Heavy smog engulfs parts of northern China

Parts of northern China have suffered the worst bout of smog since July, pushing pollution well past healthy levels and reducing visibility.

Beijing, Tianjin and Hebei province have issued an orange alert, the second highest.

The elderly and those with heart or lung problems have been advised to stay indoors.

China has seen harmful levels of pollution for years caused by the use of coal to generate electricity.

Authorities expect the smog to last until Saturday in parts of northern China

The government has declared a "war on pollution" and vowed to cut the use of coal in some areas

The number of cars in big cities in China's also contributes to unhealthy levels of pollution

The number of cars and factory emissions have also contributed to the smog, while state news agency Xinhua said the latest pollution was partly caused by farmers burning their land after the autumn harvest.

The levels of PM2.5 particulates, which can cause the biggest harm to human health, reached as high as 454 on Thursday, the Beijing Municipal Environment Monitoring Centre said on its website.

The World Health Organisation recommends exposure over a 24-hour period of no higher than 25.