Most are "small, old, and lack advanced pollution control equipment, " the study found.

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Many Midwest coal plants were built back in the Eisenhower and Kennedy years, and have not yet been retrofitted with modern pollution control equipment.

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By installing pollution control equipment that cut all of its releases that include mercury, it avoided significant legal battles while also improving its system reliability.

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These companies, and some unions that see the rules as a potential economic catalyst, say that the industry has had years to prepare for tougher regulation and that the rules will generate jobs for workers who make, install or operate pollution control equipment.

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William Hite, general president of the United Association of Plumbers and Pipe Fitters, says that while his union is "certainly concerned" about the potential for job losses, the mercury rule could also create thousands of jobs for workers who build and install pollution control equipment.

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Consequently, pollution control technology manufactures cannot provide guarantees that companies installing their equipment will be in compliance.

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