It suggests that a little-noticed provision of the law could affect far more employers than Congress had assumed.
In the raucous debate over health care, Democrats and Republicans focused on a provision under which employers would generally have to offer coverage to employees or pay penalties, starting in 2014.
As they study the law, employers are discovering another provision that got much less attention. If a company offers coverage but requires any full-time employees to pay premiums that amount to more than 9.5 percent of their household income, the coverage is deemed unaffordable, and the employer may have to pay a penalty.
A goal of the law, pushed through Congress by President Obama and Democratic leaders with no Republican support, is to give all Americans access to affordable insurance.
The Mercer survey found that one-third of employers had some workers for whom coverage might be “unaffordable,” meaning that the workers’ share of premiums — in the absence of federal assistance — would consume more than 9.5 percent of their household income.
Employers with fewer than 50 employees are generally exempt from penalties.
Beth Umland, Mercer’s research director for health and benefits, said employers with 50 or more employees were more likely to offer unaffordable coverage than to offer no coverage at all.
Tracy Watts, a partner in Mercer’s Washington office, said it would be difficult for employers to know in advance exactly how many workers might find their health plans unaffordable.
“Employers rarely have access to information on their employees’ household income,” which may include the earnings of a spouse or children, interest from savings accounts and dividend income from stocks and mutual funds, Ms. Watts said.
If an employer’s health plan is deemed unaffordable, the worker may qualify for a federal tax credit, or subsidy, to buy coverage in a new state-based marketplace known as an insurance exchange. A person claiming a credit must disclose income information to the exchange. The exchange will then notify employers if any of their workers qualify for subsidies.
Democrats say the subsidies will be a boon to low-wage workers. But the subsidies can also lead to monetary penalties for employers.
An employer offering unaffordable coverage is subject to a penalty of $3,000 a year for each full-time employee who gets government assistance to buy insurance in an exchange. The maximum penalty is $2,000 times the total number of full-time employees in excess of 30.
Andy R. Anderson, an expert on employee benefits at the law firm Morgan Lewis, said, “A lot of employers, particularly those with low-wage work forces, will run into difficulty with the affordability requirement.”
Retailers and restaurants with large numbers of low-wage workers may be most affected.
Michael C. Gibbons, chairman of the National Restaurant Association, said, “The cost of health care reform will be devastating to our industry.” At the same time, he said Friday that the association was working with the UnitedHealth Group to devise affordable health plans for restaurant workers. The association says four million to six million restaurant employees are uninsured.
In deciding whether insurance is unaffordable, Ms. Umland said, it is not entirely clear whether the government will look at the cost of coverage for an employee alone or the cost of family coverage, which is usually higher. The government plans to issue regulations to explain what happens when a worker can afford individual but not family coverage.
To avoid the penalty for unaffordable coverage, employers could respond in several ways. They could increase their contributions to premiums. They could reduce the workers’ share of premiums but recoup the money in other ways — for example, by increasing co-payments or deductibles. They could offer lower-cost health plans, with less generous coverage.
Or they could charge lower premiums to workers with lower wages. Employers now often charge the same amount to all employees in the same health plan, regardless of their wages.
Employers are obliged to offer affordable coverage to full-time workers, defined as those who work at least 30 hours a week, on average.
If an employer with 50 full-time employees offers coverage and 10 of those workers receive premium credits, or subsidies, the employer would face a penalty of $30,000. If 30 workers receive subsidies, the penalty would be $40,000.
Under the law, employers cannot dismiss or discriminate against employees because they are receiving subsidies.
But Amanda L. Austin of the National Federation of Independent Business, a trade group for small businesses, said some employers would be tempted to consider household income in making personnel decisions.
“To measure the cost of an employee,” Ms. Austin said, “some employers will want to know the worker’s household income, so they can know whether the employee will qualify for a subsidy. If an employee is getting a subsidy, the employer might not want to keep that employee.”