Health-care exchanges in some states offer few insurance plans from which to choose.

So few insurers offer plans on some of the new government health insurance exchanges that consumers in those states may pay too much or face large rate increases later, insurance experts say.

An average of eight insurers compete for business in 36 states that had exchanges run or supported by the federal government last month, the Department of Health and Human Services says. (Idaho has since started its own exchange.) But just because an insurer sells in a state, it doesn’t mean it sells in every area of a state —so many residents have far fewer options.

Many state-run exchanges also have far fewer than the HHS average, which is weighted based on the number of uninsured residents in an area. Vermont has two, Kentucky has three and Nevada and Maryland each have four.

Some insurers pulled out of the exchanges required by the Affordable Care Act as the Oct. 1 launch approached. That leaves an uneven patchwork of providers —ranging from one insurer in New Hampshire and West Virginia to16 in New York.

The difference also leads to a wide disparity in the numbers of plans, from just seven in Alabama to 106 in Arizona, accord- ing to the HHS analysis. But HHS spokeswoman Joanne Peters says the situation is still much better than it was before the law took effect.

“In the past, consumers were too often denied or priced out of quality health insurance options, but thanks to the Affordable Care Act consumers will be able to choose from a number of new coverage options at a price that is affordable,” she said in an email.

About a third of insurance companies opted out of participating in the exchanges in states where they were already doing business, according to a recent report by McKinsey & Co.

About half of states — which include about a third of the nonelderly insured population — will see a “material decline” in competitors, says McKinsey, while the other half of states will have about the same or more insurance choices on the exchanges.

“When there are too few carriers, down the road there will be issues with rate increases that make plans unaffordable for average Americans even with rate subsidies,” says Bryce Williams, managing director of Towers Watson Exchange Solutions, which operates private insurance exchanges for companies. “We need competitive insurance markets in all states (and) multiple carriers competing hard.”