Carbon dioxide (CO 2 ) ) is a trace component of the atmosphere. Its content in ambient air has steadily increased from 300 ppm in the 19th century to more than 400 ppm in our time. 1 The German occupational exposure limit (OEL) allows 5000 ppm CO 2 as time-weighted average and 10 000 ppm CO 2 as a maximum value for short-term exceedance at workplaces. 2 Exposure to elevated levels of CO 2 is an occupational health hazard currently being discussed, for example, in the transportation sector. Concentrations up to 20 000 ppm CO 2 were measured during food transportation with dry ice. 3 However, potash and coal miners can be exposed to elevated CO 2 levels up to 20 000 ppm or higher. 4,5 As exercise confounds any effects induced by CO 2 inhalation, the OEL was derived based on the additive effects of respiratory