Hong Kong's air pollution has soared to record levels, the Environmental Protection Department says, warning that a toxic stew enveloping the city was a danger to the public.

The government agency said it found Air Pollution Index (API) readings yesterday that in one case were double the level that the general public is advised to stay indoors or avoid prolonged exposure to heavy traffic areas.

On the mainland, sandstorms whipped across a wide swath of China, forcing residents to don masks and scarves to protect themselves.