China’s environmental authorities are criticizing Beijing for failing to implement contingency plans as the city grapples with its second bout of smog in a week.

The Ministry of Environmental Protection (MEP) issued a statement on Sunday, saying that the severe air pollution enveloping the city over the weekend had been exacerbated by emissions within city, particularly out-of-town traffic.

According to the MEP, Beijing‘s 5.7 million vehicles accounted for 40 to 50 percent of the smog.

The MEP also said that Beijing failed to implement contingency plans to restrict air-polluting activities like construction and illegal trash burning in Beijing suburbs during the smog attack.

Beijing raised its air pollution alert warning from blue to yellow on Friday afternoon.

The City uses a four color alert system to warn its residents about smog conditions. The highest alert, red, has only been issued twice in the city.

Under the current regulations established last February, a red alert for air pollution will only be issued in Beijing if the local air quality index (AQI) is forecast to exceed 500 for one day, 300 for two days in a row, or 200 for four consecutive days.

Emergency measures implemented during previous red alerts in Beijing include immediately reducing city traffic by half, cancelling classes for all schoolchildren, and banning all construction work in the city.

Although the capital’s air pollution has originated from surrounding areas often named as some of the country’s worst polluters, Xinhua reported that the air pollution that hit Beijing this past weekend had a local source due to the high concentration of PM 10 particles.

The annual heating season will begin soon in Beijing, and winter is a time of year that typically sees spikes in air pollution.