Why is the cabin air polluted?

Even in a new, well-maintained aircraft, there is always some degree of (albeit light) cabin air pollution. However, this involves several highly harmful substances, including various organophosphates. Below is a list of the main causes of this pollution. Especially the so-called “VOCs” (volatile oil particles) tend to deposit as a greasy layer, causing both the air ducts and the cabin interior to become increasingly polluted with harmful substances over time. This also increases the risk of a Fume Event.

What causes Fume Events?

“Fume Event” is the term used for incidents in which severe contamination of the cabin air occurs. During a Fume Event, significantly larger quantities of harmful substances are suddenly released. This often (but not always!) comes with visible smoke/mist in the cabin and/or an unpleasant odor. The typical smell is often described as “dirty socks” or “wet dog.”

Main causes of polluted cabin air

1) Bleed Air System

In all current jet aircraft (except the B787 Dreamliner), cabin air is supplied by the so-called Bleed Air system. This system allows various harmful substances to easily enter the cabin, including:

1.1 Engine Oil

The engine oil used in jet engines contains various chemical additives, including 3-5% organophosphates such as tricresyl phosphate. These substances are among the most toxic chemical compounds we know of. Inhaling organophosphates is highly detrimental to health.

1.2 Hydraulic Oil

The fluids used in hydraulic systems also contains various (very) toxic substances, including TBP (tributyl phosphate). These substances can enter the cabin, for example, through leakage or overfilling.

1.3 Carbon Monoxide

The Bleed Air used is heated in the compressor to about 400°C. During this process, incomplete combustion of present oil particles occurs, leading to the formation of carbon monoxide.

2) De-icing Fluids

De-icing fluids are used in winter to keep the aircraft free of snow and ice. This is necessary to maintain flight performance. If this fluid is sprayed too close to the air inlets of the APU or the engines, it can subsequently enter the cabin through the air conditioning system.

3) Insecticides

Some countries require airlines to spray insecticides in the cabin of the aircraft. This is done sometimes before passengers board and sometimes during the flight. These toxins also contribute significantly to air and cabin interior pollution.

4) APU

The Auxiliary Power Unit (APU) is a small turbine engine at the rear of the aircraft. It provides electricity for onboard systems and air for air conditioning while the engines are not running on the ground. The APU is often the cause of contaminated cabin air, for example, due to leaks of engine or hydraulic oil or excessive oil refilling.