Nitrogen trichloride is a highly volatile chlorination disinfection by-product, very commonly found in the air of indoor swimming pools. The aim of this work is to characterize the hazard associated with it and to determine the concentration at which health effects appear, for application in health risk assessments for users of indoor swimming pools. Hazard identification was based on a literature survey and analysis of animal and human studies, with special attention paid to their methodological quality and to reports of a dose-response relationship. A toxicity reference value was derived for respiratory effects, based on human data from both general and occupational data. We selected a lowest-observed-adverse-effect-level of 0.355mg/m(3) based on objective measurements rather than self-reported effects. Two uncertainty factors were applied to take into account both intra-species variability and the use of a concentration with an effect rather than a no-observed-adverse-effect-level. A toxicity reference value of 4x10(-3)mg/m(3) for nitrogen trichloride is proposed for repeated short exposures. Alternative values based on animal data range from 0.01 to 0.03mg/m(3).