One of the government's top scientists says much more research is needed to determine the possible impacts of shale gas drilling on human health and the environment.
Dr. Christopher Portier of the federal Centers for Disease Control and Prevention says studies should include all the ways people can be exposed, such as through air, water, soil, plants and animals.
Portier says there isn't currently enough information to say with certainty whether shale gas drilling poses a threat to public health.
Duke University researcher Rob Jackson, another scientist, says it may take three to five years to get a better understanding of the issues involved.