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Elizabeth Renstrom for TIME

When you land in the emergency room, navigating your complicated health insurance is often the last thing on your mind. But even if you have to foresight to go to a hospital covered by your plan, not every doctor who interacts with you may be in that same network, potentially leaving you with a surprise bill later on.

In new research published in the New England Journal of Medicine, a pair of Yale University economists wanted to find out the scope of surprise emergency room billing in the U.S. They looked at 2014-2015 claims data from an unidentified large commercial insurer, capturing 2.2 million visits to ERs across the country for people under age 65, totaling about $7 billion in emergency care.

They found that while the vast majority of patients visited hospitals within their coverage network, 22% were treated by an out-of-network doctor. Many incurred sizable surprise expenses as a result, the researchers say.

“It turns out to be something that’s happening way more frequently than I would ever have guessed,” says Zack Cooper, one of the authors of the study and assistant professor of public health and economics at the Yale School of Public Health. “These bills can be hundreds, thousands or even tens of thousands of dollars.”

Assuming that the insurer would only pay the in-network rate, Cooper and his co-author calculated that those treated by an out-of-network doctor might receive a bill averaging $622.

Not everyone agrees that coverage discrepancies between doctors and hospitals are to blame. In a statement, the American College of Emergency Physicians (ACEP) took issue with the study, saying that people often don’t realize how little coverage they have until a big medical event like an ER visit. “Most emergency physicians prefer to be ‘in-network,’ as long as insurance companies pay fairly,” says Dr. Rebecca Parker, president of ACEP. “The study does not discuss that insurance companies are misleading patients by selling so-called ‘affordable’ policies that cover very little until large deductibles are met—then blaming physicians for charges.”

The problem is worse in some parts of the country than in others, the authors of the new study found; in certain geographic areas, like parts of Florida and Texas, so-called “surprise billing” was rampant, but in some parts of Colorado and Indiana, it was nearly non-existent. “If there are places where this just isn’t occurring, it means that this is something that doesn’t need to be occurring anywhere,” Cooper says. “We need to do more research to figure out where and why this is happening.”

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