Maine’s Board of Environmental Protection voted unanimously Thursday to allow the Dragon Products Co. LLC cement plant on Route 1 in Thomaston to increase its mercury emissions from 25 pounds per year to the federal annual limit of 42 pounds.

An official from the state Department of Environmental Protection said the amendment to the company’s air emission license means that Dragon Products would be allowed to emit 42 pounds per year, but it would only occur if the company increased cement production above current levels.

The board’s decision came under immediate criticism from one of the state’s leading environmental advocacy groups.

The Natural Resources Council of Maine expressed “deep disappointment” in the decision, which it said could lead to an increase in emissions of “toxic mercury by 70 percent” at the plant.

“In recommending support for this request … DEP is effectively surrendering the state’s ability to limit mercury emissions at Dragon, allowing the plant to pollute at the highest level permitted nationwide,” the organization’s statement said.

The Environmental Protection Agency increased the maximum mercury emissions limit for cement plants in 2010, based on an analysis of emissions produced by some of the best-controlled cement kilns in the country, said Marc A.R. Cone, director of the DEP’s Bureau of Air Quality. The DEP supported Dragon’s emissions-limits request.

Cone said market conditions will dictate how much cement the company produces. A 2008 state law had capped mercury emissions at 25 pounds a year for the plant. “We feel this standard (42 pounds) is protective of the public’s health,” Cone said Thursday night. “If they remain at the lower levels of production, their emission limit will be lower.”

Cone said he was told that production hours at the plant have dropped off this year, but Dragon’s environmental manager, Michael Martunas, could not be reached Thursday night.

In a memo addressed to the BEP, Cone says that the “proposed mercury emission limit meets the most stringent emission limitation that is achievable and compatible for existing cement plants.”

Under its new emissions license, Dragon Cement will be required to install a mercury emissions monitor in its emission stacks, a new regulation that will provide the DEP with “real-time data,” Cone said.

Mercury can harm nervous, respiratory and immune systems, especially those of children and developing fetuses. Maine residents have been warned to limit their consumption of fish caught in inland waters because of mercury pollution.

According to the Natural Resources Council of Maine, Dragon Cement emitted at least 13 pounds of mercury into the air in 2011-12.