China may adopt a flexible working hours system and suspend school classes in case of the highest air pollution warnings, a government circular said on Wednesday. The circular was issued by the Ministry of Environmental Protection as China has been under growing pressure to curb air pollution, with heavy smog frequently smothering large cities. It also urged every possible compulsory measure to reduce emissions when the highest warning of air pollution is issued. The measures include suspension of factories, restriction of vehicles on the road, dust controls and a ban on outdoor barbecue. Large-scale outdoor activities should be stopped. Middle and primary schools as well as kindergartens should suspend classes. Enterprises and institutions should adopt flexible working hours, according to the circular sent to local environmental protection departments. The smog has caused a severe impact on public health, especially when coal-fired heating in winter results in large emissions of pollutants, according to the circular. It called for rigid control of pollution caused by coal burning and encouraging the use of natural gas. The circular also urged strengthened management of pollution caused by industries and motor vehicles. China has built 668 air quality monitoring stations in 114 cities, and the figure could rise to 884 stations in 161 cities by the end of the year, according to the ministry. Any interference, tampering or falsification during monitoring will be severely punished, ministry officials said. Since the beginning of 2013, 74 cities including Beijing, Tianjin and those in neighboring Hebei Province, the Yangtze River delta, the Pearl River delta, have been monitoring and reporting major pollutants including sulfur dioxide, nitrogen oxides, inhalable particles, ozone, carbon monoxide and fine particles. A pilot air quality warning system in the cities of Beijing, Tianjin and surrounding areas has been used since Nov. 1. When the air quality index is set to go beyond 500, a level one warning, the highest, will be issued. A level two warning will be triggered when the index is expected to read between 300 and 500 for three days in a row, and a level three warning for readings of 200-300. In Beijing, cars with odd and even license plates will be allowed on roads on alternating days and schools will close when a level one alert is issued, according to the city's emergency response program.