Vented dryers suck air in from a room, warms the air up and pushes it through your clothes, blowing the moist air through a dryer tube to an outside vent.
The dryers are often placed in basements or mud rooms. People rarely think of, or do not know that the dryer is pulling air from a vent on the unit and blowing that same air into their clothes like a filter. The air intake vent is usually located behind the unit, where people rarely clean, because it cannot easily be reached. Since the air is pulled from the inside of the home and is then blown outside, it creates a negative pressure. Air needs to come from somewhere and homes are often air tight. This result in air getting sucked from your walls through light fixtures, outlets and wall switches. If someone has fiberglass behind their walls it will suck out microscopic glass fibers. If you have a vented dryer, that is a dryer that has a hose which runs to the outside where air is blown out, look for the air intake vent on your unit and be aware that the air filtering through your clothes is coming from that vent.
Ventless Dryers - An Alternative to Vented Dryers
Ventless dryers do not suck air from the room. Instead they have an internal loop that heats a small amount of air, blowing it through the laundry. It is then pushed past the clothes taking the moist air with it to a coil that is cooled with tap water. The water falls out of the air, just like it does in a dehumidifier. The water is then drained away. This same air is then reheated and the process starts all over again. Air is not sucked into the dryer in order to dry the clothes and no air is ever vented to the outside of the home. Therefore no negative pressure is created and dirty air is not filtered through the laundry.
This is a picture of the back of a ventless washer, dryer combo. The machine first washes the clothes and then goes into the dry cycle without the need to switch the clothes from one unit to the other. Notice the lack of a dryer vent both intake and out. It only needs water, power and a drain tube.