Second-level, or "orange", pollution alerts was issued in at least nine cities in China's industrial heartland of Hebei, Shandong, Henan and Shanxi in response to a wave of smog expected to blanket the region, state radio reported on Friday.
The alerts are expected to be in effect until January 14.
Industrial sites including steel mills, coke plants and aluminium smelters were ordered to curtail output by at least an average of 30 percent during the alerts.
Diesel-fuelled trucks will also be restricted from transporting commodities.
Linfen in the coalmining hub of Shanxi province, identified by the environment ministry as the most polluted place among 169 monitored cities across the country last year, is under a first-level smog alert, state radio said.
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