Vehicles run on a smog-shrouded road in Beijing, capital of China, Oct. 14, 2016. (Xinhua/Luo Xiaoguang)

BEIJING, Oct. 14 -- Beijing raised its alert for air pollution to yellow on Friday afternoon as smog is forecast to persist another two days.

The Air Quality Index (AQI) rose above 300 on Friday, with hazardous fine particulate matter PM2.5 being the major pollutant.

Residents are advised to take protective measures, and construction sites and factories have also been ordered to reduce emissions.

The Chinese capital will see a small respite from the smog on Saturday, when light rain is forecast to arrive, said Qiao Lin, head of the municipal meteorological center.

The weather is expected to improve starting Sunday evening with the arrival of a mild cold front, said the Beijing Environmental Protection Monitoring Center.

On Thursday afternoon, the city issued a blue alert for air pollution.

China has a four-tier color-coded system for severe weather, with red being the most serious, followed by orange, yellow and blue. The authorities issue alerts when the AQI reaches 200 and severe air pollution is forecast to last at least one day.

Beijing aims to keep average PM2.5 levels below 60 micrograms per cubic meters in 2017.

The density of PM2.5 in Beijing decreased by 12.5 percent year on year to 63 micrograms per cubic meter from January to August. In 2015, the level was 80.6 micrograms, down from 95.7 micrograms in 2012.