evidence: Source & ©: WHO Regional Office for Europe [...] As noted in the introduction, threshold points in dose-response relationships are not readily established, based on either experimental or observational data. For the acute effects of NO2, the evidence comes primarily from human exposure studies at concentrations in the range of several hundred µg/m3 and higher. In general, current exposures in Europe are below this range. Thus, human exposure studies carried out with relatively small numbers of health[y] volunteers do not provide any evidence for the existence of thresholds in a range relevant to current standard setting. [...] See also: General Issues and Recommendations on Air Pollutants: