### Theory:

The pathway travelled by the air in humans
The air that is present in the environment is inhaled by us with the help of the two openings in our nose, known as the nostrils. This air passes to the nasal cavity and reaches the lungs through the windpipe.

The process of inhalation and exhalation
Lungs are found in the chest cavity, which is surrounded by the ribs. The floor of the chest cavity is formed by a muscular sheet known as the diaphragm, during the process of breathing the diaphragm and the ribcage moves.
At the time of inhalation, the ribs move outwards, and the diaphragm moves downwards to increase the space in our chest cavity. Due to this, the air rushes inside, thereby filling the lungs.

At the time of exhalation, the ribs and the diaphragm are brought back to their original positions, i.e., the ribs move inwards, and the diaphragm moves upwards so that the size of the chest cavity is reduced. Due to this, the air is pushed out of the lungs.
We can feel all these various movements that occur during the process of inhalation and exhalation.
The air we inhale contains a mixture of gases in which$$21%$$of oxygen and$$0.04%$$of carbon dioxide is present. The air we exhale includes $$16.4%$$ of oxygen and $$4.4%$$ of carbon dioxide.
The reason behind sneezing
The air we breathe in contains a wide range of unnecessary particles such as smoke, dust and pollens. These particles usually get trapped in the hair found in our nasal cavity during inhalation. Sometimes these particles irritate the lining of the nasal cavity resulting in sneezing. Thus sneezing helps us to get rid of the unwanted particles out of the nasal cavity.
Important!
During sneezing, one should always cover the nose so that others do not inhale the foreign particles which are expelled.
Smoking
It is a harmful activity of humans that damages the lungs. It also causes cancer. Hence, smoking should be avoided.
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