PORTLAND – A publicly owned waste incinerator in Maine’s largest city has the uncomfortable distinction of being the leading source of mercury pollution generated from within the state.

The Regional Waste Systems plant releases an estimated 41 pounds of mercury a year, or 7 percent of the estimated 588 pounds released from all in-state sources, according to the Department of Environmental Protection.

Sources of the mercury include battery-operated toys, electrical switches, thermostats, hearing-aid batteries and fever thermometers.

Out-of-state sources of mercury, such as coal-fired power plants, have the biggest overall impact on Maine and are responsible for warnings that pregnant women and children refrain from eating freshwater fish because of mercury contamination, experts say.

But the Regional Waste Systems plant on the outskirts of Portland may be a greater contributor to contamination in the communities that surround it.

“The most toxic [forms] of mercury are the heaviest ones, and they’re the ones that drop out first. That’s why you end up with these hot spots,” said David Evers of the BioDiversity Research Institute in Gorham.

No one has determined where the mercury goes after leaving RWS’ stack or whether it is causing problems in surrounding communities. But tests of lakes and streams have shown that some of the most contaminated surface waters in the Northeast are in the Portland area, according to a recent study by the institute.

Mark Arienti, environmental manager at RWS, said the plant has come a long way since installing technology to take mercury out of its exhaust. In 1998, when mercury emissions were uncontrolled, the plant released an estimated 300 pounds a year.

He also said some tests indicate that the plant’s emissions total 25 to 30 pounds a year, amounts that still leave the plant at the top of Maine’s list.

None of Maine’s other three waste incinerators appears to put nearly as much mercury into the air.

Maine Energy Recovery Co. in Biddeford and Penobscot Energy Recovery Co. in Orrington both burn at least 50 percent more trash than RWS does, but each releases an estimated 7 pounds of mercury a year, according to the DEP. Those two plants were designed with equipment that shreds and separates incoming trash, removing metals and other mercury-containing hardware from the waste that goes into the fire.

At RWS and Mid-Maine Waste Action Corp.’s incinerator in Auburn, virtually everything that comes in goes into the furnace. They must control mercury emissions with expensive filtering technology in the exhaust system.

Mid-Maine, however, releases only an estimated 3.5 pounds a year, the DEP says. Along with burning less trash, the Auburn plant has a system of baglike filters that may be more effective in trapping mercury than the technology RWS uses.