Until 1985 all commercial airlines completely replenished the cabin air in planes in flight once every 30 minutes  Since then the rate has been once every hour
The less frequently cabin air is replenished in a plane in flight, the higher the level of carbon dioxide in that plane and the easier it is for airborne illnesses to be spread
In 1980 the level of carbon dioxide in the cabin air on a two-hour commercial airline flight was lower than it is today on a similar flight.