts) or mixed phases (aerosols). Precise criteria for these terms exist based
on particle size and phase, but are unnecessary for the present discussion.
Where a particulate is
present in air and contains a volatile component, the volatile components will
evaporate at a rate dependent on individual vapour pressures. However, depending on the amount of
particulate present in air, it is possible to exceed the vapour pressure of an
individual contaminant. Where a
contaminant has a low vapour pressure, particulate exposure is more important
than exposure to vapour.
Therefore, particulates
containing a large proportion of volatile components will evaporate quickly
(sometimes even before settling), indicating that the vapour phase of the
contaminant is more important.
Particulates containing poorly volatile components will stay in
particulate form for a long time, until gravity or turbulence causes them to
settle. Once settled, particles
coalesce onto or adhere to surfaces, and any remaining volatile components
become subject to evaporation through their vapour pressures. Where evaporative pressures are low, long
term, low-level contamination leading to residual exposures will occur.
Further, because
particulates can settle on exposed skin and be subject to absorption through
skin, sometimes after airborne exposure has ceased, it is important to consider
both the inhalational and skin routes when estimating exposure.
Particulates are not
amenable to the same sampling and collection methods that are required for
gases and vapours. They require
specialised sampling, usually by filtration or gravimetric methods. Further, because particulates can exist in
different sizes and diameters, an estimate of that fraction of the particulate
that is taken into the respiratory system may be more critical than an
estimation of the total concentration of particulate. Consideration of the type of airborne contaminants, whether in
vapour, particulate or mixed phases is quite critical for the success and
relevance of a monitoring program.
Issues Related
to Combustion and Pyrolysis
Any chemical or
chemical mixture is subject to degradation processes, such as oxidation or
reduction. Over time, these can cause
substantial loss of original chemical structures and properties. This process occurs more rapidly at higher
temperatures and pressures, in accordance with the laws of thermodynamics.
However other breakdown
processes are possible, such as, a material subject to a source of heat energy
can burn. This is called thermal
degradation, or thermolyis. The process
of thermal degradation is a chemical process in which oxygen and energy are
used to transform the original chemical into its oxidised form. For example, carbon containing materials
will, in the presence of energy and oxygen, produce the two oxides of carbon:
Carbon dioxide (CO2) and Carbon monoxide (CO). The first of these (CO2) is
produced in the presence of an abundance of oxygen, the second (CO), where
stoichiometric concentrations of oxygen are lacking (usually in conditions of
incomplete combustion). Both of these
oxides are gases, one (Carbon monoxide) is indeed toxic even at low
concentrations, causing toxic asphyxiation.
Single or short term exposure to CO insufficient to cause asphyxiation
produces headache, dizziness, and nausea; long term exposure can cause, among
other effects, memory defects and central nervous system damage.
Where oxygen is
completely lacking, the process of thermal degradation can still proceed, but
this time, any carbon in a material, will be reduced from the chemical form it
is located, to molecules containing proportionally more carbon (and
proportionally less volatile components) and ultimately, carbon atoms. This process is called pyrolysis. Both oxides of carbon are gases, but
elemental carbon is a solid (usually seen as smoke or soot). Further, the process of reducing carbon
containing materials to carbon depends on the chemical nature of the source
material, and will produce different pyrolysis products as the reaction process
proceeds. Pyrolysis products may be
fairly pure in carbon content, but are more usually found with other organic or
inorganic breakdown products. The
processes inherent in pyrolytic degradation are very complex, and vary
depending on the source materials, the temperature and duration of combustion,
and the progressive combustion of pyrolysis products that occur in the thermal
degradation process.
Many combustion and
pyrolysis products are toxic. The toxic
asphyxiants, such as carbon monoxide or hydrogen cyanide were discussed
above. Some thermal degradation
products, such as acreolin and formaldehyde are highly irritating. Others, such as oxides of nitrogen and
phosgene, can produce delayed effects.
Still others, such as particulate matter (for example, soot) can carry
adsorbed gases deep into the respiratory tract where they may provoke a local
reaction or be absorbed to produce systemic effects.
Of course, in a
situation where a fire occurs, all three processes can occur. Where there is no oxygen, pyrolysis products
(such as smoke) will be formed, where there is incomplete combustion carbon monoxide
will form, and where there is complete combustion, carbon dioxide is
formed. Further, these process may
proceed sequentially, as oxygen becomes available to the burning material.
Therefore, as well as
particulate and gas/vapour phases, consideration of the type of airborne
contaminants, whether in unchanged, degraded, combusted or pyrolised forms is
also critical for the success and relevance of a monitoring program.
Issues related to Exposure in an
Airplane
If an airplane engine
leaks in flight, and leaking engine oils contaminate air flowing to the flight
deck or passenger cabin. There are two
possible exposure scenarios:
m
exposure to the
oil;
m
exposure to a
thermally degraded oil and its by-products.
In such circumstances,
exposed crew and passengers are exposed to airborne contaminants that are
leaking directly into air, and they are unaware of the toxicity of the
contaminants they are inhaling. There
is little control of exposure.
If exposure is to an
oil, it will be at least partially in a particulate (mist) form, where it can
attain higher airborne concentrations than might be predicted from vapour
pressures (even at elevated, but rapidly cooling, temperatures). Also, the potential for skin exposure is
greatly increased, as the mist can settle onto exposed skin, where it will then
be available for dermal absorption.
Further, the emission
of oil vapours/smoke/mists into the passenger cabin would produce contamination
of the cabin. Particulates would settle
out onto surfaces (such as ducting, cabin walls, furniture and equipment),
which would thereafter slowly vapourise, the rate of evaporation being
dependent on individual contaminant vapour pressures. This residual contamination would continue until cleaned off or
until it had evaporated.
While the toxicity of some aircraft fluids has been
established, little is known about the possible transformations that may have
occurred in an oil while in operation.
A leak of such an oil from an engine operating at altitude would see
most of the oil pyrolise once it leaves the confined conditions of temperature
and pressure operating in the engine.
While it seems reasonable that any ingredients with suitable
autoignition or degradation properties that allow such a transformation after
release from the engine could be radically transformed, it is possible to
speculate in only general terms about the cocktail of chemicals that could
form.
Presumably this would include:
m
combustion gases
such as carbon dioxide and carbon monoxide;
m
other irritating
gases, such as oxides of nitrogen;
m
partially burnt
hydrocarbons (including irritating and toxic by-products, such as acreolin and
other aldehydes); and
m
any specific toxic
contaminant such as Tricresyl phosphate that are fairly stable at high
temperatures or thermal degradation products, such as highly toxic oxides of
carbon, nitrogen or phsophorus.
These contaminants will be in gas, vapour, mist and
particulate forms.
If the exposure is to a thermally degraded oil then as well
any exposure to the oil mist (as outlined above), exposure can also include
particulates such as soots; thermally degraded chemicals such as acreolin, and
combustion gases such as carbon monoxide.
Chemicals used in
Aviation are commercially useful products.
They are known to contain toxic ingredients. While the continued use of toxic materials is always a matter
requiring caution and forethought, a full deliberation of risks and benefits
may overcome such considerations.
An increasing number of
oil leaks in the 1990’s around the world and the increase in a number of flight
attendants and flight crew reporting signs of toxicity after such events
suggests the toxicity of the jet oils should be reconsidered:
m
Firstly, the
exposure scenario at altitude is utterly different from conventional exposures
to the such products while using them in maintenance situations. Exposed individuals do not know to what they
are being exposed, exposure by inhalational and dermal exposures can occur, the
possibility of escape is absent, the possibility of cleaning or decontamination
is absent).
m
Secondly, options
for the control of exposure are all but absent. Switching off an engine or bleed air system may offer some
assistance, but is less useful if an entire ventilation system is contaminated.
m
Thirdly, the
exposure may be not only to gases and vapours, but also to particulates (such
as oil mists or soots) that can be in proportionally greater concentrations
than they would be for vapours.
m
Fourthly, the
exposure may vary from unchanged oil mists, or to combusted or pyrolised
contaminants. The chemical make up of
such a mixture would be difficult to deduce; the toxicity of exposure to such a
mixture would be difficult to predict.
However, these
contaminants could not be classified as being of low toxicity. The interactions of such effects with a
specific toxic exposure is not known, but not presumed to be benign. The possible problems that might arise from
exposure to such a cocktail cannot be dismissed without proper consideration.
Effects of exposure
Studies of exposures in
airplanes where cabin contamination occurs show common symptoms of irritancy
and toxicity. The range of symptoms in
these studies is quite broad, affecting many body systems. These include:
m
neurotoxic symptoms: blurred or tunnel
vision, nystagmus, disorientation, shaking and tremors, loss of balance and
vertigo, seizures, loss of consciousness, parathesias, numbness (fingers, lips,
limbs), parathesias;
m
neuropsychological symptoms: memory impairment, light-headedness, dizziness,
confusion and feeling intoxicated, forgetfulness, lack of co-ordination, severe
headaches, sleep disorders;
m
gastro-intestinal symptoms: nausea, vomiting, salivation, diarrhoea;
m
respiratory symptoms: cough,
breathing difficulties (shortness of breath), tightness in chest, respiratory
failure requiring oxygen, susceptibility to upper respiratory tract infections;
m
skin symptoms: skin itching and rashes, skin blisters (on uncovered body
parts), hair loss;
m
cardiovascular symptoms: chest pain, increased heart rate and
palpitations;
m
irritation of eyes, nose and upper airways;
m
sensitivity: signs of immunosupression, food and alcohol intolerances,
chemical sensitivity leading to acquired or multiple chemical sensitivity
m
general: weakness and fatigue (leading to chronic fatigue),
exhaustion, hot flashes, joint pain, muscle weakness and pain.
Some of these
effects are transient, others appear more permanent. A preponderance of these
symptoms are related to exposure to irritants.
However, the presence on symptoms related to central nervous system
dysfunction, hair loss, muscular and gastrointestinal problems, suggests the
possibility of a component of systemic toxicity. Neurotoxicity is a major flight safety concern, especially where
exposures are intense. The exacerbation
of pre-existing health problems by toxic exposures is also highly probable.
Many of the signs and
symptoms of exposure being reported by exposed flight crew (and to a lesser
extent, passengers) appear consistent with the toxicity of some of the ingredients
of the oils. These include hydrocarbon
neurotoxicity from exposure to organic chemicals, COPIND from organophosphate
exposure, or long term low level toxicity from exposure to carbon monoxide. These health problems need to be evaluated
with more care than is apparent in the aviation industry at present.
This is a hidden
issue. These health effects present
significant issues with regard to the health of pilots, cabin crew and
passengers, but most notably with regard to air safety if pilots are incapacitated
and cabin crew cannot supervise cabin evacuations during emergencies.