Objective To investigate short-term effects of size-fractionated particle number concentrations (PNCs) on daily respiratory diseases outpatient visits in Wuhan City, China.

Methods Data on daily respiratory diseases outpatient visits from several municipal hospitals and PNCs (0.25-10 μm in aerodynamic diameter) in Wuhan were collected between 1 January 2014 and 31 December 2014. Associations between daily PNCs and daily outpatient visits for respiratory diseases were studied by time-series analysis with semi-parametric generalized additive model.

Results In the single-pollutant model, with an interquartile range (IQR) increase of PNC_0.25-0.5, the respiratory diseases outpatient visits were correspondingly increased by 5.60% relative risk (RR)=1.056 0, 95% confidence interval (CI): 1.004 5-1.1102 for total population and 8.63% (RR=1.086 3, 95%CI: 1.0270-1.1489) for males; with an IQR increase of PNC_0.5-1.0, the respiratory diseases outpatient visits were correspondingly increased by 2.42% (RR=1.024 2, 95%CI: 1.0067-1.0421) for total population and 3.29% (RR=1.0329, 95%CI: 1.0134-1.0528) for males. The excess relative risks of daily outpatient visits for respiratory diseases were increased by 4.45% (RR=1.044 5, 95%CI: 1.0134-1.0765) for total population and 3.89% (RR=1.038 9, 95%CI: 1.003 6-1.075 4) for males associated with an IQR increase in PNC_1.0-2.5. In the two-pollutant model, associations between PNC_0.5-1.0 and daily outpatient visits of respiratory diseases for total population and males were generally stable after adjustment for mass concentrations of particulate matters with an aerodynamic diameters of≤10 μm (PM₁₀) or 2.5 μm (PM_2.5), sulfur dioxide, nitrogen dioxide, carbon monoxide, and ozone (P < 0.05). However, only PNC_0.25-0.5 was significantly associated with the daily outpatient visits of respiratory diseases for males (P < 0.05).

Conclusion PNC_0.25-1.0 is significantly associated with daily outpatient visits of respiratory diseases for total population and males after adjustment for the mass concentrations of particulate matters and gaseous pollutants.