sneeze, involuntary violent expiration of air through the nose and mouth. It results from stimulation of the nervous system in the nose, causing sudden contraction of the muscles of expiration. The stimulus can include any irritating factor in the nose—inflammation of the tissues as the result of a cold or infection, allergic irritants (hay fever), or irritating substances such as dust or pollutants in the air. An occasional sneeze usually has little significance. Repeated sneezing indicates that some condition of the nose or in the immediate atmosphere requires attention.