ObjectiveTo understand the prevalence, source, characteristics of indoor PM_2.5 and its correlation with outdoor PM_2.5 in Putuo District of Shanghai, and to provide evidence for establishing standards and controlling pollution.

MethodsA total of five apartments in different areas in Putuo District were selected.We conducted 24-hour indoor and outdoor PM_2.5 mass concentration monitoring for five consecutive days in the four quarters of 2018.In addition, we recorded the running condition of kitchens, air purifiers, air conditioners, and ventilation, and indoor smoking.

ResultsThe indoor PM_2.5 concentration was determined to be (49±33) μg/m³, with a median of 43 μg/m³.The outdoor PM_2.5 concentration was (53±29) μg/m³, with a median of 47 μg/m³.The indoor PM_2.5concentration was higher in spring, summer and winter; similarly, the outdoor concentration was higher in winter, followed by spring and summer.The indoor PM_2.5 concentration was lower from 0: 00 to 6: 00, whereas higher from 7: 00 to 23: 00.Comparatively, the outdoor concentration was lower from 7: 00 to 17: 00 and higher from 18: 00 to 6: 00.There was a significantly positive correlation between indoor and outdoor PM_2.5 concentrations (rs=0.825, P < 0.001).The I/O ratio was 0.93±0.53, with a median of 0.87.Smoking, cooking, ventilation time and running of air purification equipment were determined to be the factors associated with the I/O ratio.

ConclusionThe indoor and outdoor PM_2.5 concentration in the apartments is generally acceptable. Indoor and outdoor PM_2.5 concentrations are significantly correlated, with spatial, seasonal and diurnal change.According to different sources of PM_2.5 pollution, increasing ventilation time or using air purification equipment can effectively reduce the I/O ratio.