John Kiely, the owner of Allied Cleaning Service in Manhattan, has encountered more than his fair share of office refrigerators during his forty years in the cleaning business, and he doesn’t mince words when describing some of the horrible things he’s seen and smelled.

“If you can wear a mask, sometimes you’ll be okay,” he says. “But sometimes the stench is just too much. It’ll make you lightheaded. I’ve actually been scared a few times. After cleaning enough of these fridges, now I know what a decomposing body smells like.”

It may sound like an exaggeration, but anybody who’s shared a refrigerator with their co-workers knows exactly what he’s talking about. Take a peek inside your company’s communal fridge and you’ll see some of the most pernicious atrocities this side of a Roger Corman movie. Saran-wrapped sandwiches that seem to have a pulse, take-out leftovers in the early stages of evolution, and cartons of milk with the consistency of molten lava.

There’s always an excuse, of course. And Kiely has heard them all. “My favorite is when they won’t throw out a meal because it has sentimental value,” he says. “Usually because it was made by their mom or a new girlfriend. I want to tell them, ‘You know that new girlfriend you’ve got? I hope you’re not thinking about marrying her for her cooking skills. Because let me tell you, her food stinks!'”

The communal refrigerator has been a timeless source of dread for the working professional — talk to anyone who’s shared office space with more than three people and they’ll likely have a horror story for you — but it’s become especially frightening during the recent recession, which has inspired more and more workers to “brown-bag” their lunches rather than dine out. With 70% of Americans now eating at their desks, that means bigger lines for the break-room microwave and a refrigerator packed tighter than a fat guy in spandex.

And that translates to increased health dangers. According to a study conducted by the American Dietetic Association and ConAgra Foods, 44% of office refrigerators are cleaned once a month and 22% are cleaned only once or twice a year. In other words, the bathrooms in your building are probably cleaner than the fridge. If you’re looking for a relatively bacteria-free place to store your lunch, you might want to consider the office urinals.

What’s the worst that can happen if you regularly store food in your office refrigerator? Maybe nothing. Or maybe it’ll require calling in a hazmat team. That’s what happened last year at an AT&T call center in San Jose, California, when a helpful employee decided that somebody really ought to clean the company fridge. When she cracked it open like a Pharaoh’s tomb, the noxious fumes sent seven of her co-workers to the hospital and forced authorities to evacuate the building. “It was like a brick wall hit you,” one of the employees recalled later about the stench. And yes, guys in hazmat suits were indeed needed to clean up the mess.

So what types of foods are the most likely to turn your office refrigerator into a scene out of The Hurt Locker? According to the USDA, the biggest culprits are casseroles, luncheon meats, poultry, and of course, yogurt and sour cream.

“One of the worst problems is food left in the fridge that everyone is encouraged to eat,” says Alice Henneman, a registered dietitian with the University of Nebraska Cooperative Extension. “Say, leftovers from an office meeting or group lunch. Who knows how long they sat out before they were refrigerated? I am aware of one company where over thirty people became sick because leftover food in a deep container didn’t cool fast enough.”

In some cases, decaying food may be your best case scenario. Henneman, who’s devoted much of her career to studying the office refrigerator menace, is still shocked by what some people consider acceptable to put in a communal fridge.

“Several years ago, I did a survey on what were some of the worst things people encountered in office refrigerators,” she said. “The two scariest examples were human stool samples stored in the same refrigerator as employee lunches, and cow manure samples refrigerated next to food items.”

She adds that the stool samples “probably came from some type of company involved with laboratory procedures” and there was no mention of any workers getting sick, but that’s small comfort for anybody who’s ever taken a long whiff of their office refrigerator and muttered, “What smells like crap in here?” Turns out, you might be more correct than you think.

But even if you think that the mysterious piece of Tupperware in the back of the office fridge could contain something hazardous, or even scatological, don’t be too quick to throw it out. You might just be breaking the law. In 2007, the University of Texas ordered the “aggressive cleaning” of a messy chemistry professor’s office, including his personal lab refrigerator. The professor sued, and the 5th U.S. Circuit Court of Appeals ruled that all “government employees must receive adequate notice before their personal belongings can be tossed out.” Do the same rules apply to your co-worker’s long-forgotten and increasingly toxic tuna casserole?

Dennis P. Ortwein and J. Stephen Kreglow, Pennsylvania trial lawyers, don’t think throwing out a fellow employee’s tuna casserole is grounds for legal action. And if an oversensitive co-worker does complain when his lunch science experiment has gone missing and threatens to sue, “the damages are probably nil,” says Ortwein. “What is a moldy tuna casserole worth? Then again, since all tuna now has a BP oil base, maybe there is some value.”

“Why make a federal case out of a lousy, green tuna casserole?” Kreglow adds with a laugh. “What do you think the response of the District Attorney or the local police will be when you phone in this theft of ‘personal property’?”

With office refrigerators continuing to be a source of controversy, both as a breeding ground for disease and “hands-off-my-sandwich” infighting, some companies have started looking for creative solutions. At Google and PepsiCo, for instance, they’ve introduced organic corporate vegetable gardens, where employees can harvest their own lunch rather than fill the break-room fridge with food they’ve brought from home and will inevitably abandon.

And then there’s the Flip-Stacking Office Fridge, the creation of a New York industrial designer named Spencer Schimel. It’s a Lego-style stack of mini-refrigerators that allows all of your co-workers to have their own individualized lunch storage space.

“An important problem addressed in my design, where I believe most of the problems stem from, is personal accountability,” says Schimel. “Because the office refrigerator is unowned by any individual, no one feels responsibility to take care of it, and certainly no one wants to take time out of his or her busy workday to do so.” But with this new design, those responsible for creating a “range of unappetizing sights and smells” will have to contend with the consequences of his or her own actions.

Some of your co-workers will likely scoff at the very idea of taking any responsibility for the cleanliness of an office fridge. “That’s why we hire a service,” they’ll say. “Let them get rid of all that moldy food!” Don’t be so sure. According to Martin Friedman, the owner of Blue Chip Cleaning Services in New York, hosing down the office fridge is not necessarily part of their job description.

“Our standard rate only includes the exterior of appliances,” he says. “Not the inside of the fridge. If they want that included, we’ll bill them with a separate invoice.”

If your company is willing to pay the extra fee, the Blue Chip cleaners will take a blitzkrieg approach to sterilizing the refrigerator. Everything goes, with extreme prejudice.

“Sometimes the customer asks us to empty out the entire refrigerator, wash everything down and then sort through their stuff and put anything that’s still edible back in,” Friedman says. “We don’t do that. We’ll use common sense. If there’s an unopened bottle of soda, that obviously goes back. It doesn’t take a genius to figure that out. But beyond that, we’re not going to make a subjective judgment call about the quality and freshness of your food. If it’s in there and you didn’t toss it, it’s garbage.”

He’s had complaints, of course. Office workers have called him the morning after a cleaning in a panic, demanding to know what happened to their blue cheese salads or unmarked tubs of Tupperware. Friedman rarely apologizes.

“If that’s what you’re eating,” he says, “then you’ve got bigger problems than I can help you with.”

(This story originally appeared, in a slightly different form, in the July 15 2010 issue of Bloomberg Businessweek.)