However , other mines under its ownership were able to remain in operation due to the implementation of new equipment and procedures that reduced sulfur dioxide emissions .
The passage of the Clean Air Act amendments in 1990 prompted the closure of some Peabody mines .
Other mines under Peabody 's ownership were able to remain in operation due to the implementation of new equipment and procedures that reduced sulfur dioxide emissions .
The passage of the Clean Air Act amendments in 1990 prompted the closure of some Peabody mines .