Abstract: Objective： To analyze the effect of domestic high-efficiency particulate air (HEPA) puri-fiers on the concentrations of indoor fine particulate matter (PM2.5) and its elementary constituents in 20 residences in a district of Beijing during winter. Methods: From November 2015 to January 2016, 20 residences in a district of Beijing were selected, where indoor and outdoor PM2.5 data were collected simultaneously in three time periods according to the operating of air purifiers (Group 0 h: 24 hours before operating; Group 24 h: 24 hours after operating; Group 48 h: 24 to 48 hours after operating). The content of 21 elements in PM2.5 samples were determined by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICPOES). Indoor/outdoor particle concentration ratio (I/O ratios) and ΔI/O ratios were used to describe the pollution levels and the variation range of PM2.5 and its 21 elementary constituents. One-way analysis of variance (ANOVA) for repeated measurement data was applied to compare the I/O ratios of PM2.5 and its elementary constituents among the different groups, and Bonferroni method was used for comparison in pairs. Wilcoxon signed rank test for paired-samples was used to compare ΔI/O ratios of 21 elementary consti-tuents with that of PM2.5. Results: The median I/O ratios of PM2.5 in the three groups were 1.27 (P25-P75: 0.50-2.68), 0.45 (P25-P75: 0.27-1.03) and 0.36 (P25-P75: 0.28-2.48), respectively. Compared with Group 0 h, the I/O ratios of PM2.5 in Group 24 h (P=0.042) and Group 48 h (P=0.006) decreased significantly. However, there was no significant difference between Group 24 h and Group 48 h. Significant differences were found comparing ΔI/O ratios of aluminium, ferrum and titanium to that of PM2.5, in both Group 24 h and Group 48 h (P<0.05). No significant change was found in the I/O ratios of these three elements among the three groups before and after air purifier operating (P>0.05). Distances from residences to traffic arteries could affect I/O ratios of some elements from trafficrelated source (P<0.05). Conclusion: Domestic HEPA air purifiers could effectively reduce indoor PM2.5 concentration, and the pollution level of PM2.5 tend to be stable after the purifier operating for a time. The purifiers had different effects on different elements, among which most showed statistical significances.

Key words: Air pollutants, Particulate matter, Environmental exposure, Elements, Air pollution, indoor, I/O ratio