=Sulphur Dioxide= (SO₂) is a colourless, pungently smelling gas which, acting in low concentration or for a short period, causes cough and irritation of the mucous membrane of the respiratory passages and of the eyes; acting for a longer period, it sets up inflammation of the mucous membrane, bronchial catarrh, expectoration of blood, and inflammation of the lungs. As Ogata and Lehmann have proved by experiments—some of them made on man—a proportion of 0·03-0·04 per thousand of sulphur dioxide in the air has a serious effect on a person unaccustomed to it, while workmen used to this gas can tolerate it easily. As sulphur dioxide probably does not affect the blood, treatment by oxygen inhalation is useless. Otherwise the treatment spoken of as applying to acid poisonings in general holds good. =Sulphuric Acid= (H₂SO₄). Concentrated sulphuric acid occasionally splashes into the eye or wets the skin, causing severe irritation and corrosion, unless the liquid is quickly washed off or neutralised. If the action of the acid persists, the corrosive effect becomes deepseated and leads to disfiguring scars. =Nitrous Fumes, Nitric Acid.=—Nitric oxide (NO) oxidises in the air with formation of red fumes composed of nitrogen trioxide (N₂O₃) and nitrogen peroxide (NO₂). These oxides are contained in the gases evolved from fuming nitric acid and where nitric acid acts upon metals, organic substances, &c. Industrial poisoning by nitrous fumes is dangerous; unfortunately it frequently occurs and often runs a severe, even fatal, course; sometimes numerous workers are poisoned simultaneously. The main reason why nitrous fumes are so dangerous is because their effect, like that of most other irritant gases, is not shown at once in symptoms of irritation, such as cough, cramp of the glottis, &c., which would at least serve as a warning to the affected person; on the contrary, generally no effect at all is felt at first, especially if the fumes are not very concentrated. Symptoms of irritation usually appear only after some hours’ stay in the poisonous atmosphere. By this time a relatively large quantity of the poisonous gas has been absorbed, and the remote effect on the blood induced. The first symptoms of irritation (cough, difficulty of breathing, nausea, &c.) generally disappear when the affected person leaves the charged atmosphere, and he then often passes several hours without symptoms, relatively well. Later severe symptoms supervene—often rather suddenly—difficulty of breathing, fits of suffocation, cyanosis, and copious frothy blood-stained expectoration with symptoms of inflammation of the bronchial tubes and lungs. These attacks may last a longer or shorter time, and in severe cases can lead to death; slight cases end in recovery, without any sequelæ.