The air around us has a various types of unwanted particles, such as amoke, dust, pollens etc. When we inhale, the particles get trapped in the hair present in our nasal cavity. Howeever, sometimes these particles may get pass the hair in the nasal cavity. Then they irritate the lining of the cavity, as a result of which we sneeze. Sneezeing expels these foreign particles from the inhaled air and a dust - free, clean air enters our body.