Residents worry about the impact on their health. Even as the government says it is taking steps to cut down on air pollution, campaigners say it is not enough.
Hong Kong's air pollution can be dangerous, even deadly. On bad days, city health officials urge people to stay indoors.
Environmental authorities put the latest smog alert partly down to little or no wind but campaigners say officials should be looking beyond the weather.
The government has taken steps to tackle air pollution over the years, including measures to reduce motor vehicle emissions. In 2013, the environment bureau brought in a clean air plan for Hong Kong.
According to roadside air quality recordings, however, the number of hours smog posed a "high" health risk in 2017 was double the year before.
TRT World's Samantha Vadas reports from Hong Kong.