The module closes with the cabin's own hazards: a poison gas that rides in on the heating system, a reactive gas waiting at high cruise levels, the radiation that thin air no longer filters, and the desert-dry air the pressurisation system delivers. None announces itself; all four are exam material, and the first, carbon monoxide, is the deadliest of them.
Carbon monoxide
Carbon monoxide (CO) is produced by the incomplete combustion of carbon-containing fuels: engine exhaust above all, with cigarette smoke adding a steady background dose in a smoker's blood. The classic aviation pathway is the light piston aircraft whose cabin heat comes from a shroud around the exhaust muffler: a crack in the muffler lets exhaust into the heated air, and the heater delivers the poison straight to the cockpit. The gas is colourless, odourless and tasteless, so nothing warns the crew except the exhaust smell of its dirtier companions, when there is one.
Its weapon is the previous lessons' own molecule. CO binds to haemoglobin far more readily than oxygen, over 200 times as readily, forming carboxyhaemoglobin: every haemoglobin molecule occupied by CO is removed from oxygen transport, and the bond releases only slowly. The result is a shrinking oxygen-carrying capacity while breathing continues normally: the anaemic-hypoxia mechanism of lesson four, produced chemically. Because the loss is gradual and painless, the symptoms build insidiously: blurred thinking and a feeling of uneasiness, then dizziness and headache, with nausea, impaired vision and judgement and weakness following, and eventually unconsciousness; the classic hypoxia signs mingle in, since tissue oxygen starvation is exactly what is happening. Affected pilots may be completely unaware of the exposure, which is why cockpit CO detectors exist.
The syllabus wants the drill on suspicion of CO in flight, and the logic behind each step: shut off the cabin heater (removing the likely source), open the fresh-air vents and windows where possible (diluting the gas), descend to a lower altitude (giving the remaining haemoglobin the richest air available), breathe 100 per cent oxygen if available (competing the CO off the haemoglobin), and land as soon as possible. On the ground the poisoning is treated with continued oxygen, in severe cases hyperbaric oxygen: elimination is slow on air, hours of half-life, and high-pressure oxygen shortens it dramatically. A pilot who has been meaningfully exposed is not fit to fly again until the carboxyhaemoglobin has cleared and a medical assessment says so.
Ozone
Ozone, the triatomic form of oxygen, is created by ultraviolet light acting on oxygen in the stratosphere. Its proportion increases with altitude toward a stratospheric peak far above airline levels, and concentrations are negligible at and below about 40,000 ft, which is why the problem belongs to the highest cruise levels and polar routes where the tropopause dips low. Breathed at concentration, ozone irritates the airways: coughing, chest discomfort, reduced lung performance. The defences are heat, which destroys ozone, and equipment: aircraft flying where ozone lives can be fitted with special ozone removers, catalytic converters in the air-conditioning supply that break the gas down before it reaches the cabin.
Radiation at altitude
The atmosphere is a radiation shield, and cruising aircraft sit above much of it. The radiation reaching them, cosmic radiation, is the collective term for two sources: the solar component from the sun, rising sharply during solar flare events, and the galactic component arriving from outside the solar system. The dose rate grows with altitude and toward the poles, where the earth's magnetic field gives least protection. Excessive exposure to ionising radiation damages living cells and their chromosomes: the listed effects run from cell and tissue damage through an increased long-term cancer risk to genetic effects and risk to an unborn child, which is why crew doses are monitored and pregnant crew members' duties are limited.
Cabin humidity
The last hazard is dryness. The relative humidity of any air depends on two factors: its moisture content and its temperature, since warmer air can hold more water. Cruise-level outside air is extremely cold and holds almost no water; drawn in and heated to cabin temperature, its relative humidity collapses to a few per cent, far drier than any desert the passengers have visited. The listed effects of that dryness: spurious thirst, and dry eyes, skin and mucous membranes. The listed measures match one for one: drinking water, using eye drops and aqueous creams. The dryness is uncomfortable rather than dangerous on a single flight, but for crews it is a career-long exposure, and contact-lens wearers feel it first.