It has also been established, both in vitro and in animal models, that PM causes a number of biological effects that may be important in explaining the adverse health outcomes that occur in the presence of air pollution (reviewed in Ref. 42). Other components of polluted air, such as nitrogen dioxide, which are correlated with adverse health outcomes have only been conclusively shown to have biologic activity alone in model systems at very high doses.39 Thus they are seen generally as surrogate markers for active pollutants rather than the