Pro-democracy protesters may have inconvenienced commuters yesterday, but by blocking roads in some of the city's busiest areas they also managed to clear the skies. The air quality was better than usual for a weekday in Causeway Bay, Mong Kok and Central, where the protests are centred, according to Environmental Protection Department data. The health risk posed by air pollutants in the three areas was "low", as opposed to the usual "high", the department's website said. And it remained "low" all day yesterday, except in Mong Kok, where it changed to "moderate" at 3pm. Kwong Sum-yin, chief executive of the Clean Air Network, said the link was clear. "The roadside stations mainly detect traffic emissions. They are less susceptible to air pollution from the Pearl River Delta region. So it is quite clear that the protests blocking road traffic have improved the city's air quality," Kwong said. In Causeway Bay, the level of nitrogen dioxide, an air pollutant emitted by vehicles, more than halved - from 125 micrograms per cubic metre of air at 10pm on Sunday night to about 60 micrograms after 9am yesterday. In Mong Kok, the level fell from 126 micrograms at midnight on Sunday to 70 micrograms after 9am. And in Central, pollution levels dropped after protesters began occupying Harcourt Road on Sunday afternoon - going from about 110 micrograms per cubic metre of air at 7pm on Sunday to about 60 micrograms after 9am yesterday. A department spokeswoman confirmed that reduced traffic in the areas had helped lessen air pollution levels at the roadside monitoring stations. Kwong said the temporary improvement showed the effectiveness of a low-emissions zone proposed by the government, in which polluting vehicles would be banned from busy districts. Bus companies are already encouraged to use greener vehicles in busy areas to reduce air pollution, but Kwong said this had proved ineffective. Secondary school pupil Christy Leung, who lives in Mid-Levels and joined the protest in Admiralty after school yesterday, said the air quality was "more acceptable" than usual. Another protester, University of Science and Technology student Tommy Wong, also noticed the difference but said "extreme action" was not the answer to improve the city's air quality.