Photo taken on Jan. 13, 2013 shows the fog-shrouded buildings at the Central Bussiness District in Beijing, capital of China. Dense fog on Sunday hit China's east and central regions from the northeast to the south, causing serious air pollution. [Xinhua]

Beijing's air pollution reached dangerous levels yet again on Sunday, marking the third consecutive day of severe smog, municipal environmental authorities said.

The municipal meteorological station issued the city's first orange fog warning Sunday morning due to decreased visibility caused by the heavy smog.

Monitoring data released on Sunday showed that air quality indexes in most regions of Beijing had hit 500, the indexes' highest level.

"Beijing implemented its emergency response plan for hazardous pollution for the first time on Sunday," said Yu Jianhua, director of the air quality department under the municipal environmental protection bureau.

The plan, issued last year, includes multiple measures to combat and respond to hazardous levels of air pollution.

The plan calls for construction sites to limit construction activity that creates large amounts of dust, as well as asks industrial enterprises to reduce emissions, during hazardous pollution days.

The plan also requires municipal traffic authorities to limit government vehicle usage during smoggy days, with the goal of reducing such usage by 30 percent compared to normal days.

The plan also calls for education authorities to instruct schools to limit or completely stop outdoor activity during periods of heavy air pollution.

The municipal environmental monitoring center said readings for PM2.5, or airborne particles measuring less than 2.5 micrometers in diameter, had reached more than 700 micrograms per square meter at several monitoring stations in Beijing, reaching as high as 993 Saturday evening.

"These figures represent extremely bad pollution. Pollutants have gradually accumulated over the course of recent windless days, making the air quality even worse," said Zhu Tong, a professor from the college of environmental sciences and engineering at Peking University.