A team of toxicologists at the University of Pennsylvania are mounting a major study on the health effects of fracking, the controversial method of removing natural gas from deep within the earth. The university’s Center of Excellence in Environmental Toxicology has organized a working group with researchers at other top universities includingColumbia, Johns Hopkins and the University of North Carolina to investigate and analyze reports of nausea, headaches, breathing difficulties and other ills from people who live near natural gas drilling sites, compressor stations or wastewater pits. “There is an enormous amount of rhetoric on both sides,” said Trevor M. Penning, head of the Penn toxicology