Thousands of gallons of toxic drilling fluid used in hydraulic fracturing spilled into a Pennsylvania waterway Tuesday night after a blowout at a natural gas well. The spill occurred in Bradford County, Pennsylvania, at a well operated by Chesapeake Energy. Local residents in LeRoy Township had to be evacuated from their homes. The blowout could heighten concerns about the safety of a controversial process to extract gas from shale rock known as fracking. House Democrats recently issued a report warning that 29 chemicals used in natural gas drilling are human carcinogens or considered hazardous under federal clean air and water rules. For years, companies—including Halliburton—have attempted to keep secret the list of chemicals used in fracking.