More than 8 million people travel by air every day. From small propeller driven aircraft to large commercial jetliners, aviation transport is a mainstream industry that facilitates tourism and generates economic growth.

Have you ever wondered, though, about the air you breathe when you’re flying? Probably not, as we tend to be on auto-pilot, taking everything for granted, assuming that cabin air is clean and safe to breathe in.

The fact is, cabin air isn’t as clean as you would think. Cabin air comes from the engine, which sucks in air and spreads it throughout the aircraft fuselage with the help of the Environmental Control System (ECS, a fancy name for the air conditioning system aboard aircraft). The air taken from the engine is called bleed air. The more bleed air you take from an engine, the less efficiently the engine operates, i.e. the flight becomes more expensive.

Most airlines use something called cabin air recirculation to save cost. On average, up to 50% of cabin air is recirculated. The other 50% is drawn in from bleed air, filtered, and then mixed with the existing cabin air.

Budget airlines though, don’t stick to 50% recirculation. Instead, companies like EasyJet and Ryan Air, recirculate at least 75% of the cabin air, with the aim of lowering cost.

A recent study by the WHO revealed that in 2015 more than 3.5 billion airline passengers were exposed to contaminated cabin air, containing low levels of engine oil.  The study stated that consequences of contaminated cabin air include health issues such as respiratory tract problems, eye, nose and throat irritation, respiratory, cardiovascular and neurological symptoms, fatigue, cancer, aerotoxic syndrome and chemical sensitivity.

Newer aircraft such as the Airbus A350, Boeing 787, Airbus A320neo etc, are all equipped with High Efficiency Particulate Air (HEPA) filters, similar to those used in hospital operating theatres, to remove bacteria, viruses and other particles from cabin air before it is mixed with outside air. Older airplanes however (think pre-2008) don’t have such HEPA filters.

How many newer aircraft do we fly? Considering airplanes last 25 years, I’d say not many. Ever come down with the flu shortly after flying? Now you know why.