Beijing is experiencing its worst air pollution this year, with monitoring sites throughout the city reporting extremely hazardous levels of pollutants.

City authorities issued a rare orange alert, the second highest of four danger levels. Schools suspended outdoor activities and polluting factories were required to reduce production.

The city reported the level of tiny PM2.5 particles in the air to be more than 600 micrograms per cubic meter late Monday afternoon. The U.S. Embassy in Beijing reported the level to be 625 micrograms per cubic meter at 5 p.m.

The World Health Organization considers the safe level to be 25 micrograms per cubic meter on a 24-hour average basis.