In a typical commercial cabin air recirculation system, the air supplied into the cabin consists of approximately 50% of outside air and approximately 50% of recirculated air.
The origins of ozone and VOCs in cabin air are various. Some of them come from outside, particularly ozone (O3) when the aircraft is flying at high altitudes near the stratospheric ozone layer. The level of ozone in aircraft cabins depends on ambient concentrations, the presence (or absence) of control devices and their effectiveness, the rate of outdoor air supply, and the rate of ozone loss through within-cabin transformation processes, such as reactions with interior surfaces, including those associated with occupants (Nazaroff and Weschler, 2010). Ozone levels outside the aircraft vary with seasons and depend on flight altitude, tropopause height, and on meteorological processes that affect vertical mixing between the lower stratosphere and the upper troposphere.
When ozone is present in an aircraft cabin, ozone-derived reaction products are also present, both in the gas phase and on surfaces. The fact that ozone-initiated chemistry could meaningfully increase the levels of certain volatile and semivolatile organic compounds in aircraft cabin air was first demonstrated in a Boeing-funded study (Wisthaler et al., 2005) conducted in a simulated section of a B-767 at the Technical University of Denmark.
Others pollutants can contribute to the degradation of cabin air quality such as VOCs and nitrogen oxides (products of combustion engines), ethanol (alcoholic beverages) or formaldehyde (cabin materials). As a consequence, passengers and crew in aircraft cabins may be exposed to a combination of elevated ozone levels and a mixture of various VOCs. Exposure to ozone in cabin air has potential health significance for flight crew and for the general flying population. Acute effects from short-term exposure to ozone range from breathing discomfort, respiratory irritation, and headache for healthy adults (Strøm-Tejsen et al., 2008) to asthma exacerbation and premature mortality for vulnerable populations (Gent et al., 2003; Bell et al., 2004). Chronic exposure effects may include enhanced oxidative stress (Chen et al., 2007), reduced lung function in young adults (Tager et al., 2005), and adult-onset asthma in males (McDonnell et al., 1999). There is no established safe level of ozone exposure (Bell et al., 2006).
This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 687014.