The recirculated air usually passes through high-efficiency particulate (HEPA) filters before delivery into the cabin.

CNN: Commentary: Flu and your health on a plane

To minimize the risk of disease spread by aircraft, regulations requiring HEPA filters for any aircraft that uses recirculated air should be seriously considered.

CNN: Commentary: Flu and your health on a plane

But he said there was no evidence air conditioning systems in buildings, or the recirculated air on planes, was involved in the spread of the disease.

BBC: Scientists home in on mystery bug

Ventilation can involve either 100 percent fresh air in which outside air enters and leaves the cabin in a single pass or a system in which various fractions of air are recirculated from the aircraft cabin and mixed with fresh air.

CNN: Commentary: Flu and your health on a plane