Almost half of working-age adults in the U.S. had inadequate health insurance or no coverage at all last year, a widening deficit that the Affordable Care Act should mitigate, according to data from the Commonwealth Fund.

About 84 million were uninsured or underinsured, 3 million more than when the 2010 health law was signed and 20 million more than in 2003, the New York-based nonprofit group, which advocates for better health care, said today in a report. About 80 million adults who had medical conditions said they chose not to see a doctor or fill a prescription because of the cost.

At least 85 percent of these people will become eligible for some type of subsidized or government health insurance under the U.S. health law when the core parts of it take effect in January, the Commonwealth Fund said. Younger adults are benefiting from existing provisions as the proportion of people ages 19 to 25 who were uninsured at some point in the year fell to 41 percent from 48 percent in 2010.

“It will be critical to continue to monitor the effects of the law as the major provisions go into effect in 2014 and beyond to ensure it achieves its goal of near-universal, comprehensive health insurance,” Sara Collins, the vice president for affordable health insurance at the Commonwealth Fund, in a statement.

Of the 84 million people cited by the group, about 55 million were uninsured during 2012. About 30 million were underinsured, meaning they had coverage that provided “inadequate protection from health-care costs,” measured by the amount spent out-of-pocket for care compared with their income, the group said.

The U.S. health law is projected by the Congressional Budget Office to extend health insurance to about 27 million people by 2021. The Commonwealth Fund calls it “a fundamental transformation in their health insurance system.”

The potential success of the health-care system overhaul relies heavily on a series of state-run exchanges that are supposed to open in October. The exchanges will serve as marketplaces for people who lack insurance at their jobs and help them enroll in subsidized plans for 2014. Medicaid, the federal-state health system for the poor, also will be expanded in many states to cover adults earning wages near the poverty level, who are largely ineligible now unless they have children.

A requirement already in effect that made insurers let families keep children in their plans until age 26 led to a 1.9 million drop from 2010 in the number of young adults without insurance, according to the report.