Northern smog is forecast to last a week

The year's first round of smog began blanketing much of the nation's northern region on Friday and is forecast to last for a week, environmental officials said. Thirty-one cities including Beijing and Tianjin issued an orange alert, the second highest, on Friday.

Residents in the capital who have enjoying blue skies for weeks have been told to take the warning seriously. The severe air pollution is expected to peak on Tuesday with concentrations of PM2.5 climbing to 250 micrograms per cubic meter or higher, the Ministry of Environmental Protection said.

When the level of PM2.5 - hazardous fine particles - reaches that level, it means the air quality index would be above 300.