How Cabin Air Systems Work

Aircraft Ventilation Systems

In a typical commercial cabin air recirculation system, the air supplied into the cabin consists of approximately:

• 50% outside air from either the engine's compressor stage (engine 'bleed air') or the Auxiliary Power Unit (APU) mixed with approximately
• 50% of filtered, recirculated air.

(Note however, that some aircraft designs are now moving to 'bleed air free' ECS but these aircraft still use HEPA and HEPA/odor filters in the system)

Where are Cabin Air Filters Located

Cabin air filters are normally located in the recirculation loop, near the mixing unit either underneath or over the main passenger cabin. However, these only treat the recirculated air.

Some particulate contamination, odor causing compounds and volatile and semi-volatile organic compounds (VOCs and SVOCs) may enter from the outside air system. Air purification systems should therefore be installed to treat both the outside air ('bleed air'), as well as the recirculated air.

Why Do You Need Cabin Air Filters?

Due to the large number of passengers in the aircraft cabin, there may be high concentrations of dust, fibers, bacteria and other microorganisms. All of these are potential health hazards to crews and passengers.

In order to enhance passenger and crew health and comfort the HEPA filters must be fine enough to control these contaminants. 

Examples of contamination type and removal technologies are shown below.