Air quality in Beijing will be at hazardous levels on Friday, according to a blue alert issued by the city's government on Thursday.

Friday's warning is the second this week, Beijing News reported

Four colors, blue, yellow, orange and red, denote the severity of pollution.

Eight of the city's 12 monitoring sites for PM2.5 - airborne particles measuring less than 2.5 microns - in urban areas reported readings of more than 230 at 2 pm on Friday, according to Beijing Environmental Monitoring Center.

So far, the number of days with air quality at levels above moderate pollution was up to 15 in October, the Beijing News said.

The continued smog is expected to come to an end during the weekend, it said.