Schools in China's Shanghai city have been directed to shut down during heavily polluted days.

The directive will apply on all Kindergartens, primary and high schools of the city, Xinhua reported Wednesday.

Smog in Chinese cities this winter has come as a warning for the country to intensify its campaign against environmental damage and for local governments to take more effective measures to curb air pollution.

According to a heavy air pollution emergency response plan issued Wednesday by the municipal government, Shanghai will adopt a four-tier colour alert system for air pollution.

A red alert, the highest level, will be issued if the air quality index (AQI) exceeds 450 the next day. A level one emergency response will be initiated, the plan said.

Under level-one emergency response, schools will suspend classes. Fifty percent of the government cars, not abiding by laws, will not be allowed on the road.

In addition, large-scale outdoor public activities will be suspended. High polluting industrial companies will be asked to halt production and construction sites to be closed temporarily.

According to the Ministry of Environmental Protection, AQI of over 300 is defined as "serious pollution" and an AQI between 201 and 300 as "heavy pollution".