Why does a room that has been locked for weeks become dusty?

Atmospheric air usually always contains a certain amount of dust. Virtually dust-free air is found only over vast seas and high mountains. In Europe, there are on average 100-200 dust particles in 1 cm3 of air. In cities with even more polluted air, this number can reach thousands. The presence of dust can be easily verified even without instruments: in the sunlight falling through the slots of the lowered window blind, countless glittering dust particles can be seen. If there is no significant air movement, most of them will settle. Larger particles are faster and smaller ones are slower. There are millions of grains of dust in a single cubic meter of air that becomes visible when deposited on the surface of objects.

In an airtight enclosure, only as much dust as was originally present in the enclosed air can settle on the surfaces. However, ordinary windows and doors do not provide an airtight closure, we know this well from experience. The air in a living room is completely exchanged in one hour through the gaps in the doors and windows. This is also necessary because people sleeping in an airtight room would suffocate in the morning, for example, so a “room that has been locked for weeks” is constantly flowing with atmospheric air, the dust content of which increases the thickness of the deposited dust layer over time.

Some materials also electrostatically attract dust. This is striking in the case of plastics. The temperature difference also plays a role: dust condenses on colder surfaces.

It means in a locked room certain area may become dust Ex hazardous from the air easily.