The McKinsey researchers also found that about half of the people eligible for zero-premium plans were under 39 and uninsured. The Obama administration has been emphasizing the affordability of its plans for young people, a critical group because their participation in the marketplaces will help keep overall premiums low.

It is impossible to know who will actually sign up, and whether they will choose a zero-premium plan.

For many people, paying slightly more for a silver plan may be a much better option, experts said. Ninety percent of those who will have the option of buying the no-cost plans make less than 250 percent of the federal poverty level, which is $28,725 for an individual, and $58,875 for a family of four. People earning below those thresholds are eligible for the most generous assistance, but only if they choose a silver plan.

About a million of those who will qualify for free coverage will be able to buy a silver plan for no monthly cost. McKinsey, which is releasing a report about the new insurance marketplaces, estimates that the cost of silver plans for the people who qualify for a zero-premium bronze plan will range from $40 to $50 a month.

“They may be getting zero premiums, but they’re also leaving a lot of money on the table if they don’t enroll in a silver-level plan,” said Sabrina Corlette, a professor at Georgetown University’s Health Policy Institute.

All plans, including bronze policies, limit annual out-of-pocket costs to $6,350 for individuals and $12,700 for families. But insurers and advocates said out-of-pocket costs — even those under that limit — can be daunting to people with low incomes.

For Mark and Elisabeth Horst, both artists in Albuquerque, the risks of signing up for a bronze plan were outweighed by the prospect of getting it free. The Horsts, who make $24,000 a year between them, qualified for $612 in monthly subsidies, but the cost of a bronze plan was $581 a month.