The inner lining of nostril is lined by nasal hair and remains wet due to mucus secretion. When air passes through the nasal passage, the nasal hair and mucus trap the dust particles and other impurities present in the inhaled air. When dust laden air is inhaled, the dust particles get trapped in the nasal hair and irritate the lining of the cavity. This causes sneezing as a result of which the dust particles are expelled and clean air enters our body.