As more and more Mainers buy their health insurance on the marketplace offered by the Affordable Care Act, those who stick with their grandfathered individual plans are being hit with double-digit premium increases.

Many of the roughly 7,000 consumers in the state who are buying a grandfathered plan are probably paying more than they should, experts say.

The grandfathered plans are those that did not meet ACA standards – such as high-deductible plans or those that did not cover services required by the ACA – but were permitted by the federal government to continue being offered to people who wanted to hang onto their old plans.

But staying with a grandfathered plan comes with a price.

Anthem Blue Cross Blue Shield, which sells the vast majority of grandfathered plans in Maine, is seeking a 19.9 percent premium increase for 2016 for about 7,000 Mainers remaining on the plans. That’s on top of a 13 percent increase that was approved by the Maine Bureau of Insurance for 2015.

In contrast, ACA premiums for plans sold by Community Health Options increased by 0.5 percent for 2016 and decreased by 0.8 percent for 2015. Community Health Options dominates the ACA individual marketplace in Maine, having sold about 80 percent of the 75,000 plans purchased in the state. Marketplace plans sold by Anthem and Harvard Pilgrim also have increased at modest rates during the past two years.

The Bureau of Insurance does not set rates for large group plans offered by employers, so it’s unclear whether those premiums will increase. The rates for small group plans, offered on the marketplace for employers with fewer than 50 workers, varied widely, with a handful of companies raising rates by 3 percent to 8 percent and one company – Anthem – reducing its rate by 9 percent.

The ACA marketplace, operated on the Web as healthcare.gov, is where people who have no insurance – often part-time or self-employed workers – can obtain benefits. About 87 percent of Mainers who buy plans on the exchange qualify for a subsidy.

Emily Brostek, executive director of Consumers for Affordable Health Care, an Augusta-based nonprofit advocacy group, said that as more people switch to ACA plans, the pool of people remaining in the grandfathered plans shrinks. With fewer people in the insurance pool – perhaps older or sicker people – the financial risk for the insurer increases, driving up premiums.

While in most cases switching to the health insurance marketplace is a better deal for consumers, some are sticking with their grandfathered plans because they are familiar with what their plan offers, and choosing from a myriad of marketplace plans can be daunting, Brostek said. Others assume that because they don’t qualify for the subsidies, marketplace plans would be unaffordable. But Brostek said many of the plan choices are actually more affordable than the grandfathered plans.

At some point, Brostek said, the grandfathered plans will be cost prohibitive and Anthem will likely no longer offer them, even as a continuation of existing coverage.

“These plans are probably going to be phased out,” she said. “It was really intended as a transition to the ACA marketplace, to give people time to weigh their options.”

For those who don’t qualify for subsidies – which are available to a family of four earning up to about $90,000 per year – it may not be immediately clear that the marketplace plans can be more cost-effective.

In one example emailed to the Portland Press Herald by a person who wished to remain anonymous, the premium for a grandfathered Anthem plan is set to increase from $678 to $798 per month, with a $2,500 deductible.

But a marketplace plan offered by Anthem, even without subsidies, may offer better coverage at a competitive price, said Rory Sheehan, an Anthem spokesman.

“People need to look at all of their options,” Sheehan said. “I’m not saying that in every single case it will be a better deal, but people should gather as much information as possible.”

Sheehan said he looked up plans for an individual similar to the example emailed to the Press Herald and found plans with premiums between $430 and $570 per month, with deductibles ranging from $1,400 to $2,600.

Open enrollment for 2016 plans runs from Nov. 1 to Jan. 31, 2016.

Sheehan said Anthem will work with consumers still on the grandfathered plans to help them decide on a plan. Also, health insurance “navigators” are available to provide free assistance as consumers look for a plan. To find a navigator, go to www.enroll207.com.

“We will work with this small group of members during the next open enrollment period to help them understand their options and the importance of finding the plan that best fits their health needs and budget,” Sheehan said in a written statement.