[nitrogen]

I lived in an apartment two floors above a pool and barbecue area for a year. I regularly had severe air quality issues that did not affect the floors below me. I could watch the smoke from the barbecue grills rise up to my level, then travel horizontally across and into my poorly sealed windows due to persistent local air currents.

I moved to another apartment building for the two years after that, and continued to have measurable AQ issues because the dryer, bathroom, and kitchen vents were flowing in reverse due to a poor building ventilation design that used a single central vent shaft and didn't adequately account for wind or the height of the building. I was on a side and floor of the building that had negative relative pressure much of the time.

Air isn't a perfectly homogenized uniform fluid -- there are very localized effects, and because it's often invisible, those local effects are often dismissed.

IMO the only sane and responsible way to develop apartments in a challenging environment (by a highway, by a fire or bbq environment, in an area where weed is legal, or on a toxic site) is for every single unit to have its own air handling system to maintain air quality and positive pressure in that individual unit.