Objective To explore associations between short-term exposure to ambient nitrogen dioxide (NO₂) and hospital outpatient visits.

Methods Daily outpatient visit data were obtained from 5 hospitals in Beijing, Shanghai, and Guangzhou, the three major cities in China.The study period was from 2013 to 2015.The causes of disease were determined in accordance to the International Classification of Disease 10th version, with the codes A00-R99.Daily air pollution data were from the nearest state-controlled environmental monitoring stations.City-level environmental factors were acquired, including temperature and relative humidity.A time-series generalized additive model (GAM) was adopted to explore the effect of daily NO₂ exposure on hospital outpatient visits, after controlling the time trends, day of week (DOW), holiday, and environmental factors.Then a random-effect meta-analysis was used to pool the results across cities.We also explored the lag structure, two-pollutant models, and potential seasonal effect.

Results During the study period, the average daily outpatient visits were 3 949.The average NO₂ exposure level was 55.3 μg/m³.The lag analysis showed that NO₂ exposure had the greatest effect on outpatient visits every day. For a 10 μg/m³ increase in NO₂, outpatient visits remarkably increased by 0.85% (95%CI:0.51%-1.19%).The associations of NO₂ remained robust after controlling co-pollutants, including fine particulate matter, ozone, sulfur dioxide and carbon monoxide.The effects of NO₂ were more in cool seasons, and the exposure-response curves between NO₂ and outpatient visits were nearly linear.

Conclusions Short-term exposure to NO₂ increases the risk of hospital outpatient visits in major cities, suggesting a need to further lower exposure levels of NO₂ in China.