We do tend to live in closed-up homes, as we don’t readily open our windows, with our heating in the winter and the air conditioning in the summertime. Before central air conditioning, in particular, people would open their windows to let in fresh air, a practice that is seldom done today.
The ventilation system in your house carries the warm and cold air from your furnace and air conditioner to the rest of your home. Over a period they collect dust, dirt, dust mites, allergens, and mold.
Mold can be especially distressing, as it can cause severe respiratory distress if inhaled over a period. Mold comes from tiny spores that float through the air, but when the spores find a place that is conducive to their growth, the mold can take hold and grow quite rapidly. Of course, this creates more spores.