China’s weather observatory has issued a yellow alert as huge volumes of dangerous smog have rolled into the northern regions.

The alert lasts from Sunday evening to Monday evening and affects Beijing and the provinces of Hebei, Henan, Shandong, and Jiangsu — parts of the capital are supposed to get heavy smog, according to a Xinhua report.

A cold front should disperse the smog by Thursday, but until then the choking smog will linger, making outdoor activities unhealthy for residents.

As a result of the yellow alert, Beijing is forcing schools to keep children indoors and out of the smog as the alert is expected to climb from yellow to orange in the coming days, according to Channel News Asia.

It wil have a big impact on hundreds of factories, which will have to shutter. Many children will also skip school. Construction sites that produce a lot of dust will be dormant as authorities wait for the smog to blow over.

China’s extensive vehicle exhaust emissions and coal burning have created big smog problems for the country. It has led to public discontent and China has been trying to figure out how to get a handle on the problem. Authorities have promised to upgrade the coal power plants, and the State Council has announced plans to slash major pollutants by 60 percent from power plants by 2020.

Smog is dangerous because it contains ozone, a pollutant that can be harmful to people’s health. It is odorless and colorless, and has three oxygen atoms. Ozone can aggravate asthma, inflame and damage the lining of the lung, and have other negative health effects, especially for those who have chronic lung diseases. The people who are most sensitive to the ill effects of smog are children, adults who are active outdoors, those who have respiratory diseases, and individuals with a sensitivity to ozone.