YORK COUNTY — South Carolina’s New Indy paper mill… releases more of a powerful neurotoxin into the air than any other major paper plant in the country, a new report says. The amount of mercury leaving New Indy’s York County plant is detailed in a study that examines air pollution at the nation’s largest 185 paper and pulp mills. A major finding is that many paper mills aren’t fully reporting the amount of greenhouse gases they release into the air because of a loophole in federal law. The report says they are burning fuels that release too much air pollution. But the report also provides insight about the discharge of mercury. The study, conducted by the Environmental Integrity Project, calls New Indy’s plant “the worst polluter in the nation’’ for mercury at paper mills, as well as for zinc, another type of metallic waste.