Most residential forced-air systems don't bring in air from outside (intentionally at least). They recirculate the air in the building through the heating/cooling unit (typically a combination updraft furnace with AC evaporator sitting on top).

Modern systems (since late 80s, maybe?) have return air ducts so that you get even heating even if interior doors (e.g. bedroom doors) are closed, but older systems may only have one return vent per floor or even just a single one in the house, and depend on gaps under doors and such for return air circulation.

Bigger commercial forced-air systems have had exterior air intakes and heat exchanger systems for a while, but are pretty new for residential (I've never seen one in the wild myself).

Heat recovery ventilators [1] are reasonably common in Canada. Most (all?) EnergyStar homes [2] have one, and they includes most if not all the houses I've looked at (or friends have bought) in the past decade or so.

I lived in a house built around 1985 that had one, which was R-2000 [3], so they're at least that old.

[1] https://en.m.wikipedia.org/wiki/Heat_recovery_ventilation

[2] https://www.nrcan.gc.ca/energy/efficiency/housing/new-homes/...

[3] https://www.nrcan.gc.ca/energy/efficiency/homes/20575

Oops, I meant houses that were built in the last ~decade are mostly EnergyStar.

You can add filters on the blower to clean up the air. Especially important in cities.

Also important in the middle of nowhere when the clean, forest air turns to smoke each August.