My dad has an old lo-rise apartment building with hot-water heating rather than central air. He is thinking of replacing his old Pearson slider windows with new high efficiency windows. I'm worried that with their tight seal, the windows will have an unhealthy effect on air quality. Each bathroom has a ceiling air vent (6"x6" vent opening) that goes to an exterior vent on the roof, but aside from that there are no other vents. The vents are passive (no fan). If he goes ahead with the windows, will air quality be a concern? If so what are the ways to handle it? What if he just replaces some of the windows and not all of them?