The MSA SSR 90 is a chemical oxygen apparatus. It supplies the wearer with oxygen which is generated from a chemical (KO2). In a closed circuit the exhalation air of the user is regenerated and enriched with oxygen. The carbon dioxide and humidity of the exhalation air react with the chemical, whereby carbon dioxide is removed and oxygen is generated. The oxygen is always generated according to the demand of the wearer. For instance, at heavy work loads and intensive breathing more oxygen is generated than at low ventilation rates as for example when waiting for rescue.