Seven counties in southern Maine will be required to sell reformulated gasoline next year if a proposal before the federal Environmental Protection Agency is approved.

The proposal, which was requested by the state of Maine, would become effective next year in York, Cumberland, Sagadahoc, Androscoggin, Kennebec, Lincoln and Knox counties.

It would require fuel refiners, importers and distributors to begin selling reformulated gasoline, or RFG, by May 1, 2015, and for it to be available to retailers and consumers by June 1, 2015.

Reformulated gasoline, which once contained methyl tertiary-butyl ether, better known as MTBE, was banned by the state in the late 1990s over concerns the colorless additive posed a health threat to groundwater supplies.

Mainers now use a “boutique” gasoline, called RVP 7.8, that is only sold in a few parts of the country, according to the state. Most gasoline sold in Maine also contains 10 percent ethanol, an alcohol-based fuel additive made by fermenting and distilling the sugars found in agricultural crops and wood fibers.

“The use of RFG in many areas of New England has contributed to cleaner air in the region,” Curt Spalding, regional administrator of the EPA’s New England office, said in a statement. “We are pleased that Maine also wants to implement this measure in its southern counties.”

States like Maine, which are designated by the EPA as part of the Ozone Transport Region, have the ability to opt-in to the federal RFG program. Once the proposed rule is published in the Federal Register, interested parties will have 30 days to comment on the change before it is adopted. The EPA would make its decision within a few weeks after that.

The purpose of the federal RFG program is to improve air quality through the use of a gasoline that is reformulated to reduce motor vehicle emissions that lead to the formation of ground-level ozone. Ground-level ozone can cause breathing problems, aggravate asthma and pre-existing lung diseases, and make people more susceptible to respiratory infections.

According to the EPA, reformulated gasoline is already being used in 17 states, including Connecticut, Rhode Island, Massachusetts, southern New Hampshire and the District of Columbia.

There is not expected to be significant opposition to the changeover since the Legislature enacted a law in 2013 establishing the use of RFG in the southern Maine counties. That law was supposed to take effect May 1, 2014, but was postponed to give suppliers the time they needed to prepare.

“Certainly, we all want cleaner air, but we didn’t want people to have to pay more for gas,” said Maine Sen. James Boyle, D-Gorham, who chairs the Legislature’s Environmental and Natural Resources Committee.

Boyle’s committee weighed a lot of factors, including whether its actions might drive gas prices up, before voting to opt-in to the federal RFG program in 2013.

He doesn’t believe that RFG will drive prices up, but he also doesn’t believe they will go down. The key factor was the environmental impact.

“The people I have talked to around my district want clean air. And I believe that excessive ozone levels can be bad for tourism,” Boyle said.

Marc Cone, director of the Bureau of Air Quality for the Maine Department of Environmental Protection, said the proposal to change gasoline in seven counties came from the Maine Energy Marketers Association, which represents more than 300 heating oil, propane and motor fuel businesses.

“They felt that reformulated gas was more readily available,” Cone said.

The “boutique” gas blend being used in the seven Maine counties is available in only a few other areas of the country, Cone said. He wouldn’t speculate on whether RFG would result in lower gas prices at the pumps or improved vehicle mileage.

“Gas prices are volatile,” Cone said. “But we are not anticipating a huge price increase.”

Jamie Py, president and chief executive officer of the Maine Energy Marketers Association, said he is prohibited by anti-trust laws from commenting on whether RFG will affect gas prices. He said his organization supports the use of RFG because it will make distribution more uniform and will lead to cleaner air.

Py said the members of his association will continue to lobby for RFG being available in all 16 Maine counties.

“Our goal as an association is to have one gas for Maine,” Py said.