Who are the Americans still lacking health insurance?

With Aetna pulling back from the Affordable Care Act’s health-insurance exchanges​, more than 800,000 of its current customers will be scrambling to find new coverage.

While that’s a blow to the federal Obamacare program and the customers who relied on Aetna’s plans, there are also an estimated 24 million people in the U.S. who still lack health insurance. That means the uninsured outnumber the 20 million people who have gained health insurance in the nearly three years since the health-insurance marketplaces first opened, according to a recent report​ from The Commonwealth Fund.

The uninsured likely aren’t making the decision lightly, given that they are not only uncovered in case of a medical emergency but also must pay a penalty at tax time for failing to buy insurance. Hints about why those 24 million people are opting to go without insurance can be gleaned from the traits they share: They tend to be poor, live in states that decided against expanding Medicaid, or may be undocumented immigrants.

“There have been notable shifts in the demographic composition of the uninsured since [Obamacare’s] major coverage expansions went into effect in 2014,” the Commonwealth Fund report noted. “Latinos have become a growing share of the uninsured, rising from 29 percent in 2013 to 40 percent in 2016.”

The ACA bars people who aren’t legal U.S. residents from buying insurance through the marketplaces, which may explain why the share of uninsured Latinos has grown, the study noted. The study didn’t document what percentage of uninsured Latinos aren’t legal residents, but the report noted that the Census Bureau has estimated that about half of Latinos without health insurance aren’t citizens.

There are also 19 states that haven’t expanded Medicaid for people with income up to 138 percent of the poverty level. That means an estimated 3 million people across those states are effectively living in a “coverage gap” -- they make too much money to qualify for Medicaid in their states, yet may not have enough money to feel they can afford insurance.

More than 8 of 10 of the uninsured said they didn’t enroll because they couldn’t find an affordable plan, according to the Commonwealth Fund, a private nonpartisan foundation that supports health research. More than half had incomes that would qualify them for subsidies, while one-third made less than the poverty line.

Lastly, the uninsured tend to be young. About half of the 24 million uninsured Americans are between the ages of 19 and 34, the survey found. “Like the overall adult population, the majority of uninsured young adults (96 percent) have incomes that make them eligible for marketplace subsidies or Medicaid,” the report noted.

The reluctance of millennials to participate in Obamacare may be key to why insurers like Aetna are pulling back from the marketplaces. The health insurance law was built on the idea that younger, healthier Americans would sign up for participating plans, offsetting the higher costs of older, sicker enrollees. But with many young Americans opting to pay a tax penalty instead of buying insurance, some insurers are finding the economics aren’t working in their favor.

More young Americans may sign up for insurance as the penalties increase. In 2014, the penalty for poor households who opted against insurance was just $285, but that’s slated to jump to more than $2,000 this year​, which would hit families at tax time in early 2017, according to the Henry J. Kaiser Family Foundation. 

“For some eligible uninsured people, enhanced subsidies and lower cost-sharing in marketplace plans may be required to facilitate enrollment, particularly for moderate-income households and those near the income-eligibility thresholds,” the Commonwealth Fund noted.

It added, “Immigration reform would help increase the numbers of people who are eligible for coverage, as would a loosening of the law’s restrictions on the eligibility of undocumented immigrants.”