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.

toxicology principles
aircraft atmosphere contamination




In aviation, the unique toxicological environment is primarily limited to an enclosed environment. Thus, this chapter’s focus is on aircraft cockpit exposures.

acute toxicity

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:

  • Suddenly incapacitating exposures.

  • Subtle, performance-decrement exposures.

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 relationship

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.

physiochemical factors

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 kidneys.

entry points

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.

individual variability

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 bodily impairment

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.

body detoxification

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 hours.


contamination overview

The interior of an aircraft may contain various contaminants that could present a risk.  Aircraft atmosphere contamination can include—

  • Exhaust gases.

  • Tetraethyl lead.

  • Carbon monoxide.

  • Engine lubricants.

  • Oxygen contaminants.

  • Jet-propulsion fuels.

  • Coolant fluid vapours.

  • Fluorocarbon plastics.

  • Hydraulic fluid vapours.

  • Fire-extinguishing agents, including halogenated hydrocarbons.

exhaust gases

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, radial-engine airplanes.

carbon monoxide

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 exposure.

A 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.

A 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—

  • Tremors.

  • Headache.

  • Weakness.

  • Joint pain.

  • Hoarseness.

  • Nervousness.

  • Muscular cramps.

  • Muscular twitching.

  • Loss of visual acuity.

  • Impairment of speech and hearing.

  • Mental confusion and disorientation.

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.

aviation gasoline

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.

  • Restlessness.

  • Excitement.

  • Disorientation.

  • Disorders of speech, vision, or hearing.

  • Convulsions.

  • Coma.

  • Death.

tetraethyl lead in aviation gasoline

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 narcotic effects.

hydraulic fluid and vapours

A 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.

  • Headache.

  • Vertigo.

  • 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 irritated.

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.

engine lubricants

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:

  • Headache.

  • Nausea.

  • Vomiting.

  • Irritation of the eyes and upper-respiratory passages.

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—

  • Halogenated hydrocarbons.

  • Carbon dioxide.

  • Aqueous film forming foam.

halogenated hydrocarbons

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 layer.

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.

carbon dioxide

As 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—

  • Drowsiness.

  • Headache.

  • Increasing difficulty in respiration.

  • Vertigo.

  • Indigestion.

  • Muscular weakness.

  • Lack of coordination.

  • Poor judgment.

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

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.

oxygen contamination

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.

protective measures

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 altitudes.

  • 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.