The heavily industrial Midwest has long been afflicted with some of the worst air and water pollution in the country — but now that distinction has been handed off to the Sun Belt, according to a report released yesterday by the U.S. Public Interest Research Group. The report tracked toxic releases from large industrial plants across the U.S., breaking down the data into ZIP-code regions and including information about potential health effects. Nationwide, more than 138 million pounds of toxins affecting development and 50.8 million pounds of toxins affecting reproduction were released in 2000. Southern states accounted for more than 75 percent of reproductive toxins, 59 percent of developmental toxins, and half of toxins that may cause neurological problems. Although the amount of toxic releases continues to drop nationally, the South’s share has grown steadily since 1987.