Ozone is produced by ultraviolet light from the Sun hitting the Earth’s atmosphere, lightning, certain electric devices (i.e. air ionisers), and as a byproduct of other types of pollution. Ozone exists in greater concentrations at altitudes commonly flown by passenger jets. Ozone itself is also irritating to lung tissue and harmful to human health. Larger jets have ozone filters to reduce the cabin concentration to safer levels. Outdoor air used for ventilation may have sufficient ozone to react with common indoor pollutants as well as skin oils and other common indoor air chemicals or surfaces. Particular concern is warranted when using “green” cleaning products based on citrus or terpene extracts as these chemicals react very quickly with ozone to form toxic and irritating chemicals as well as fine and ultrafine particles.