“I never open my windows. How do my draperies, window treatments and windows get so dirty?”
The word is convection: the movement of air because of differences in density due to temperature. Warm air is less dense, and therefore lighter, than cool air. When the sun heats the space between your windows and window treatments, the warm air in the space rises. As it does, cooler air is drawn in at the bottom. Conversely, when the air space cools, the flow is reversed. There can actually be quite a bit of air flowing across your window treatments and window glass... even though the windows are closed!
Your window treatments filter the air, as does carpet: they work just like the filter in your heating or air conditioning system.
Some of the substances that stick to your draperies and blinds cause pitting, corrosion, or oxidation, and ruin them. Mildew and mold feed on soil. Dirt, dust, lint, pollen, bacteria, grease, allergens, smoke - all of these can stick to your blinds, draperies or shades, create a film on glass, then bake on in the heat of the day. Much of what collects on your window treatments can be irritating, or cause allergic reactions.