Impact of air pollution changes and meteorology on asthma outpatient visits in a megacity in North China Plain

The effects of air pollution and meteorology on asthma is less studied in North China Plain. In the last decade, air quality in this region is markedly mitigated. This study compared the short-term effects of air pollutants on daily asthma outpatient visits (AOV) within different sex and age groups from 2014 to 2016 and 2017–2019 in Tianjin, with the application of distributed lag nonlinear model. Moreover, relative humidity (RH) and temperature as well as the synergistic impact with air pollutants were assessed. Air pollutants-associated risk with linear (different reference values were used) and non-linear assumptions were compared. In 2014–2016, PM10 and PM2.5 exhibited a larger impact on AOV, with the corresponding cumulative excess risks (ER) for every 10 μg/m3 increase at 1.04 % (95%CI:0.67–1.40 %, similarly hereafter) and 0.79 % (0.35–1.23 %), as well as increased to 43 % (26–63 %) and 20 % (10–31 %) at severe pollution. In 2017–2019, NO2 and MDA8 O3 exhibited a larger impact on AOV, with a cumulative ER for every 10 μg/m3 increase at 1.0 (0.63–1.4 %) and 0.36 % (0.15–0.57 %), with corresponding values of 7.9 % (4.8–11 %) and 5.6 % (2.3–9.0 %), at severe pollution. SO2 associated risk was only significant from 2014 to 2016. Cold effect, including extremely low temperature exposure and sharp temperature drop could generate a pronounced increase in AOV at 9.6 % (3.8–16 %) and 24 % (9.1–41 %), respectively. Moderate low temperature combined with air pollutants can enhance AOV during winter. Higher temperature in spring and autumn could trigger asthma by increasing pollen levels. Low RH resulted in AOV increase by 4.6 % (2.4–6.9), while higher RH generated AOV increase by 3.4 % (1.6–5.3). Females, children, and older adults tended to have a higher risk for air pollution, non-optimum temperature, and RH. As air pollution-associated risks on AOV tends to be weaker due to air quality improvement in recent years, the impact of extreme meteorological condition amidst climate change on asthma visits warrants further attention.