The skies over Beijing cleared today and the city lifted the code red smog alert imposed two days earlier amid heavy pollution.

Schools restarted and the streets opened to all traffic on schedule from midday, after the predicted arrival of a cold front overnight that blew away the worst of the pollution.

The PM2.5 index, which measures the level of particularly harmful fine particulate matter in the air, was at around 100 microgrammes per cubic metre, down from close to 300 in recent days, but still more than the World Health Organisation safe maximum of 25.

Schools were closed when the top-level code red was declared on Tuesday for the first time since a four-colour code system was imposed in 2013, and traffic restricted to driving alternate days by odd and even number plates. Some factories were reportedly suspended.

The restrictions were effective in mitigating the pollution, state-run news agency Xinhua said. The traffic controls were particularly helpful, but more needs to be done to reduce coal burning, it cited scientists as saying.

China’s booming economy depends on coal for 64 percent of its primary energy, but the country has been trying to clean up or close the smaller, more polluting of its coal power plants, according to financial news agency Bloomberg .

- dpa