About 1 in 12 U.S. adults have unpaid medical bills of at least $250, with people in the South and rural areas reporting the greatest burden of medical debt, according to a study from the Peterson Center on Healthcare and KFF.

The big picture: Medical debt remains a big problem, even for insured patients who struggle to afford co-pays and deductibles. People in poor health, those with lower incomes and those lacking insurance are more likely to struggle to pay medical bills.

Zoom in: South Dakota (17.7%), Mississippi (15.2%), North Carolina (13.4%), West Virginia (13.3%) and Georgia (12.7%) had the highest shares of adults with medical debt on average between 2019-2021.

• During that time, West Virginia was the only one of those states with expanded Medicaid coverage for low-income adults. (South Dakota and North Carolina have since joined the Affordable Care Act expansion.)
• Hawaii (2.3%), Washington, D.C. (2.7%) and California (3.9%) had the lowest shares of adults reporting medical debt.

Of note: Though it's hard to pin down exact numbers on medical debt, the report estimates that Americans owed at least $220 billion at the end of 2021.

Go deeper: New York joins cities erasing medical debt