Hong Kong was smothered in pollution for a second day on Thursday. Pollution readings were "very high" in business and shopping districts such as Central, Western, Causeway Bay and Mongkok.
The readings are the highest since March 2010 when a sandstorm in northern China covered Hong Kong in dust.
Views of the skyscrapers of Hong Kong Island and the Peak were obscured from across the harbour in Kowloon.
The pollution comes largely from coal-fired power stations and traffic, as well as the factories in the Pearl River Delta of China.