Boston.com reports that Massachusetts state officials have made good on their pledge to reduce school bus exhaust emissions by 90 percent over the next three years. Ed Coletta, spokesman for the Department of Environmental Protection, said that the state will spend $22.5 million over the next three years in order to install exhaust filters on all school buses and regional transit authority buses throughout the state of Massachusetts. The state has roughly 8,000 school buses and 600 regional buses that will be retrofitted to reduce emissions. The promise stemmed from environmental agreements made with federal authorities in the 1990s in anticipation of the fact that the completion of the Big Dig would bring more cars through Boston, and making buses cleaner was one way to offset the environmental impact…