Combined effects of increased O-3 and reduced NO2 concentrations on short-term air pollution health risks in Hong Kong

The reduction of NOx emissions in a VOC-limited region can lead to an increase of the local O-3 concentration. An evaluation of the net health effects of such pollutant changes is therefore important to ascertain whether the emission control measures effectively improve the overall protection of public health. In this study, we use a short-term health risk (added health risk or AR) model developed for the multi-pollutant air quality health index (AQHI) in Hong Kong to examine the overall health impacts of these pollutant changes. We first investigate AR changes associated with NO2 and O-3 changes, followed by those associated with changes in all four AQHI pollutants (NO2, O-3, SO2, and particulate matter (PM)). Our results show that for the combined health effects of NO2 and O-3 changes, there is a significant reduction in AR in urban areas with dense traffic, but no statistically significant changes in other less urbanized areas. The increase in estimated AR for higher O-3 concentrations is offset by a decrease in the estimated AR for lower NO2 concentrations. In areas with dense traffic, the reduction in AR as a result of decreased NO2 is substantially larger than the increase in AR associated with increased O-3. When additionally accounting for the change in ambient SO2 and PM, we found a statistically significant reduction in total AR everywhere in Hong Kong. Our results show that the emission control measures resulting in NO2, SO2, and PM reductions over the past decade have effectively reduced the AR over Hong Kong, even though these control measures may have partially contributed to an increase in O-3 concentrations. Hence, efforts to reduce NOx, SO2, and PM should be continued. (C) 2020 Elsevier Ltd. All rights reserved.