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.

Other affected cities, including Tianjin, Shijiazhuang in Hebei province, Linfen in Shanxi, Dezhou in Shandong and Zhengzhou in Henan are expected to see smog peak on Tuesday as well.

The pollution is forecast to ease a little on Wednesday, worsen again on Thursday, and finally start to fade when a cold front arrives next Friday, the ministry said.

Some of the orange alerts issued by the 31 cities took effect on Friday and some, including Beijing's, take effect on Saturday, according to local environmental protection authorities.

The four-color response system is red, orange, yellow and blue, with red being the most serious pollution. Under the orange alert, tough restrictions are put in place, limiting the production of heavily polluting factories as well as the use of vehicles.

To ensure implementation, the ministry requires over 100 inspection teams to visit the 28 major regional cities and conduct field checks.

"The inspection results will be released on a daily basis, showing problems uncovered in the inspections," the ministry said in a statement on Friday.

More smog is likely in coming weeks: The joint forecast of the National Climate Center and China National Environmental Monitoring Center show a trend of warmer, windless days in late January and the first days of February.

The cluster of northern cities often share smoggy days because they are in the same air flow field, said He Kebin, dean of the School of Environment at Tsinghua University, the last time the region issued a similar group of orange alerts on Nov 4.

Joint controls managed to cut emissions by 20 percent in the previous round, and they are expected to be effective this time as well, said Chai Fahe, senior researcher at the Chinese Research Academy of Environmental Sciences.

In addition to the short-term response, governments in the smog-prone region continue to phase out polluting companies as a long-term solution.