Grandpa had winter air quality problems in his house -- it was always too dry and an occasional back draft would fill the room with smoke. Other than that, pollution or the absence of air didn't pose a problem. Why? His drafty old house worked like a fresh running stream -- it continually carried away ordinary pollutants and supplied him with lots of cold, fresh outdoor air. He might hire a mason next time to avoid the backdraft, but dry winter air was a way of life whether he understood it or not. (search keyword "relative humidity" for the title "WHAT IS RELATIVE HUMIDITY?")
When we seal a modern house the way it should be sealed we lose those cleansing cold-air drafts (and we save a lot of money). At the same time, things that have always been floating around in our household air can build up to dangerous concentrations. (Even water can be bad for your health if you get too much of it -- it's called drowning.) As well, without a steady supply of fresh air, we begin to use up things that we need in the air like oxygen and negative ions. I call this air starvation.
Luckily for us, with the exception of the ions, the pollution can be diluted to the same acceptable levels Grandpa had in his house and the oxygen supplied too, simply by properly controlled air changes or ventilation. (The overview "Insulation & The House As A System" found in the Nuts & Bolts section of the site explains how to ventilate properly -- here I want to talk about the air quality itself.)