Surprise hospital bills—when a patient is asked to pay for an out-of-network service during a visit to a hospital that’s in their insurance network—have gotten a lot of attention from consumer advocates and lawmakers in recent years.

But a new study shows just how prevalent the problem is: Nearly 40 percent of people who go to a hospital get hit with one of these bills, and the average amount of the bills crept up year by year.

Researchers from Stanford University examined more than 5 million inpatient admissions and more than 13 million emergency room visits between 2010 and 2016, crunching insurance data to find out how many Americans are leaving the hospital with liabilities they didn’t expect.

Of the ER visits, 39 percent of people got an out-of-network bill—the average amount of which rose from $220 to $628 over the six-year period, according to results published in the Journal of the American Medical Association Internal Medicine on Monday. For inpatients, 37 percent got at least one out-of-network bill, and the average more than doubled from $804 to $2,040 over that period. In addition, the study found that the percentage of both groups getting surprise bills rose between 2010 and 2016.

The bills could have serious consequences for those without savings: A recent survey showed that four in 10 Americans would be unable to pay an unexpected expense of $400 without borrowing money or selling something.

“It is disgusting to think that the rate of these cases is actually going up,” Claudia Knafo, who became a patient advocate after getting hit with a surprise bill, told The Daily Beast. “We have to empower patients to know exactly what their rights are.”

The study found that taking an ambulance to a hospital was particularly risky; 85 percent of all ambulance trips saddled patients with out-of-network bills. 

 “Ambulances almost never write contracts with insurance companies,” says J.B. Silvers, a professor of health-care finance at Case Western Reserve University. “And they’re counting on the fact that you’re not going to get off a gurney and ask if you’re covered.”

Emergency room patients were most likely to get surprise hospital bills from the radiology, medical transport and cardiology departments.

Silvers said the status of hospital employees is the culprit. Full time staffers are usually covered under the same insurance package as the hospital, but temporary employees who are pulling in extra shifts or filling in for a sick doctor may not.

“The responsibility belongs to hospitals to ensure everyone who works there is under the same insurance,” said Silvers. 

Silvers has some advice for patients who get a surprise bill: Negotiate. With everyone.

“Talk to the hospital, the insurance company, the doctors. It’s a Middle Eastern bazaar and there’s a carpet between you and the other guy,” he said. 

Sean Barry, of the American Hospital Association, says his group has limited data on surprise billing but said that “hospitals and health systems are committed to protecting patients from surprise bills and support a federal legislative solution to do so.”

Legislative solutions are in motion; as of June, at least nine states had passed laws to deal with the issue and four  states had laws that hadn’t taken effect yet, according to Peterson-Kaiser’s Health System Tracker. President Trump issued an executive order in June authorizing the U.S.  Department of Health and Human Services to draft legislation to protect consumers against surprise bills.