Theory:

Water vapour
Air contains water vapour. When the air is in contact with a cold surface, it condenses and then drops of water appear on it.
 
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Water vapour in the air plays a vital role in the process of the water cycle. Water vapor rises in the air then gets cooled and condenses to form clouds that bring us rain.
 
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Dust and smoke
Smoke consists of dust particles and gases which are harmful for the environment. The main cause of the smoke is the burning of fuel.
Long chimneys are used in factories to take away the harmful smoke from our nose. But, this affects the birds living in the surroundings.

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The traffic policemen wear masks in the crossroads of a crowded city to prevent themselves from the harmful gases emitted by the vehicles.
Presence of dust in the air
In a dark room, open a small slit on a door or a window. Make sure that the door should be facing the sun to allow sunlight inside the room. A tiny shining particles, called dust, can be seen in the sunlight.
 
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During winter, the dust particles can be seen in a beam of sunlight passing through the trees. Thus, we can conclude that air also contains dust particles and smoke. It may vary from time to time, and also from place to place. The dust particles can be seen clearly on a transparent glass window.
 
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Prevention of dust in the body
Most often we inhale air through our nose and not by mouth. This is because fine hair and mucus are present inside the nose to filter the dust particles. It prevents the dust particles from getting into the respiratory system. It is not advisory to breathe through the mouth, because harmful dust particles can easily enter in the body to affect our health.
Reference:
https://image.shutterstock.com/image-photo/focusing-transparent-glass-drops-water-600w-1741223831.jpg
https://image.shutterstock.com/z/stock-vector-the-water-cycle-illustration-infographic-vector-1601995063.jpg
https://image.shutterstock.com/z/stock-photo-polluted-smoke-from-coal-power-plant-129709403.jpg
https://image.shutterstock.com/z/stock-photo-dust-on-dirty-glass-window-for-background-1827350183.jpg