Theory:

The Human Respiratory System:
The system involved in the exchange of respiratory gases and helps in breathing is known as the respiratory system. The human respiratory system consists of nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, and lungs. It also helps in the movement of air inside and outside of the body.
 
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Organs of the respiratory system:
Nostrils:
These are the two openings found at the end of the nose. Air is inhaled and exhaled through the nostrils.
 
Nasal cavities:
It is a large space behind the nose and is filled with air. In these cavities, the temperature and the humidity of the air we breathe in is regulated.
 
Larynx:
It is also known as the voice box as it is the organ involved in the production sound. It is the structure that connects the throat (pharynx) to the trachea.
 
Trachea:
It is also known as the windpipe as it serves as the passage for air. The trachea is a tube that is supported by cartilaginous rings. It connects the pharynx and larynx to the lungs, moistens and warms the air before it passes into the lungs. It finally divides into the right and left bronchi and enter the lungs.
  
Bronchi:
It is a structure that serves as the main passageway into the lungs.
 
Lungs:
These are the most important respiratory organs. They are found in the chest cavity. Inside the lungs, the left and the right bronchi divide further and end in small air sacs called alveoli. The lungs are covered by a double-layered membrane known as the pleura. The diffusion of gases such as O2and CO2 occurs across the alveolar membrane.
What happens to the air that we inhale?
NostrilsNasalcavityPharynxLarynxTracheaBronchusBronchioleAlveolus
 
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 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.