Fig. 4.

Associations of mean concentrations of nitrogen dioxide (2016–2019) and other air pollutants with spreadability of COVID-19, divided by area. (a) The basic reproductive number R₀ was positively associated (meta χ² = 13.74, p = 0.0082) with the average NO₂ value from 2016 to 2019. (b)–(f) There were no significant associations between other air pollutants (SO₂, CO, O₃, PM_2.5, and PM₁₀) and R₀ (meta χ² < 9.09, p > 0.06). We controlled the effects from temperature and relative humidity in the multiple linear regression models.