The air quality in Beijing and nearby regions hit dangerous levels Thursday, Beijing's environmental authorities said.

A motorcyclist wearing a mask rides on a fog-shrouded road in Beijing on Feb. 28. [Chinanews.com]

Calm winds, temperature inversion, pollutants transformed from eastern and southern regions and large-scale dust from Inner Mongolia are behind the hazardous air pollution in Beijing, Tianjin, Hebei and east China's Bohai Bay, the Beijing Municipal Environmental Monitoring Center said Thursday.

Readings for PM 2.5, or airborne particles measuring 2.5 microns or less in diameter that can deeply penetrate the lungs, reached more than 400 micrograms per cubic meter, or Level VI, which is considered "dangerous."

Smog in southern Beijing reduced visibility to 900 meters earlier this morning.