30/09/2026
Here's a few reasons why pilots should not smoke or drink alcohol before flying. This Human Factors theory is relevant to recreational as well as GA flying.
How oxygen reaches the body’s tissues
Hypoxia occurs when the body’s tissues do not receive, or cannot use, enough oxygen to maintain normal function. Before oxygen can be used by the cells, it must enter the lungs, pass into the bloodstream, bind to haemoglobin, circulate around the body and reach the tissues. The cells must then be able to use the oxygen for normal metabolism. A failure at any stage of this process can produce hypoxia. Aviation medicine generally recognises four forms:
1. hypoxic
2. hypemic
3. stagnant
4. histotoxic hypoxia
Although each has a different cause, they can produce similar effects because the final result is an inadequate supply or use of oxygen at the tissue level.
Hypoxic hypoxia
Hypoxic hypoxia occurs when there is insufficient oxygen available to enter the bloodstream through the lungs. It is the form most commonly associated with aviation because it develops as atmospheric pressure decreases with altitude. The proportion of oxygen in the atmosphere remains close to 21 per cent as an aircraft climbs, but the total atmospheric pressure falls. The partial pressure of oxygen therefore falls as well, reducing the pressure gradient that normally moves oxygen from the alveoli in the lungs into the blood. As a result, less oxygen becomes available to the body with each breath. Unpressurised flight at altitude is the most common aviation cause of hypoxic hypoxia, although it may also result from a loss of cabin pressurisation or a fault in an oxygen system. A blocked, kinked or disconnected hose, an incorrect flow setting, a malfunctioning regulator or a poorly fitting mask may all reduce the amount of oxygen reaching the pilot. The body initially attempts to compensate by increasing breathing rate and heart rate. These responses may delay the onset of noticeable symptoms, but they cannot fully overcome the continuing reduction in oxygen partial pressure as altitude increases.
Hypemic hypoxia
Hypemic hypoxia occurs when the blood cannot carry its normal amount of oxygen. It may result from a reduction in the amount of haemoglobin available or from a substance that prevents haemoglobin from carrying oxygen effectively. The term anaemic hypoxia is also sometimes used for this condition. Haemoglobin is contained within the red blood cells and transports oxygen from the lungs to the tissues. Anaemia, blood loss and recent blood donation can reduce the amount of haemoglobin available, lowering the blood’s oxygen-carrying capacity. These conditions may produce few noticeable effects at sea level but can increase susceptibility to hypoxia as altitude increases. Carbon monoxide poisoning is a particularly important aviation cause of hypemic hypoxia. Carbon monoxide can enter the cabin through a leaking exhaust system or a fault associated with the cabin-heating system. It binds to haemoglobin much more readily than oxygen, forming carboxyhaemoglobin and preventing the affected haemoglobin from transporting oxygen.
Smoking also introduces carbon monoxide into the bloodstream. A smoker may therefore begin a flight with some haemoglobin already unavailable for oxygen transport, making the effects of altitude more pronounced and causing symptoms to appear at a lower altitude. Carbon monoxide exposure may cause headache, nausea, dizziness, weakness, confusion and impaired judgement. Because these symptoms can resemble altitude-related hypoxia, carbon monoxide should be considered when several occupants develop similar symptoms, when cabin heat is being used or when there is any reason to suspect an exhaust-system fault.
Stagnant hypoxia
Stagnant hypoxia occurs when oxygenated blood is available but circulation does not deliver enough of it to the tissues. The problem is therefore not the amount of oxygen in the lungs or blood, but the movement of that blood around the body. High positive G can cause blood to move towards the lower parts of the body, reducing the flow of oxygenated blood to the brain. If cerebral blood flow falls sufficiently, the pilot may experience grey-out, blackout or G-induced loss of consciousness. Shock, severe dehydration, heart or circulatory problems and local restriction of blood flow can also contribute to stagnant hypoxia. Severe blood loss may produce both hypemic and stagnant effects because it reduces the amount of haemoglobin available while also reducing the volume of blood being circulated. Cold causes blood vessels in the extremities to constrict as the body attempts to preserve its core temperature. Although this response protects the major organs, it reduces circulation to the fingers and toes and may impair dexterity.
Histotoxic hypoxia
Histotoxic hypoxia occurs when oxygen reaches the tissues but the cells cannot use it normally. In this case, the lungs, blood and circulation may all be functioning, but cellular metabolism has been impaired by a toxic substance. Alcohol is the most relevant aviation-related example. It interferes with normal brain and cellular function and compounds the impairment caused by reduced oxygen availability at altitude. An amount of alcohol that produces limited noticeable impairment at sea level may have a greater effect when combined with altitude.
Some drugs, medications and chemical toxins can also interfere with normal cellular function or the body’s use of oxygen. A medication that appears to cause little difficulty on the ground may produce more significant impairment when combined with altitude, fatigue or other physiological stresses. Pilots should seek appropriate aeromedical advice before flying while taking medication and should not assume that meeting a minimum waiting period after alcohol consumption means that all residual effects have disappeared.
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