HOUSTON -

A Dallas-based medical group has asked the Texas environmental agency to cut air pollution from coal-fired power plants by strengthening rules and requiring equipment upgrades in older facilities.

The Dallas County Medical Society filed a petition Wednesday with the Texas Commission on Environmental Quality asking for a rule change. The agency will have 60 days to respond.

The society said research shows that three old coal-fired power plants contribute to air pollution in North and East Texas. These facilities have not been required to meet the same emissions standards as newer facilities also owned by Energy Future Holdings, one of Texas' largest power generators.

The medical group cited studies that link pollution to a variety of illnesses, and demands the old plants lower emissions.