Beijing issued a severe air pollution warning Friday afternoon, following an AQI reading of nearly 400.

The yellow alert was issued around 3:30 pm and followed a blue alert, which was issued the day before.

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.

On Thursday, the Ministry of Environmental Protection released a report that described air quality as having improved in a number of cities across China this year. Over the first nine months of 2016, Beijing experienced days with “good” air quality 56 percent of the time, a year-on-year increase of six percent. PM 2.5 and PM 10 recordings are said to have dropped 10 and 15 percent.

However, the report listed six of the top ten worst Chinese cities for air pollution as being in Hebei, the province adjacent to Beijing.

Winter marks an annual bump in air pollution in China due to the use of central heating. Previous announcements have said coal use, the main factor in causing air pollution during heating season, will be either banned or restricted in urban areas like Beijing.

A cold front forecast for Sunday is expected to alleviate current smog conditions in Beijing.