One of the biggest obstacles to “Medicare for All” and other ambitious health reform proposals is that a majority of Americans already have private health insurance through employers.

These job-based plans have been around a long time and they are relatively popular, judging by the polls. Anytime the national conversation turns toward regulating or eliminating them ― or taking other sweeping action that might affect employer plans indirectly ― voters get skittish.

That goodwill makes it easy to assume the plans provide comprehensive, reliable coverage. They frequently don’t, as a new study from the nonpartisan Commonwealth Fund makes clear.

Roughly 24 million Americans with employer plans are struggling with “high medical costs” annually, the researchers concluded after examining two years of U.S. Census data. And that number is bound to increase in the future, at least as long as the price of medical care keeps going up.

“High medical costs” is an inherently subjective term and the definition that the researchers used is complex. For the majority of Americans, the study considered a “high” cost as anytime somebody with an employer plan had to spend more than 10% of their household income on either out-of-pocket expenses or the employee share of premiums.

For Americans living at or below twice the federal poverty line, out-of-pocket expenses of just 5% of household income counted as high medical costs ― on the theory that, at such low incomes, even relatively small copayments or deductibles were enough to cause financial hardship.

The study’s authors chose those benchmarks because, based on past research, the numbers seemed likely to correspond roughly with the point at which people would begin rationing their own health care, by delaying or skipping treatments, because they could not afford it.

“It’s not precise, but it’s a measure of affordability where people start to make different decisions about getting health care,” co-author Sara Collins, the Commonwealth Fund’s vice president for health care coverage and access, told HuffPost. “They have higher rates of reporting not getting needed health care because of cost, not filling prescriptions, delaying going to the doctor or getting a follow-up visit.”

Some 24 million Americans might not sound that many, given that they represent only one-sixth of the total population with job-based coverage. But only a fraction of people have serious medical expenses in a given year, which means that, among those who actually need their insurance, a much higher proportion find employer coverage inadequate.