There are many health policy studies that link uninsurance to worse health outcomes. A forthcoming paper in the Journal of Health Economics adds an important layer to that relationship: It finds that high rates of uninsurance mean worse outcomes even for those with coverage.

Health-care economist N. Meltem Daysal compared outcomes for insured heart attack patients in California over a six-year period, 1999 to 2006, when the state saw a 19 percent reduction in mortality rates for such cases. Across the state, however, there was huge variation in how health outcomes were improving: San Francisco and Los Angeles had decreases between 26 percent and 30 percent, while in Sacramento, the drop was just 13 percent.

Daysal wanted to know what was different about the patients in the three cities. He controlled for basic demographic information, like age and race, as well as a set of preexisting risk factors that were present at admission, like a history of heart failure or hypertension.

What he saw was cities that treated more uninsured patients seeing worse outcomes for those who came in with coverage. Daysal estimates that, if uninsurance were eliminated, there would be 3 to 5 percent fewer deaths among those who already had coverage.

What’s happening here? Daysal describes it as “negative spillover,” where the hospitals that treat more uninsured patients end up footing a higher bill for uncompensated care. That takes away resources they could be spending on hiring better doctors, upgrading medical equipments or making a whole host of investments that could improve health outcomes. Worse care, for all patients treated at the facility, ensues.

On the flip side, increasing insurance coverage could have positive spillover effects, improving quality of care even for the already-insured. “Taken together, my results suggest that policies aimed at addressing the issue of uninsurance may have additional benefits to insured patients residing in the same communities,” Deysal concludes. That means the Affordable Care Act, which is expected to extend coverage to 32 million Americans, could have some fringe benefits for those already covered.