Background: Recent epidemiological and toxicological studies have shown the associations between fine particulate matter (PM2.5) and human health.However, the estimates of adverse health effects are inconsistent in many areas and countries.Objective: The aim of the study was to evaluate the effect of PM2.5 concentration on hospital emergency admissions (HEA)for total and disease-specific respiratory diseases under a heavily PM2.5 polluted circumstance in Beijing.Method: We collected air pollution data of 17 urban monitoring stations from Beijing Municipal Environmental Monitoring Center and weather conditions data fromChina Meteorological Data Sharing Service System.Data on 92,464 emergency visits for respiratory cause were obtained from ten large comprehensive hospitals in urban areas of Beijing.A time-series study was conducted between 1st January 2013 and 31th December 2013.The percentage change of HEA for total and disease-specific respiratory disease per 10μg/m3PM2.5increase were estimated using semi-parametric generalized additive Poisson model, controlling for higher PM2.5 levels was significantly associated with COPD among men at lag0 to lagl and lag0-1to lag0-4, with LRTI among women at lag0 and lag0-1 to lag0-3, with total respiratory disease,LRTI, and COPD among people aged≥66 years at lag0 and lag0-1 to lag0-3 (P<0.05).Conclusions: Thisstudy provides evidence that higher levels of PM2.5increase the risk of hospital emergency admissionsfor respiratory diseases, especially for LRTI and COPD in Beijing.People with different gender and age vary intheir susceptibility to PM2.5 pollution, time trends, seasonality,day of the week, and weather variables.Subgroup analyses by gender and age groups were performed.Result:The overall mean daily concentration of PM2.5 was 102.1μg/m3 with a range from 6.7 to 508.5μtg/m3 in urban areas of Beijing during 2013.For the single-pollutant model(without adjustment for other pollutants), a 10μg/m3 increase in PM2.5 was associated with a0.20% (95%CI: 0.03%-0.38%) and 0.15% (95%CI:0.00%-0.30%) increase in number of HEA for total respiratory disease at lag0 and lag3, respectively.For the cumulative effects, an increaseof10μg/m3pM2.5 resulted in an elevation of 0.31% (95 % CI:0.11%-0.52 %), 0.35 % (95 % CI:0.11%-0.59%) and 0.44% (95%CI: 0.15%-0.73%) for total respiratory disease at lag0-1 to lag0-3days, respectively.For disease-specific analysis, increased number of admissions for LRTI and COPD were significantly associated with higher PM2.5 levels at lag0-1 to lag0-3 days (P<0.05).Subgroup analysis showed