Street protests and air pollution in Hong Kong

June 2019 saw large-scale street protests in Hong Kong that impeded traffic flow along streets in areas around to the Legislative Council building. These had the potential to reduce overall air pollutant emissions from traffic and lower their concentrations. Two roadside monitoring stations relatively close to the Legislative Council reveal that measured concentrations of nitrogen dioxide declined during the protests compared with measurements from other sites by at least 50% on many occasions. There were only subtle changes in particulate loads and no evidence of any reduction in carbon monoxide concentrations. Pedestrianisation and bus route rationalisation are often seen as methods to reduce exposure in congested areas, but the observations here suggest that the substantial improvements in the nitrogen dioxide levels might not be matched by improvements in other pollutants. Plans for changes to street layouts to improve air quality need careful investigation before they are implemented.