Alexandria won a partial victory in its years-long battle with the Mirant power plant Thursday when a state panel ruled the plant must regulate a type of pollution that is particularly harmful to humans.

A permit drafted by the state Department of Environmental Quality would have allowed the Alexandria coal-fired plant to merge its five smokestacks into two without regulating the emissions of a type of fine particulate matter — called PM 2.5 — that has been linked to asthma and heart disease in humans.