Many people find that they become ill with a cold after traveling by airplane. the air circulation systems of commercial aircraft use hepa filters that should remove any infectious agents that pass through them. what are the possible reasons for increased incidence of colds after flights?

There can be multiple reasons to this question. It is true that the air in the airplanes are filtered by HEPA filters.

But the passenger sitting next to you might be having contagious infections such as common cold or flu.

The seat, toilet and chambers still has many infectious organisms in it inspite of being circulated by pure air.

There are many people packet inside the metal body and the air that is circulated inside might contain infectious microorganisms in it.

So, a person travelling by plane have increased chances of cold after flight.