sneeze gas, poison gas used in riot control and chemical warfare. It is highly irritating to the nose and throat; inhalation of the substance causes sneezing and coughing, but rarely causes permanent injury. Mixed with CN (common tear gas, chloroacetophenone), it is used by police for dispersing crowds. Chemically, it is diphenylamine chloroarsine; sneeze gas is prepared by heating diphenylamine with arsenic trichloride.

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.