Beijing, Jan 30 (IANS/EFE): Pollution in the Chinese capital has again surpassed the worst alert levels, leading authorities to urge residents to remain indoors and to order more than 100 factories to suspend operations.

Dozens of flights have been cancelled due to the reduced visibility.

Monitors installed in the US Embassy in Beijing detected at 6 p.m. a concentration of 411 micrograms of particulates per cubic meter of air, 20 times the level deemed acceptable by the World Health Organization.

Hours before, the American measuring devices gave a reading of 517 micrograms on an index where anything above 500 is considered in the worst possible range of pollution.

New Mayor Wang Anshun, appointed Monday, has promised to make the environment his first priority.

Pollution has become an important source of popular discontent and has received unprecedented criticism in the official media.

This is the fourth strong spike in Beijing's contamination levels since Jan 12, when the Chinese capital suffered the worst day of pollution in its history, with the concentration of particulates reaching 993 micrograms per cubic meter of air, or 40 times the maximum recommended by the WHO.

Since then Beijing has announced new measures, such as taking 180,000 obsolete vehicles out of circulation, and has promised to reduce contaminating emissions by 2 percent per year.

Experts say that a large part of the pollution in the capital comes from heavy industry installed in the surrounding provinces, so that without joint action the measures adopted by the city will have a limited effect.