BEIJING (AFP) - More than 100 million people in China have been warned to stay indoors after at least 10 Chinese cities issued red alerts for smog, state media reported Thursday (Dec 24).

Pollution covered large parts of the country's east and centre as measures to curb the toxic haze were ordered to be implemented.

The alert came as broad swathes of the country suffered through their fourth wave of choking pollution this month. In addition, the eastern province of Shandong, with a population of almost 96 million and in which some of the cities 10 cities are located, issued its first ever top-level warning Wednesday (Dec 23), the provincial environmental bureau said.

It is believed to be the first time an entire province has issued a red alert.

The ten cities include the sprawling industrial hub of Tianjin in the northeast.

Counts of PM2.5 - harmful microscopic particles that penetrate deep into the lungs - in the city of Xinxiang in central Henan province, one of the 10 cities under red alert, were as high as 727 micrograms per cubic metre earlier Thursday (Dec 24), according to provincial authorities.

The reading is nearly 30 times the World Health Organisation's recommended maximum exposure of 25 over a 24-hour period.

The alerts follow Beijing's decision earlier this month to issue for the first time the highest-level warning in its four-tier system in response to scathing public criticism about the government's handling of the chronic haze.

The capital cancelled its red alert for pollution at midnight on Tuesday.

The red alert in Beijing saw factories ordered to close and half of all private cars pulled off the streets, among other measures. Meanwhile, the national environmental bureau has ordered six major cities, including Beijing and Tianjin, to evaluate their warning systems, the state-run Legal Daily said.