Dust and Smoke - Science

We all know that air is a mixture of several gases like oxygen, nitrogen, carbon dioxide and inert gases. Air also contains minute particles of dust and smoke.
The following activity demonstrates the presence of these minute particles in air. This experiment can be performed in your house.
Here is a boy sitting inside his room.
By switching off the lights, and shutting all the doors and windows he made the room extremely dark.
He opened the front door slightly to allow only a thin beam of light.
He observed that this thin beam of light highlighted the presence of many dust particles in the air which were continuously in motion. This random movement is termed as Brownian motion.
Dust and smoke form a minor constituent of air. The amount of dust and smoke in the air varies from place to place.
They are inhaled by humans through the nose during respiration. But the particles are prevented from reaching the lungs as they are blocked in the nostrils by fine hair like structures. This is one of the reasons why one shouldn't respire through the mouth.
These particles are present in large number where there is a lot of pollution.

[Henk]

Explain to me how it is scientifically possible for smoke to stay in earth’s atmosphere and how it is harmful.

[addythegreat]

This was not at all helpful pls make a longer vid