FILE - In this March 29, 2013 file photo, workers tend to a well head during a hydraulic fracturing operation outside Rifle, in western Colorado. Colorado health officials said Wednesday, March 1, 2017, that air monitoring and a dozen existing studies on air pollution from oil and gas sites indicate the risk of harmful effects on humans appears to be low, but they stressed that more study is needed. (AP Photo/Brennan Linsley, File)
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