“…These two pollutants also produce negative synergistic effects with particulate matter and SO 2 in many diseases in which their relative risks (RRs) are statistically significant and the highest [8]. Although there are some natural emissions of NOx, the majority of emissions are from anthropogenic sources [9], in which coal and biomass are regarded as the major sources of emissions in northeastern China [10], and urban-traffic NO 2 has increased the COVID-19 pandemic fatality rate [11]. O 3 pollution is also serious due to its close relationship with NOx.…”