Record ozone levels yesterday left Hong Kong resembling Asia's pollution blackspots of Bangkok and Shanghai.
The harbour was wreathed in a heavy pall of ozone-induced smog that left Kowloon virtually invisible from the island.
Ozone, known for its characteristic blue hue, can cause asthma and damage the lungs.
The air pollution index hit 118 in industrial areas - its second highest level ever - and into the 90s in urban and rural areas. Officials told people with respiratory problems to stay indoors.
Ozone readings in the industrial area of Kwai Tsing, Tsuen Wan and Kwun Tong were the highest in the territory at 269 micrograms per cubic metre for an hour in the afternoon.
The levels also broke the US Environmental Protection Agency's hourly health limit of 240 micrograms/cu m for the first time since July 1994.