Heavy roadside pollution is bad for health - of that there is no dispute. The exhaust fumes from vehicles, especially the decades-old diesel buses and trucks common on Hong Kong streets, irritate eyes and skin, exacerbate respiratory problems and, with long-term exposure, can lead to lung cancer and heart disease. Despite the risks, authorities have largely implemented voluntary schemes rather than legislation to improve air quality, leading to little noticeable change and continued risks. An unexpected result of the democracy protests was to give a glimpse of what could be expected were the government to adopt a resolute approach. Student sit-ins closed our busiest streets to traffic, causing the rerouting or cancellation of hundreds of bus services and the clearing of thousands of delivery trucks. Unsurprisingly, on September 29, a normal working day ahead of a two-day holiday break during which the protests gathered steam, the sky was blue and government monitoring station readings were better than usual in the areas where the demonstrations took place: Causeway Bay, Central and Mong Kok. Official data showed the health risk from air pollutants in the areas was low rather than the typical high; it stayed like that throughout the day in all but Mong Kok, where it shifted to moderate at 3pm. There was a steep fall in levels of nitrogen oxide, one of the harmful pollutants emitted by diesel engines. In Causeway Bay, it dropped from 125 micrograms per cubic metre of air at 10pm on Sunday night to about 60 micrograms after 9am on Monday; in Mong Kok, from 126 at midnight on Sunday to 70 after 9am; and in Central, from about 110 at 7pm on Sunday to about 60 after 9am. Roadside station readings are less susceptible to pollution from other sources like power stations and cross-border factories. The disruption of traffic by the protests has had an undeniable impact on air quality alongside streets. Without a government policy shift, after the demonstrations have ended, we will have to rely on our memories of the protest days for what clean vehicles on our roads mean for air quality.