toxic hazards in aviation
The effects of toxic chemicals in the aviation environment may lead to
human error, which is the leading cause of aviation accidents. During
flight, the exposure of aircrews to toxins can range from an acute and
suddenly incapacitating event to long-term health effects secondary to
chronic exposure. Aviation personnel must be able to understand the
dangers and recognize the often near-imperceptible onset of toxic
hazards. The flight surgeon or aeromedical physician’s assistant should
educate the aircrew in the prevention of toxic hazards and treatment of
flight personnel who are exposed to known toxic chemicals.
aviation, the unique toxicological environment is primarily limited to
an enclosed environment. Thus, this chapter’s focus is on aircraft
The greatest toxicological risk during flight is an acute, high-dose
exposure to a toxic agent. The cabin air quality may change rapidly or
insidiously. These air-quality changes can be due to the generation of
toxic substances from fluid leaks, fire, and/or variations in altitude
and ventilation rates.
Exposure to chemical fumes from burning wire insulation or rocket
exhaust can degrade a pilot’s ability to function. Acute in-flight
exposures are of two types:
Exposures to toxic chemicals have contributed to some accidents
erroneously attributed to pilot error. During the most demanding modes of
flight, the balance between critical flight tasks and human abilities is
sometimes delicate and fragile even for well-trained crews. Therefore, any
performance decrement caused by toxic substances is a cause for concern.
time and dose
With most substances, the medical effects of an exposure depend on the
duration of exposure and the concentration of the chemical. As the
concentration increases, the interval between initial exposure and the
onset of symptoms decreases. Many chemicals change their adverse
physical effects as the concentrations increase. At high
concentrations, gases, such as nitrogen dioxide, and numerous
petrochemicals and other mechanical fluids are highly irritating to
the upper respiratory tract, nasal passages, and mucous membranes; at
lower concentrations, these chemicals may have little or no effect.
Specific organs or tissues selectively absorb a chemical substance as
it enters the bloodstream. For example, fat-soluble compounds, such as
carbon tetrachloride and most aviation fuels, tend to accumulate in
the nervous system tissues. Heavy metals from lead-acid batteries tend
to produce damage at the point of exit from the bloodstream—the
Toxic agents may enter the body by inhalation into the lungs, by
ingestion into the stomach, or by absorption through the skin. The
most important route of entry in the aviation environment is
inhalation. Aircrews are often in close contact with volatile fuels
and other potentially hazardous petroleum products, oils, lubricants,
and hydraulic fluids. For example, a well-intentioned service engineer
may choose to eat while working on the engine deck without realizing
the potential danger of ingesting a toxin through contaminated food or
water. Another example is the crew member, in a hurry after an
aircraft refueling, who chooses not to wash his hands and then smokes
a cigarette or eats a meal. Acute toxic exposures are
characteristically related to inhalation or ingestion, whereas toxin
exposure through skin absorption usually produces symptoms only after
chronic, repeated exposures.
pre existing contions
People with organ impairment—such as liver or lung damage, sickle-cell
disease, or an active disease process—are usually more susceptible to
toxic agents. Various toxic agents in the presence of another specific
chemical can combine or accelerate their adverse effects on the
individual. Examples include smoking and asbestosis exposures as well
as carbon monoxide and another agent that has already reduced the
oxygen-transport capabilities in the blood. Increased altitude and
temperature can also accelerate the effects of toxic chemicals.
Allergies can influence an individual’s physical response to an
allergen. The allergic physical response to a toxic agent can vary
considerably. For example, in an environment in which several people
are in daily contact with a specific chemical at low concentration,
only one person may exhibit signs/symptoms because of his unique
genetic characteristics such as metabolic rate, retention and
excretion rates, and level of physical fitness.
allowable degree of
Even a slight degree of in-flight impairment is hazardous to the
pilot’s task. The flight surgeon, working with the industrial
hygienist, should be aware of chemicals within the flight-line area of
responsibility to ensure that personnel exposure remains within safe
limits. Several methods of quantifying the hazard risk to routine
chemical exposures have been established.
The human body has varied and intricate chemical defence mechanisms.
Upon entry of a toxic substance, the body immediately begins to reduce
the concentration of the substance by multiple processes. These
processes includes metabolism (the chemical breakdown of a substance),
detoxification, and excretion. The flight surgeon must be familiar
with the metabolic pathways of well-known poisons and understand the
physical or psychological symptoms attributable to a subtle chemical
intoxication. For example, the amount of carbon monoxide eliminated by
the body during a single exposure decreases by 50 percent every four
SECTION II — AIRCRAFT
The interior of an aircraft may contain various contaminants that
could present a risk. Aircraft atmosphere contamination can
The physical relationship of engine positioning to the cockpit is
important. Depending on the age of the aircraft and the power plant
used (jet or reciprocating), there will be a wide range of potential
cockpit air contaminants caused by exhaust gases. Single-engine,
piston-type aircraft with the engine located directly in front of the
fuselage are subject to greater contamination than multiengine
aircraft with engines situated laterally. Reciprocating engines
uniformly produce much more carbon monoxide in their exhaust than the
modern jet engine. Liquid-cooled, single-engine airplanes are also
less likely to be contaminated by exhaust gases than air-cooled,
The effects of carbon monoxide are subtle and deadly. Carbon monoxide,
a product of incomplete combustion, is the most common gaseous poison
in the aviation environment. It is also the most common unintentional
and intentional cause of poisoning in the United States. More deaths
have been attributed to CO than to any other toxic gas. Carbon
monoxide acts as a tissue asphyxiant that produces hypoxia at both sea
level and altitude. It preferentially combines with haemoglobin, to
the partial exclusion of oxygen, and thus, interferes with the uptake
of oxygen by the blood. CO has a 256-times greater affinity for
bonding with haemoglobin than with oxygen. The presence of CO greatly
reduces the oxygen-carrying capability of haemoglobin. It is a
colourless, odourless gas that is slightly lighter than air. Because
it is odourless, CO should be suspected whenever exhaust odours are
detected. Carbon-monoxide concentration in the blood is based on a
variety of factors, including the concentration of the gas,
respiratory rate, CO saturation of haemoglobin, and duration of
relatively low concentration of CO in the air can, in time, produce
high blood concentrations of CO. A person who inhales a 0.5 percent
concentration of CO for 30 minutes while at rest will have a 45
percent blood concentration of CO.
reduced concentration of oxygen in the air and increased temperature
or humidity may increase the concentration of CO-bound haemoglobin.
Any of these changes or an increase in physical activity can
accelerate the toxic effects of CO.
Production of carbon monoxide depends upon incomplete combustion of
fuel. An engine that yields complete combustion will produce only
carbon dioxide. As the fuel-to-air ratio decreases and complete
combustion increases, the percentage of carbon dioxide in the exhaust
gas rises and the percentage of carbon monoxide declines. Conversely,
as the mixture becomes richer (increasing the fuel-to-air ratio), the
carbon monoxide in the exhaust gas increases.
The effects of carbon monoxide on the human body vary. The leading
symptoms of carbon monoxide intoxication are—
The symptoms are those of hypaemic hypoxia. Of particular importance
to aviators is the loss of visual acuity. Peripheral vision and, more
importantly, night visual acuity is significantly decreased, even with
blood CO concentrations as low as 10 percent saturation.
The dangers associated with carbon monoxide rise sharply with
increasing altitudes. When experienced separately, a mild degree of
hypoxic hypoxia (caused by altitude increases and decreased partial
pressures of oxygen) or an exposure to small amounts of carbon
monoxide may be harmless. When experienced simultaneously, their
effects become additive. They may cause serious pilot impairment and
result in loss of aircraft control.
For practical purposes, the elimination rate of carbon monoxide
depends on respiratory volume and the percentage of oxygen in the
inspired (inhaled) air. Smoking one to three cigarettes in rapid
succession or one and one-half packs per day can raise an individual’s
carbon-monoxide haemoglobin saturation to 10 percent. At sea level, it
may take a full day to eliminate that small percentage of carbon
monoxide because the carbon-monoxide gas is reduced by a factor of
only 50 percent about every four hours.
When flight personnel suspect the presence of carbon monoxide in the
aircraft, they should turn off exhaust heaters, inhale 100 percent
oxygen (if available), and land as soon as practical. After landing,
they can investigate the source and evaluate their own possible
symptoms of carbon-monoxide intoxication.
AVGAS is used only as an emergency fuel. It is a mixture of
hydrocarbons and special additives such as tetraethyl lead and xylene.
One gallon of aviation gasoline that has completely evaporated will
form about 30 cubic feet of vapour at sea level. Flight personnel who
have been exposed to aviation gasoline vapours can have adverse
physical or psychological reactions.
Aviation gasoline vapours, which are heavier than air, are readily
absorbed in the respiratory system and may produce symptoms of
exposure after only a few minutes. If vapours are inhaled for more
than a short time, one-tenth of the concentration that could cause
combustion or explosion may cause unconsciousness. The maximum safe
concentration for exposure to vapours of ordinary fuel is about 500
parts per million, or 0.05 percent. Aviation gasoline vapour is at
least twice as toxic as ordinary fuel vapour. Exposure to aviation
gasoline may include—
Burning and tearing of the eyes.
Disorders of speech, vision, or hearing.
tetraethyl lead in
Tetraethyl lead, an antiknock substance, is highly toxic. Poisoning
may occur by absorption of the lead through the skin or by inhalation
of its vapours. Tetraethyl lead poisoning primarily affects the
central nervous system. Symptoms include insomnia, mental
irritability, and instability. In less dramatic cases, sleep may be
interrupted with restlessness and terrifying dreams. Other symptoms
include nausea, vomiting, muscle weakness, tremors, muscular pain, and
visual difficulty. The amount of tetraethyl lead in aviation gasoline
is so small that a lead hazard through normal handling is remote; the
amount is only about 4.6 cubic centimetres per gallon, or about one
teaspoon. Poisoning has resulted from personnel entering fuel-storage
tanks containing concentrated amounts of tetraethyl lead within the
accumulated sludge. Maintenance personnel who work (welding, buffing,
or grinding) on engines that have burned leaded gasolines can receive
significant exposure to lead compounds.
jet propulsion fuels
JP-4, JP-5, and JP-8 are mixtures of hydrocarbons, producing different
grades of kerosene. Each JP fuel has a specific vapour pressure and
flash point. JP fuels do not contain tetraethyl lead. The recommended
threshold limit for JP fuel vapours has been set at 500 parts per
million. Toxic symptoms can occur below explosive levels; therefore, a
JP fuel intoxication can exist even in the absence of a fire hazard.
In addition to being an irritant hazard to skin and mucous membranes,
excessive inhalation of JP fuels degrades central nervous system
functioning. JP fuels, in high enough concentrations, can produce
hydraulic fluid and
leak from a hydraulic hose or gauge, under pressures of up to 1,200
pounds per square inch, can produce a finely divided aerosol fluid
that diffuses quickly throughout the cockpit. Large leaks may cause
liquid to accumulate on the floor. In either case, the cockpit air may
quickly develop a high level of aerosolized hydraulic fluid. Like
other hydrocarbons, hydraulic fluid can be toxic when inhaled. In
fact, several hydraulic fluids are phosphate ester-based and have
identical actions as the military nerve agents known as
organophosphoesterase inhibitors. Increasing temperature or altitude
can aggravate the toxic effects of inhaling the aerosolized fluid. The
toxic effects may include—
Irritation of the eyes and respiratory tract.
Nerve dysfunction in the limbs.
Impairment of judgment and vision.
coolant fluid vapour
The coolant fluid used in liquid-cooled engines consists of ethylene
glycol diluted with water. Ethylene glycol is toxic when ingested.
Although volatile, its vapours rarely exert any significant acute
toxic effects when inhaled. However, with continued exposure to
ethylene-glycol vapours, the respiratory passages become moderately
Ruptured coolant lines frequently result in smoke in the cockpit,
either from the engine overheating or from leaking fluid. Smoke in the
cockpit is always a concern for pilots; some have abandoned their
aircraft because of coolant-line leaks. The flash point of pure
ethylene glycol is 177 degrees Fahrenheit; however, the fire hazard
from escaping coolant-fluid ignition is not especially great because
the ethylene glycol has been diluted with water.
The oil-hose connections in aircraft consist of the various types of
adjustable clamps in contrast to the pressure-type connections used in
the hydraulic system. Hose clamps occasionally break or loosen. When
oil escapes onto hot engine parts, smoke often forms and enters the
cockpit. Inhaling hot oil fumes causes symptoms similar to those of
carbon monoxide poisoning:
fire extinguishing agents
Fire-extinguishing agents can pose a toxic threat to the aircrew
fighting a fire, especially within an enclosed cabin or cockpit. Crew
members could come into contact with these agents by using portable
extinguishers. They may also be exposed to gaseous fire-fighting
agents in the ventilation system when automatic or semiautomatic
fire-extinguishing systems aboard the aircraft are discharged.
Ground-support personnel could also inhale fire-extinguisher agents
but to a lesser extent because of the non-enclosed environmental
conditions. The three chemical classes of fire-extinguishing agents in
use today are—
The halogenated hydrocarbon group is composed of carbon tetrachloride,
or CCl4; chlorobromomethane, or CB; dibromodiflouromethane,
or DB; and bromotriflouromethane. Because of their toxicity, these
halogenated hydrocarbons are no longer used to fight fires. The most
common halogenated hydrocarbon in current use as a fire-extinguishing
agent is Halon.
Halon is frequently seen on the flight line and used in automatic
fire-suppression systems for large electrical/computer areas. It has
excellent fire-suppression properties without chemical residuals.
Halon has specific numbers associated with it, depending on its
particular chemical composition of carbon, chloride, fluorine, and
bromide. Halon is an excellent fire extinguisher and is relatively
nontoxic to personnel except when extensively discharged in an
enclosed space. Within a confined area, Halon acts as a simple
asphyxiant (displaces oxygen from the room upon release). Under
extremely high temperatures, this gas can decompose into other more
toxic gases such as hydrogen fluorine, hydrogen chloride, hydrogen
bromide, and phosgene analogues. In addition, the discharge of Halon
from a compressed state can generate impulse-noise levels of more than
160 decibels. Halon is being removed from all but mission-essential
areas because of its strong tendency to deplete the atmospheric ozone
Phosgene (a thermal by-product of Halon), carbon tetrachloride, and
the burning plastics significantly irritate the lower respiratory
tract. Exposures to sublethal concentrations of this gas may
permanently damage the respiratory system.
a fire extinguisher, carbon dioxide becomes a hazard because large
quantities of the gas are required to extinguish a fire. At low
concentrations, carbon dioxide acts as a respiratory stimulant. Beyond
this concentration, inhaling 2 to 3 percent concentrations results in
a feeling of discomfort and shortness of breath. A person can tolerate
up to 5 percent concentrations for 10 minutes. A concentration of
about 10 percent appears to be about the maximum exposure that a
person can tolerate before performance deteriorates. A concentration
above 20 percent can induce unconsciousness within several minutes.
Initial acute exposures (less then 2 percent) of carbon dioxide may
result in excitement or increases in breathing rate and depth, heart
rate, and blood pressure. These effects are followed by—
Beginning with 10 percent concentrations, an aircrew member may
experience mental degradation, collapse, and death. When the
concentration increases slowly, symptoms appear more slowly and have
less effect because the defences of the body have time to act.
Although aware of the changes occurring, the individual may be unable
to assess the situation and take corrective action.
5-40. Because carbon dioxide is heavier than air, it accumulates in
lower positions of enclosed spaces. Normal air becomes diluted, and
the carbon dioxide acts as a simple asphyxiant. Aircrews must be
indoctrinated to the hazards of carbon-dioxide poisoning. When the
initial symptoms of carbon dioxide are detected in the cabin area, it
must be ventilated quickly. The crew should use 100 percent oxygen if
it is available on the aircraft.
aqueous film forming foam
AFFF is a protein-based material used to physically separate a
flammable liquid (fuel) from its oxygen source. It is essentially
nontoxic, even if ingested, but will irritate the eyes and skin,
similar to household soaps.
Fluorocarbon plastics are used in all aircraft as insulation on wires
in radios and other electronic equipment as corrosion-resistant
coatings. They are chemically inert at ordinary temperatures but
decompose at high temperatures. In aircraft, they pose a problem only
when a fire occurs. At about 662 degrees Fahrenheit, fluorine gas is
released. It reacts with moisture to form hydrogen fluoride, a highly
corrosive acid. Above 700 degrees Fahrenheit, a small quantity of
highly toxic perflouroisobutylene is also released. Rapid,
uncontrolled burning of fluorocarbon plastics yields more toxic
products than does controlled thermal decomposition. If a fire occurs
in an aircraft, aircrew members must wear oxygen masks to protect
themselves against the fumes from fluorocarbon plastics. These agents
are very irritating to the eyes, nose, and respiratory tract.
The experience of perceived oxygen contamination affects the
performance of aircrews who routinely fly high-altitude profiles.
Aviators have often reported objectionable odours in oxygen-breathing
systems using compressed gaseous oxygen. While not present in toxic
concentrations, these odours can produce nausea and perhaps vomiting.
In situations other than accidental or gross contamination, the
analysis of oxygen has indicated the presence of small amounts of a
number of contaminants. These include water vapour, methane, carbon
dioxide, acetylene, ethylene, nitrous oxide, and traces of
hydrocarbons as well as unidentified contaminants. Complaints of
oxygen-tank odours also have been attributed to the solvent
trichlorethylene, which has, in the past, been used in cleaning the
cylinders. The contaminants, either singly or in combination, never
seem to reach concentration levels that are toxic to humans. Often the
odours are neither offensive nor disagreeable, as indicated by such
descriptive terms as stale, sweet, cool, fresh, pleasant, and
unpleasant. Distinct symptoms that have been reported are headache,
sickness, nausea, vomiting, and in some instances, disorientation.
However, the usual problem with perceived oxygen contamination is most
often psychological rather than physiological. During flight, aviators
can become more concerned and apprehensive about their
oxygen-breathing source. This preoccupation could lead to
stress-induced hyperventilation or loss of situational awareness. If
pilots are concerned about this issue, they should land as soon as
practical to evaluate the oxygen equipment.
Key points to remember are—
Be acutely aware of the potential toxic hazards in the aviation
environment and the lethality associated with them at flight
In the working environment, use appropriate personal protective
equipment to protect yourself from inhalation, absorption, and
ingestion of toxic agents.
Always work in well-ventilated areas when using toxic materials.
Periodically analyze your own processes. If you perceive that they
are not normal or if you have a strong urge to go to sleep or feel
dizzy or unusual in any way, you may be experiencing the subtle
onset of an incapacitating toxic exposure.
Pay strict attention to physical symptoms such as a headache,
burning eyes, choking, nausea, or reddened patches of skin, which
may indicate a toxic exposure.
Most importantly, remember that your immediate action measures—such
as rapid ventilation of the cockpit, descending from high altitudes,
or landing the aircraft as soon as possible and evacuating the
aircraft—can alleviate a disaster.
Last, even if you land safely but suspect that you have been exposed
to a toxic hazard, consult your flight surgeon or another physician
as soon as possible.