Beijing and 33 other northern Chinese cities have issued orange smog alerts for the next three days. The orange alert is the second highest in China's four-tier system. Industrial factories must limit output by 30 percent to 50 percent. The Ministry of Ecology and Environment says the pollution is partially the result of increased industrial output after the end of winter restrictions. China ordered 28 northern cities to impose special restrictions on polluting industries during the winter heating season that runs from mid-November to mid-March. Now it's been mostly lifted. This is the second orange alert for March.