This Aug. 8, 2011, file photo shows the entrance to the Humana building, in Louisville, Ky.

Health insurer Humana is leaving the Affordable Care Act's public insurance exchanges next year as it regroups after ending its proposed combination with rival insurer Aetna.

Humana Inc., based in Louisville, Ky., covers about 150,000 people on exchanges in 11 states, including Tennessee.

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This screen image shows the website of Overnight Tuesday, Jan. 31, is the deadline to sign up for coverage under the federal health care law. Even if the ultimate fate of “Obamacare” is uncertain, there’s been no change for this year. About 11.5 million people had enrolled as of Dec. 24. (AP Photo)

Humana was the only insurer on exchanges in 16 Tennessee counties, according to data compiled at the start of the 2017 open enrollment period by The Associated Press and health care consulting firm Avalere. That means customers in those counties may have no way to buy coverage with help from government tax credits next year unless another insurer decides to enter those markets.

The insurer said Tuesday it had taken several actions to improve that business, but it was still seeing signs of unbalanced risk in that customer population. Humana said it has modified its provider networks, restructured its products, raised its premiums and reduced its geographic footprint to limit its losses from Obamacare plans.

Last fall, the Tennessee Department of Commerce and Insurance granted Humana a 44.3 percent increase in average rates for its individual health exchange plans in the Volunteer State.

"However, based on initial analysis of data associated with the company's healthcare exchange membership following the 2017 open enrollment period, Humana is seeing further signs of an unbalanced risk pool," the company announced Tuesday.

Health insurers have struggled to attract enough healthy people to their risk pools to balance the claims they incur from people with expensive medical conditions.

Since 2014, Humana has been a key player in Tennessee's health care exchange, at least in its major cities. The nation's fifth largest health insurer currently sells plans in the Knoxville, Nashville and Memphis markets.

Last year, Tennessee's biggest health insurer, the Chattanooga-based BlueCross BlueShield of Tennessee, withdrew individual coverage under ObamaCare in the state's three biggest markets.

Government and industry officials have said President Donald Trump's administration and congressional Republicans are weighing measures to stabilize the wobbly exchanges. Insurers have been pushing them to act soon.

"The clock is definitely ticking for the Trump administration to provide some clarity around what the rules will be," said Larry Levitt of the non-profit Kaiser Family Foundation, which studies health care issues.