How does dust get in a house when the doors and windows are always closed

Dust is more than just bits of dirt and sand from outside. In fact, any solid matter that is broken into small enough bits to be blown around becomes dust. Outside, common sources of dust are dirt, sand, pollen, and pollution. Indoors, common sources of dust include dead skin and hair cells from humans, the carcasses and waste products of microscopic creatures such as dust mites, as well as worn down bits of clothing and furniture. Dust is unavoidable because all solids slowly wear down. However, indoor dust can be minimized through a variety of techniques:

Replace carpeting with hardwood or tile floors to make it easier to remove the dust.
Frequently wash places where dead skin accumulates such as bed sheets, blankets, pillows, and couches.
If necessary, place pillows and mattresses inside zippered dust covers that trap out the dust.
Wipe dust off of furniture, frames, and fixtures using moist paper towels. Using dry paper towels, reusable cloth towels, or feather dusters does not get rid of the dust but just redistributes it to other places.
Clean floors by either mopping or vacuuming with a vacuum that has a dust filter. Sweeping tends to just mix dust around without getting rid of it.
Use a high-quality electric air filter that traps dust.
Regularly clean or replace the air filter in the air vent system.
Store clothes and stuffed animals in sealed plastic bins so that the dust they generate stays trapped.
Use a dehumidifier machine to reduce the ambient humidity. Much of indoor dust is comprised of the waste of dust mites, who need moisture to survive and reproduce.
Reduce the dust tracked into the house by using door mats and taking off shoes and coats when entering the house.
Replace venetian blinds, which collect dust easily and are difficult to clean, with roller shades which can be wiped down easily when unrolled, or cloth curtains which can be washed in a laundry machine.

Credit:https://wtamu.edu/~cbaird/sq/2013/04/29/how-does-dust-get-in-a-house-when-the-doors-and-windows-are-always-closed/