Abstract: Objective To investigate the impact of fine particle matter （PM_2.5） and ozone on outpatient visits for circulatory system diseases and explore their interactions. Methods Time series research method was used in this study. Data on daily outpatient visits for circulatory system diseases of three Level-3 Class-A hospitals in cities of Guangzhou, Foshan, and Zhuhai were obtained from Guangdong Provincial Center for Disease Control and Prevention between 2015 and 2017. Daily ambient PM_2.5 and ozone concentration data from Guangdong Environmental Monitoring Center and daily meteorological data from Guangdong Provincial Meteorological Bureau were collected in the same period. Generalized additive model （GAM） was used to estimate the excess risk （ER） for each 10 μg/m³ increment in PM_2.5 and ozone_, concentrations and further to explore the potential interactions between PM_2.5 and ozone. Additionally, a meta-analysis was conducted to quantitatively combine the ERs of the three cities. Results During 2015- 2017, a 10 μg/m³ increment in PM_2.5 concentration was associated with an increment in the daily outpatient visits for circulatory system diseases by 2.45%, 0.64%, and 0.95% of ER in Guangzhou, Foshan, and Zhuhai, respectively. And a 10 μg/m³ increment in PM_2.5 and ozone concentrations was associated with an increment in the combined ER of three cities by 1.34% （95%CI：0.25%-2.43%） and -0.17%（95%CI：-0.47%-0.14%）, respectively. Additionally, ozone modified the association between PM_2.5 and the daily outpatient visits for circulatory system diseases. Specifically, when ozone concentration was lowest, the ER for PM_2.5 on daily outpatient visits for circulatory system diseases was highest （ER=4.19%,95%CI：1.82%-6.56%）. However, the modification effect of PM_2.5 on the association between ozone and outpatient visits for circulatory system diseases was not statistically significant. Conclusion Atmospheric PM_2.5 could increase the risk of outpatient visits for circulatory system diseases in the Pearl River Delta urban area, and ozone had a modification effect on it.

Key words: Air pollution, Fine particle matter, Ozone, Time-series study