from the time the door on a plane closes to the time it opens no fresh air gets in, also being at x thousands of feet makes getting emergency treatment for Anaphylaxis somewhat difficult..
Boeing.com
“Here’s briefly how the system works:
Cabin Air System Operation
Pressurized air for the cabin comes from the compressor stages in the aircraft’s jet engines. Moving through the compressor, the outside air gets very hot as it becomes pressurized. The portion drawn off for the passenger cabin is first cooled by heat exchangers in the engine struts and then, after flowing through ducting in the wing, is further cooled by the main air conditioning units.
The cooled air then flows to a chamber where it is mixed with an approximately equal amount of highly filtered air from the passenger cabin. The combined outside and filtered air is ducted to the cabin and distributed through overhead outlets.
Inside the cabin, the air flows in a circular pattern and exits through floor grilles on either side of the cabin or, on some airplanes, through overhead intakes. The exiting air goes below the cabin floor into the lower lobe of the fuselage. The airflow is continuous and is used for maintaining a comfortable cabin temperature. About half of the air exiting the cabin is exhausted from the airplane through an outflow valve in the lower lobe, which also controls the cabin pressure. The other half is drawn by fans through special filters under the cabin floor, and then is mixed with the outside air coming in from the engine compressors.
These high efficiency filters have similar performance to those filters used to keep the air clean in hospitals. Such filters are very effective at trapping microscopic particles such as bacteria and viruses.”