I don't know if it's a cabin pressure situation or not, but the gasses that enter the cabin make me very sick for days after the flight . I fear the air quality onboard. See also, a Letter , proposing a solution. Pilots try hard to avoid that situation, but airport congestion, combined with contrary wind conditions, sometimes makes that goal difficult to achieve. It isn t possible to smell the exhaust fumes from your own plane as the intake of the air-conditioning packs is far upstream of the engine exhaust cones.
Passengers radiate body heat and exhale carbon dioxide.
It wouldn't take long for both temperature and carbon dioxide to reach unacceptable levels if new air wasn't brought into the cabin, exhausting the old in the process. Additional fans are also turned on, to facilitate the flow of air from those packs. Recirculation fans are also running to enhance the velocity of the air in the cabin. When cruise altitude is reached, it is normal for the packs to change from high flow to normal flow. That is done to reduce wear and tear on the packs and to save fuel.