In a heretofore undocumented ecological process, the Great Lakes are purifying themselves by “exhaling” decades-old toxic chemicals, according to a study released on Friday by the Integrated Atmospheric Deposition Network. Lake Ontario alone released nearly two tons of now-banned PCBs between 1992 and 1996; together, the five lakes eliminated 10 tons of PCBs and close to four tons of the pesticide Dieldren, also banned. The researchers describe the finding as a happy surprise — the lakes are cleansing themselves — and say the evaporation of chemicals is not a public health concern. However, in bleaker news, an unrelated bi-national “State of the Great Lakes” report released last week says that thousands of toxic chemicals continue to enter the lakes via rain, snow, fog, dust, and polluted air.