Thick smog will continue to blanket more than 20 cities in northern China over the next few days, weather authorities said on Sunday even as officials tried to limit polluting activity in an attempt to clear the air. Beijing and its neighbouring areas, including Hebei, Henan, Shandong, Shanxi and Tianjin, would experience severe air pollution, Xinhua quoted the meteorological authorities as saying. The warning came after Beijing issued a red pollution alert – the highest in a four-tier system – on Friday. The smog built up as winds blew in with PM2.5 pollutants – particles smaller than 2.5 microns in diameter that can cause the greatest harm to public health. Conditions were at hazardous levels last night. Pollution was expected to peak on Tuesday before a strong cold front arrived to disperse it, the authorities said. READ MORE: Beijing issues red alert as ‘worst smog of the year’ headed for northern China In response to the red alert – the second in just two weeks – Beijing has ordered half of all private vehicles off the road. But some experts say the car ban will have only limited effect in curbing pollution in the capital. Other cities’ measures, such as partially halting work in some polluting factories, might prove more effective, they said. When wind from the south is blocked by the mountains north of Beijing, pollutants from the heavy burning of coal and other industrial work in some cities south of the capital accumulate and expand within the region, which shares an “air shed”. This “regional pollution feed” contributes to more than half of the PM2.5 particles in Beijing’s air on the most polluted days – far more than local vehicle emissions, according to a recent study. Upgrading the industrial structure and using clean energy were the fundamental solutions to the capital’s air pollution woes, Zhang Dawei, head of the Beijing Environmental Emergency Centre, said. “This is a long and difficult process that will take about 30 to 50 years to see an ultimate change,” he said. READ MORE: ‘Where’s the fire?’: Chinese city cloaked in choking smog after coal-burning heating systems turned on for winter The measures to scale down the region’s economic operations could help slow down the build-up of airborne pollutants, if they were properly implemented, Zhang said. Experts have called for a joint mechanism to coordinate efforts by six provinces in the region to tackle air pollution. But Ma Jun, director of the Beijing-based Institute of Public and Environmental Affairs, said the long-term solution lay in enforcing the rule of law. “The industrial development of this region has far exceeded the capacity of the environment,” he said. “We must strictly enforce the Environmental Law, closing down the polluters that fail to meet the standards.” Ma said some of the region’s major polluters had never complied with the environmental law, let alone carry out the emergency measures to curb pollution. Such firms got away with doing so because of a lack of transparency and public supervision, he said. Despite the severe pollution, environment vice-minister Li Ganjie, said on Saturday that the average density of PM2.5 in 74 cities across the country had dropped 16 per cent so far this year, China National Radio reported. But Ma said that in evaluating the effect of pollution controls, the authorities had also to take into account changing weather conditions, which affect the diffusion of airborne pollutants.