They either idle the engines / run the APU to keep air circulating the whole time, or else they pipe in fresh air. I think it's SeaTac airport where they paint

They either idle the engines / run the APU to keep air circulating the whole time, or else they pipe in fresh air. I think it's SeaTac airport where they paint a message about "Mister Pipe" or "Mister Air Pipe" right on the jetway air pipe, along with a message about how it reduces pollution by reducing engine / APU idling, ending with "Thank You, Mister [Air?] Pipe". This "pipe" allows the more efficient (and bulky / heavy) air filtration and industrial chillers in the airport to be used to supply fresh air to the plane while the jetway is extended. If this weren't the case, the plane would get very stuffy very quickly. Imagine your share of the cabin space. Now imagine yourself sealed in a cardboard box that size waiting for the plane to board. How many minutes before your cardboard box becomes unbearable? They're always pumping in fresh air while passengers are in the airplane.