Theory:

In a dark room, open a small slit on a door or a window. Make sure that the door faces the sun to allow sunlight inside the room. Tiny shining particles, called dust, can be seen in the sunlight.
 
dust hole.jpg
Dust particles seen in the sunlight
 
During winter, these dust particles are seen in a beam of sunlight passing through the trees. Thus, we can conclude that the air present around us in the room contains some dust particles although it may vary from time to time and also from place to place.
 
tree-3812739_1280.jpg
Sunlight passing through the trees
 
Since the dust particles are too small, they are not visible to us. These shining tiny particles become visible when the sunlight falls on them. It can also be seen clearly on a transparent glass window.
 
window dust.jpg
A glass window
 
Prevention of dust in the body:
Most often we inhale air through our noses and not by mouth. This inhalation process is because of fine hair and mucus present inside the nose to filter the dust particles. It prevents the dust particles from getting into the respiratory system. It is not advisable to breathe through the mouth because harmful dust particles can easily enter the body to affect our health.
 
Long chimneys are used in factories to take away the harmful smoke and dust from our noses. But, this affects the birds living in the surroundings.
 
industry.jpg
Industrial chimneys

The traffic policemen wear masks at the crossroads of a crowded city to prevent themselves from the harmful gases emitted by the vehicles.
 
shutterstock_1715265178.jpg
Policemen wearing a mask