How long have recirculation systems been used on passenger airplanes?
Recirculation was in use before the jet age began. For example, the Boeing Stratocruiser of the late 1940s was equipped with an air recirculation system but it did not include HEPA filters. In jet airplanes, filtered or recirculated air combined with outside air came into use principally with the introduction of high-bypass-ratio fan engines. At Boeing, this began with the 747 back in 1970. Keep in mind that air recirculation is common in building ventilation systems.
