To tap the gas-rich Marcellus Shale, water, sand and chemical compounds are driven deep into the earth to shatter the rock and release the natural gas. Environmentalists say they welcome the decision by a leading natural gas company in Pennsylvania to reveal the chemicals used in the drilling process. Michel Boufadel, a professor of civil and environmental engineering at Temple, says those chemicals could pose a public health threat, but more studies are needed. "I think we should focus on understanding how these chemicals impact the environment in terms of their pathways, through the ground water, the surface water and air pollution," he says.