About a third of U.S. companies could find themselves in violation of a provision of the recently enacted healthcare reform measure because some of their employees might not able to afford the health care coverage they provide, according to an analysis by human resources consultancy Mercer.

The Patient Protection and Affordable Care Act (PPACA) defines “affordable” health insurance as coverage that costs employees no more than 9.5% of their household income. Using data from its 2009 survey of nearly 3,000 employers that sponsor health plans, Mercer estimates that 38% of the companies have some employees for whom health coverage would be unaffordable. Bigger employers are in better shape, but not immune; 20% of companies with 20,000 or more employers would have a problem.

Those percentages probably overstate the extent of the problem, Mercer says, because its data includes only average employee salaries, not total household income. “What's troubling is it will be hard for employers to anticipate even whether it's a problem, because they're missing that piece of information about the household income,” says Beth Umland, Mercer's director of research for health and benefits. “So they're doing more conservative estimates based on what they're paying the employee.”