GE has a history of some of its activities giving rise to large-scale air and water pollution. Based on year 2000 data, researchers at the Political Economy Research Institute listed the corporation as the fourth-largest corporate producer of air pollution in the United States, with more than 4.4 million pounds per year (2,000 tons) of toxic chemicals released into the air. GE has also been implicated in the creation of toxic waste. According to EPA documents, only the United States Government, Honeywell, and Chevron Corporation are responsible for producing more Superfund toxic waste sites.

Answer this question, if possible (if impossible, reply "unanswerable"): Which organization identified GE as the fourth largest corporate producer of air pollution in the US?
Political Economy Research Institute