Additionally, differences between populations with respect to age, diet, sex, ethnicity, and state of health have to be taken into account when comparing results from Beijing with other cities in China or abroad. Our findings should be interpreted cau- tiously, because exposure to particle number concentrations was based on data from a sin- gle background-monitoring site. In addition, the possibility that some effects might have occurred by chance cannot be excluded. To verify our findings with respect to different pollutants and particle fractions, further stud- ies should be conducted, including analyzing the spatial correlation of pollutant time series. Conclusions Present levels of air pollution were associated with respiratory ERV in Beijing, China. NO2 concentrations in Beijing appeared to be a bet- ter surrogate measure than PM10 or particle number concentrations for evaluating respira- tory health effects of an air pollution mixture. Ref Rencs Akaike H. 1974. 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