The quality of air in the cabins

One of my flying school instructors used to joke that nothing smelled worse than the cabin of a plane after a long haul flight.
The ground staff opening the doors are apparently assaulted by the smell of sweat, fatigue and peanut tinged farts whenever they open the doors after receiving a plane. The cabin occupants of course are blissfully unaware since being bathed in this environment for eight hours has numbed their sense of smell.

Despite the jokes there has been plenty of concern about the quality of air in the cabins from both passengers and crew. Early airliners used turbo compressors to provide pressurised air to airplane cabins. These were inefficient and were quickly replaced in the next generation of planes by the more advanced bleed air systems.

Bleed air systems work by taking air from the low pressure section of the engine compressor and feeding it to air conditioning packs. The packs take this bleed air, cool it and remove some water.

This air is then mixed with about 50 per cent recirculated air and reintroduced into the cabin. It is this recirculation that causes the most grief among passengers. While it definitely saves fuel by reducing the bleed air demand, it is widely believed that pilots deliberately reduce the amount of fresh air for economic reasons.

This is not possible in most planes. In a few planes it is possible to reduce pack flow slightly when there are very few passengers. However, all planes have a minimum flow rate that is a lot higher than you would find in most enclosed places that no one complains about.

The recirculated air is also passed through industrial grade filters that can apparently trap bacteria and viruses. Most diseases, of course, aren’t truly airborne but this might help some hypochondriacs get some sleep in flight.

Exhaust pollutants

Slightly more serious than the quantity of air is the quality. In theory the air taken from the compressor is exactly the same as atmospheric air since it is upstream of the combustor where fuel is burnt. Therefore, it is free of all the long named exhaust pollutants that cause all sorts of cancers.

The problem is that sometimes the oil and hydraulic fluids can get into the bleed air. Various groups have measured minute quantities of contaminants in the cabin. This is really only a problem for crew who spend far more time in the air than even the most committed frequent flier.

A lot of research has gone into this but, so far, no reputable study has been able to prove a definitive link between ill-health and prolonged exposure to cabin air. For this reason, there are no occupational health regulations that govern this issue. However, pilot unions and authorities are monitoring the situation should the guidance need to change.

Thankfully, as a Dreamliner pilot this issue is academic. The 787 is the only modern airliner that does not use bleed air and consequently its air is as close to natural as it can get.

Dr Ondieki is a pilot with an international airline.