ATPL human performance course: 14 illustrated chapters

Dry air is a mixture of gases whose proportions remain constant throughout the layer in which aircraft fly (and well beyond, up to about 80 to 100 km):

The two gases that matter for physiology are oxygen (consumed by the cells) and carbon dioxide (produced by them, it regulates breathing).

At 18,000 ft, as at 34,000 ft or 50,000 ft, the air therefore still contains 21% oxygen: what changes is the pressure.

Atmospheric pressure decreases approximately exponentially with altitude: each slice of air supports the weight of those above it.

The variation is therefore not proportional: for the same 5,000 ft slice, the pressure drop is greatest near the ground (≈ 170 hPa between 0 and 5,000 ft, barely 30 hPa between 45,000 and 50,000 ft).

All the physiological problems of altitude come from this drop in ambient pressure, and not from a change in the composition of the air.

The ICAO standard atmosphere (ISA) sets reference values:

The ozone layer lies in the stratosphere: it absorbs UVB much better than UVA (UVA, being less energetic, penetrate deeper into the skin).

Dalton's law: the total pressure of a gas mixture equals the sum of the partial pressures of the gases that make it up. The partial pressure of a gas is proportional to its fraction in the mixture:

PO₂ = 0.21 × P

In a climb, the fraction stays at 21% but the total pressure falls: the partial pressure of oxygen decreases in the same proportion as atmospheric pressure. At 18,000 ft, the pressure is halved, and so is the PO₂.

Example: at 33,700 ft, P = 253 hPa, so PO₂ = 0.21 × 253 ≈ 53 hPa.

Dalton's law explains altitude hypoxia: it is the partial pressure, not the percentage, that drives oxygen into the blood.

Boyle-Mariotte's law: at constant temperature, the volume of a gas is inversely proportional to its pressure (P × V = constant). It is the law usually illustrated with a balloon.

In the body, a moist gas (saturated with water vapour, like intestinal gas at 37 °C) expands even more than a dry gas, because the water vapour pressure (≈ 63 hPa) remains constant.

Consequence: gases trapped in cavities (middle ear, sinuses, teeth, digestive tract) expand in the climb and contract in the descent: these are barotraumas.

Henry's law: at constant temperature, the quantity of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid.

Think of a bottle of fizzy drink: while closed, the gas stays dissolved; when opened, the pressure drops and bubbles appear.

In the body, the tissues and blood contain dissolved nitrogen in equilibrium with the ambient pressure. If the pressure falls sharply and quickly (decompression, cabin altitude climbing above 18,000 ft, flying after diving), the tissues become supersaturated and the nitrogen returns to the gaseous state: bubbles form.

Henry's law explains decompression sickness (aeroembolism): the bends, chokes, creeps.

Charles's law (or Gay-Lussac's): at constant pressure, the volume of a gas is proportional to its absolute temperature (in kelvins). For each additional degree, the volume increases by 1/273 of its volume at 0 °C.

The general law of ideal gases: P × V / T = constant. It combines Boyle-Mariotte and Charles.

The law of diffusion: a gas passes from a zone of high partial pressure to a zone of low partial pressure, through a permeable membrane. It describes the passage of oxygen from the alveoli into the blood, then from the blood into the cells, and the reverse path of CO₂. The quantity exchanged depends on the partial pressure gradient.

A pressurised cabin protects against the three families of problems: hypoxia (Dalton), gas expansion (Boyle-Mariotte) and decompression (Henry).

Relative humidity is the ratio between the water vapour present and the maximum quantity the air could hold at that temperature (saturation).

Air taken in from outside at cruise is very cold, hence almost dry: in the cabin, average humidity is about 5 to 15%, compared with 40 to 60% for comfort. Below 40%, the body loses water, mainly through breathing. Consequences: dry eyes, nose and throat, irritated skin, subtle dehydration, fatigue. Thirst is a late signal: you must drink regularly before you feel thirsty.

Ozone (O₃, a molecule made of three oxygen atoms) is abundant at high altitude, especially at high latitudes. In the cabin, it causes eye and throat irritation, coughing, breathing discomfort, headaches and fatigue. It is removed by (catalytic) ozone converters on the bleed air.

Crews are exposed to two sources of ionising radiation:

The Earth's magnetic field deflects charged particles near the equator; they penetrate more easily near the poles. The dose therefore increases with altitude and with latitude (polar routes), as well as with flight time.

Long-term risks: cancers and, in pregnancy, congenital malformations. Under European radiation protection regulations, operators must assess and monitor the exposure of crew members likely to receive more than 1 mSv per year, adapting their schedules. For aeroplanes operated above 15,000 m (49,000 ft), ICAO (Annex 6) requires an instrument that continuously measures and displays the dose rate and the cumulative dose for each flight, and the operator keeps a record of the doses received by each crew member.