The harmful effects of ClO₂ in human organisms, including symptoms and example cases, are described. The highest admissible concentration of ClO₂ in air varies between 0.0001 and 0.0003 mg/1, values much lower than the limit of olfactory perception at 0. 003 mg/1. There is a danger-zone at which harmful concentrations of chlorine dioxide remain unnoticed. Air monitoring is necessary for very low concentrations. Practical procedures are described by the author in his next two articles (<i>Przeglad Papierniczy, 1974, pp. 158 and 466). -- AATA