Millions of lower-income workers may gain access to employer-sponsored health insurance under the Affordable Care Act—but they may decide not to purchase that coverage.

Most of the lowest-income workers currently offered health benefits elect not to enroll, according to a new study from the ADP Research Institute. While the Affordable Care Act does require large employers to offer coverage, this study raises the possibility that Americans may still decide to forgo the option.

"In 2014, there are going to be a number of people who are eligible for benefits for the first time" ADP's Christopher Ryan said. "These additional people are primarily lower-income and the critical question is: To what extent, if they have the opportunity to purchase benefits, will they participate?"

ADP looked at the enrollment rate for health benefits offered at 300 large companies, all with more than1,000 employees. They found that, for workers earning above $50,000, the majority — 81 percent — elected to buy into health insurance benefits.

Lower-income Americans, by contrast, purchased health benefits at a lower level. Among full-time employees earning $15,000 to $20,000, 37 percent elected to purchase health benefits when they were made available.

Among those earning, $20,000 to $25,000, participation increased to 58 percent.

participation rates
"For them, it's an affordability issue," Ryan said, noting that lower-income employees generally have to put a bigger chunk of their salary to buying benefits than higher earners.

The health reform law does include some protections for affordability. It requires employers with 50 or more employees to offer their workers a benefit plan that costs less than 9.5 percent of their income. If they do not, they will face thousands of dollars in federal penalties.

There also will be new penalties for not purchasing coverage: In 2014, there will be a $95 tax penalty for not buying insurance coverage. That will rise to $695 by 2016.

Still, its unclear whether those will be enough to encourage lower-income workers to move into insurance coverage.

For one, most of those who do purchase coverage now already do so well-below the affordability threshold: The ADP study found that, in its sample, only 8.6 percent of employees spent more than 9.5 percent of their income on health benefits.

The $95 financial penalty for not purchasing coverage will be significantly smaller than the price of buying into an employer-sponsored health plan.

What happens if these people don't sign up? Most obviously, the insurance expansion gets smaller, with some not electing to pay for the benefits that their companies will be required to offer.

Experts at ADP also suggest that having fewer of these lower-income workers in insurance plans would result in higher premiums. That's due to the fact that lower income workers tend to be younger, with less expensive health care costs.

"One of the key issues is how do we engage that population, get them to enroll in health care," Ryan said. "I think its going to be a significant challenge."