NEW YORK (CNNMoney.com) – There may be a bigger elephant in the living room than under-funded company pensions: under-funded healthcare benefits for retired workers.

That's a concern not only for workers, retired workers and their families – which together total tens of millions of people who have been banking on company-paid health benefits in retirement.

It will become a more obvious concern for investors as well since changing accounting standards will soon shine a brighter light on companies' healthcare benefit costs.

In a report released Monday, credit rating agency Standard & Poor's found that among S&P 500 companies, they have a combined under-funded balance of $292 billion for their OPEB (Other Post Employment Benefits) obligations, which include company-paid health insurance premiums and drug coverage plans for retired workers.

By contrast, the combined under-funding in the pensions of S&P 500 companies totals $150 billion.

"Under-funded" in this sense measures how much more companies need to set aside today to meet 100 percent of their estimated obligations. Currently, S&P 500 companies have enough set aside in assets to meet 88 percent of their full pension obligations, but only 22 percent of their OPEB obligations.

That 22 percent may overstate the case a bit. That's because a lot of companies pay their retiree health benefits on a pay-as-you-go basis. Companies are not required, as they are with pensions, to set up a separate trust for OPEB assets nor are they subject to minimum annual funding requirements to pay for OPEB.

If a company uses a pay-go system, though, Standard & Poor's assesses them to be 100 percent under-funded. That doesn't necessarily mean they won't be able to meet their OPEB obligations, just that they have no money earmarked for OPEB per se, said Howard Silverblatt, an equity analyst at S&P.

There are some companies who have set up separate OPEB trusts. But those that have "fund them to significantly lower levels than are required under pension funding rules," the S&P report noted.

What's more, there is growing concern that the true picture of a company's ability to fund future promises to retirees may be masked by current accepted accounting techniques that can make a firm's assets look greater than they really are relative to future liabilities.

And retired workers' healthcare benefits are a growing liability for companies due to rising health care costs, increased longevity and a slew of early retirements in sectors where mergers have reduced companies' workforces, Silverblatt noted.

Indeed, investor concern over corporate liabilities may increase as companies may soon be required to provide greater transparency in their OPEB reporting.

In November, the Financial Accounting Standards Board (FASB) voted to undertake a review of pension and OPEB accounting.

As part of that undertaking, by the end of 2006, it's likely to issue a standard that companies will have to add to their balance sheets the net status (assets minus liabilities) of their pensions and OPEB. Previously, such information was relegated to footnotes.

Inclusion of that information on the balance sheet will have an effect on ratios involving debt and equity that investors use to assess a company, Silverblatt said.

In its report, S&P said, "We believe this FASB project will have a significant impact on stock evaluations, income statements and balance sheets, and will become the major issue in financial accounting over the next five years."

S&P found that the S&P 500 companies with the biggest OPEB under funding problems are GM (Research), with a $61 billion shortfall; Ford (Research) with a $32.4 billion shortfall; Verizon (Research) with a $22.5 billion shortfall; AT&T (Research) with a $16 billion shortfall; and Boeing (Research), with an $8 billion shortfall.

Just recently, both Ford and GM negotiated reductions in healthcare benefits for retired workers.

Only three S&P 500 companies have over funded their OPEB obligations: Comerica (Research), PerkinElmer (Research) and Principal Financial (Research).