At the height of the smog Beijing authorities said readings for PM2.5 - particles small enough deeply to penetrate the lungs - hit 993 micrograms per cubic meter, almost 40 times the World Health Organization's safe limit.
Experts quoted by state media blamed low winds, saying fog had mixed with pollutants from vehicles and factories and been trapped by mountains north and west of Beijing. Coal burning in winter was also a factor, they added.
In an editorial Monday the state-run Global Times called for more transparent figures on pollution an urged the government to change its "previous method of covering up the problems and instead publish the facts".
(Top) the Beijing skyline during severe pollution on January 14, 2013, and the same view (side right) taken during clear weather on Febuary 4, 2012. Dense smog shrouded the city with pollution at hazardous levels for a fourth day and residents were advised to stay indoors. (Ed Jones/AFP/Getty Images)