One in seven families in the U.S. are struggling with debt from medical expenses, despite being covered by health insurance in most cases, a study found on Wednesday.

That amounts to 20 million families reporting difficulties paying for basic needs like food and shelter because of problems with debt from medical care, the nonprofit Center for Studying Health System Change found in its poll of 25,400 families.

“It was kind of a surprise that it was that high,” said study author Peter Cunningham, a senior health researcher at the think tank.

One striking finding in the study is that two-thirds of those facing medical bills they cannot pay already have health insurance.

Unpaid medical bills comprise roughly half of all personal bankruptcies, according to studies.

Medical costs are soaring at twice the rate of general inflation and faster than incomes. The primary cost drivers for health care premiums - prescription drugs, hospital care and doctors - rose 7.4 percent in 2003.

As employers grapple with these costs, they are shifting more out-of-pocket costs onto consumers, studies say. Fees charged to patients for doctors’ visits and prescription drugs, which are not fully paid for by insurance, have risen significantly for several years.

That is likely a factor driving the trend, and with health care costs rising so steeply, it is likely to burden more families in the future, Cunningham said.