Present day aircraft recirculate some of the cabin air. Cabin air is however, much fresher than air in say for instance, a big office building. There is always some fresh air coming into the cabin. BTW, cockpit air is all fresh air. We get our own supply.In the old days, before smoking was outlawed on passenger planes, the area aft of the outflow valves usually had a long trail of brown tar smeared along the fuselage. Sometimes the valves would get stuck due to tar buildup. Thankfully those days are over.