In ExternE the working hypothesis has been to use the DRFs for particles and for O3 as basis. Effects of NOx and SO2 are assumed to arise indirectly from the particulate nature of nitrate and sulfate aerosols, and they are calculated by applying the particle DRFs to these aerosol concentrations. But the uncertainties are large because there is insufficient evidence for the health impacts of the individual components or characteristics (acidity, solubility, ...) of particulate air pollution. In particular there is a lack of epidemiological studies of nitrate aerosols because until recently this pollutant has not been monitored by air pollution monitoring stations. All DRFs for health impacts have been assumed linear at the population level, in view of the lack of evidence for thresholds at current ambient concentrations. By contrast to the homogeneous populations of cloned animals studied by toxicologists, the absence of a no-effect threshold is plausible for real populations because they always contain individuals with widely differing sensitivities (for example, at any moment about 1% is within the last nine months of life and thus extremely frail).