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Twenty-five cities across northern China are under red pollution alerts, the highest pollution warning in the Chinese system. Due to the thick haze, thousands of flights have been cancelled. Beijing is under a yellow alert, the second highest pollution warning level. The pollution is due to stagnant air hanging over the northern half of the country. The jet stream has dropped down over southern China, as is normal in winter, but the lack of air circulation bordering the stream, coupled with generalized poor pollution controls in factories and power plants throughout China, has led to this extreme situation.
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