Environmentalists have repeatedly pressed regulators to compel oil and gas companies to report what chemicals they use in the drilling and fracking process. Drilling companies add these chemicals to perform particular functions (for example, to prevent corrosion or give the fluid the right consistency), or leave them in because they're too expensive to remove. According to a 2011 congressional report, many of the chemicals used can pose a serious health risk. No one knows the exact makeup of the frack mixture, drilling muds and other stuff used at well sites (which change from well to well), but this list breaks down the main ingredients revealed so far. Read more