The density of key air pollutants PM2.5 and PM10 shot up in China by 55.7 percent and 30.1 percent respectively in December, the Ministry of Environmental Protection said Friday. The 74 major Chinese cities monitored nationwide had on average more than 70 percent of the days in December failing to meet ministry air quality standards. East China's Yangtze River Delta faced an even worse situation. Twenty-five cities there reported air pollution on 81.6 percent of the days, including heavy or very heavy pollution on 34.6 percent of the days. The 10 cities with the most serious problem last month were Xingtai, Shijiazhuang, Handan, Baoding, Hengshui, Xi'an, Tangshan, Huai'an, Nanjing and Wuhan -- six of which are located in north China's Hebei Province. In a Friday statement, the ministry called for a four-level alert system for days of heavy air pollution, with color codes of blue, yellow, orange and red in ascendent order of severity. While blue is accompanied by suggestions for health protection, compulsory measures would be imposed for higher alerts, such as suspending schooling and industrial operations, banning outdoor barbecues and straw burning as well as restricting use of vehicles. However, the ministry said these measures only serve to temporarily relieve air pollution, while the adjustment of industrial and energy structures is the only solution. According to the statement, officials who fail to enforce emergency measures or fabricate pollution data will be held accountable by supervisory departments.