BEIJING: China on Saturday issued a yellow alert for smog as the pollution choked most regions in the country's north with no signs of abating.On Saturday morning, PM 2.5 density - particulate matter with size equal to or less than 2.5 micron and the main gauge to identify pollutant particles in the air - in most parts of Beijing ranged between 260 and 400 micrograms per cubic meter, far above the safety standards set by the WHO, the National Meteorological Centre (NMC) said in a statement.Other regions such as Hebei, Shandong, Henan and Shaanxi provinces will also be shrouded by heavy smog today before a cold front gradually disperses the pollution, the NMC forecast said, adding that heavy rain will continue in the south.The statement reminded people to pay more regard to safety on the roads, state-run Xinhua news agency reported.On December 7 last year, Beijing issued the first-ever red alert as the city of over 22 million people faced the worst ever smog, forcing authorities to shut down schools and putting restrictions on factories and traffic.China has a four-tier warning system for severe weather, with red being the most serious followed by orange, yellow and blue.