Twenty-three cities in China, mostly in northern region including capital Beijing have activated red alerts after the sky turned grey due to pollution today. <br/><br/>Odd-even rules has also been enforced to restrict traffic. Other measures included suspension of construction and classes for kindergartens and primary schools as well as reduced emissions guidelines for factories.<br/><br/>Beijing activated its first red alert for smog this winter as the air began to turn hazy yesterday. The PM 2.5 density exceeded 200 in the downtown areas of the capital, indicating the air is heavily polluted. Beijing's environmental and housing authorities have also strengthened inspection of vehicles and construction sites and factories.