The 161 million Americans with employer-sponsored health insurance are facing substantial increases in out-of-pocket costs, according to a study published this week.

The study, authored by researchers from the National Opinion Research Center and Watson Wyatt Worldwide and funded by The Commonwealth Fund, examines trends in employer-sponsored insurance from 2004 to 2007. It found rising rates of underinsurance and unaffordability, particularly for poorer and sicker people.

In 2007, adults with employer coverage faced an average of $729 annually in out-of-pocket costs for medical services, including deductibles and other forms of cost sharing such as copayments and coinsurance. That represents a 34 percent increase from 2004, when the average out-of-pocket burden was $545.

Health plans covered a slightly smaller percentage of overall expenses in 2007 than 2004, but growth in overall health spending was the chief culprit behind rising out-of-pocket costs, according to the study.

"The years from 2004 through 2007 were a period of economic expansion, yet rising health care costs still eroded the value of employer-sponsored coverage," said lead author Jon Gabel. "Historically, employees have been asked to shoulder even more of the cost-sharing burden during difficult economic times such as the United States is now experiencing. Hence, it is imperative that health care reform include constraints on health spending, or else health insurance will become unaffordable for low- and middle-income Americans, and reform itself will be unsustainable."