Beijing has upgraded its alert for air pollution from orange to red on Monday afternoon.

This is the first time the capital has issued a red alert. The red alert, which is the most serious warning on a four-tier system, means authorities have forecast more than three consecutive days of severe smog.

Readings of PM2.5 particles reached 300 micrograms per cubic meter on Monday and are expected to continue rising before the air begins to improve with the arrival of a cold front on Thursday. The World Health Organization says that the safe level for the tiny, poisonous particles is 25 micrograms.

According to the city’s emergency management headquarters, schools are advised to suspend classes, but some were choosing to stay open if they felt they had adequate air filtration systems.

Also, outdoor operations of construction sites are banned and some industrial plants are required to limit or stop production.

Car use will be limited allowed to drive only on alternate days, depending on their license plate numbers.