BEIJING, Feb. 22 (UPI) -- A cloud of smog shrouding Beijing and surrounding areas has become thicker, authorities said.
The smog covered about 869,920 square miles of central and eastern China as of Friday, prompting authorities in Beijing to raise the city's four-tiered alert system to "orange," China's state-run Xinhua News Agency reported.
China's Ministry of Environmental Protection said the levels of PM2.5 particles in 19 cities was higher than 150 micrograms per cubic meter, compared with China's air quality standard of 75 micrograms per cubic meter.
PM2.5 particles are particles with a diameter less than 2.5 microns, and have been a major contributor to the smog surrounding Beijing.
The heavy air pollution is expected to linger over Beijing and part of nearby Hebei province until at least Thursday, Xinhua said.