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This article reviews (i) the role of nitrogen dioxide (NO2) in respiratory diseases, including asthma, and (ii) appraises whether current guidelines for NO2 confer an adequate margin of safety for asthmatics. Medline and Excerpta Medica (1966-1997) were searched to retrieve original epidemiological studies which examined the relationship between NO2 and respiratory diseases including asthma. Previously reported reviews of controlled clinical studies were also utilised. The results of this search were then related to current NO2 guidelines. The review of 23 outdoor and 36 indoor studies suggests that respiratory effects are associated with levels of NO2 that may be encountered in common domestic and outdoor settings. Meta-analyses of studies among the general population of children demonstrate a 20% increase in respiratory problems, including increases of asthma by 20%, and wheezing by 12% in households with gas cookers. Epidemiological evidence also indicates that hourly NO2 values of 80 ppb are likely to cause respiratory symptoms in the general population of children. Such levels are frequently present in domestic and school settings. Critical appraisal of the epidemiological evidence indicates the need for a revision of NO2 guidelines to protect both asthmatics and the general population, especially children. [ABSTRACT FROM AUTHOR]