BEIJING -- 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 or limiting of enterprises' production, 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 the setting of strict controls on coal-fired pollution while guaranteeing the use of natural gas.

The circular also urged strengthened management of pollution caused by industries and motor vehicles.