How Bad Is It Really to Eat Food That's Been Out for Hours?

Have you ever scarfed down a slice of pizza from a box that's sat on the counter overnight? Had a brunch so leisurely that a few hours in, you started picking at the eggs and bacon on the table for a second round?

Food safety experts say this kind of behavior is ill-advised and a recipe for getting a foodborne illness.

After you've made dinner, ordered takeout or boxed up your leftovers from a meal at a restaurant, that food should be stashed in the refrigerator within two hours, per the USDA.

And, if the temperature outside is warm, the timeline shrinks. When it hits 90 degrees Fahrenheit, food can only safely be left out for an hour, per the USDA — keep that in mind at your next picnic.

There is a very good reason for this two-hour guideline: Food that sits at room temperature creates a breeding ground for unfriendly bacteria.

"Bacteria can grow rapidly on food when it is left at temperatures between 40 to 140 degrees Fahrenheit," says Toby Amidor, RD, FAND, nutrition and food safety expert and best-selling cookbook author of ‌The Create-Your-Plate Diabetes Cookbook‌.

In fact, that temperature range is typically referred to as the "danger zone" because it's so hospitable to the growth of bacteria, such as Staphylococcus aureus, Salmonella Enteritidis, Escherichia coli and Campylobacter, per the USDA.

"These bacteria can cause foodborne illness if ingested," says Tamika Sims, director of food technology communications at the International Food Information Council (IFIC).

This guideline for how long you can leave food out of the fridge also applies to your groceries, Amidor says.

"This is why if you go food shopping, you should plan your trip so that the market is your last stop right before you head home," she says.

On hot days, put perishables in the backseat instead of the trunk, so that food can be in an air-conditioned space, she recommends. You can also stash food in a cooler.

Anyone who's been a victim of foodborne illness is all too familiar with why it's best avoided: It's both painful and unpleasant.

"The most common symptoms [of foodborne illness] include diarrhea and/or vomiting, as well as abdominal cramps and pain, nausea, fever and fatigue," Sims says.

The precise symptoms of foodborne illness can vary depending on the microorganisms or toxins that were consumed, notes Amidor. Botulism, which is caused by ‌Clostridium botulinum‌ bacteria, can lead to difficulty swallowing and breathing, blurred vision and paralysis, she says.

"Something like E. Coli, which can happen when you handle raw ground beef and then slice a watermelon or raw vegetables with the same knife, can lead to hemolytic uremic syndrome, which can ultimately lead to kidney failure," Amidor says.

Foodborne illness is a big problem. Each year, 48 million people get sick due to foodborne illness, according to the Centers for Disease Control and Prevention (CDC). More than 100,000 people wind up in the hospital and 3,000 people die each year as a result, estimates the CDC.

Of course, not all these cases of foodborne illness are due to food that's been left out too long — many are the result of undercooking meat or eating produce with pathogens. (Leafy greens, in particular, have been a source of many recent foodborne illness outbreaks, per the USDA.)

If you believe you may have foodborne illness, tell your doctor or a health care provider, says Sims.

Even if you reheat leftovers that have lingered outside of the fridge too long, you can't guarantee that you'll be killing the harmful bacteria.

"Reheating will not destroy ‌all‌ the potentially harmful bacteria that have grown on the food and neither will freezing or refrigerating (which do not destroy bacteria, they just reduce the rate of growth)," Amidor says.

The safest thing to do is throw away food that's been left out too long, agree both Amidor and Sims.

And while everyone knows to be cautious around potato salad at picnics, it's not the only food to watch for. "Other foods are also problematic as bacteria like to thrive on them," Amidor says.

"These are foods that are high in carbs and protein, high in moisture and have a slightly acidic to neutral pH," she says. These include cooked fruits, rice and vegetables, raw or cooked poultry, meat, fish, eggs, milk and dairy and tofu and soy-based foods.

Many of us have a friend who boasts of an iron stomach. Or maybe it's you: You find that you're impervious to tummy troubles, and have eaten late-night, left-out pizza tons of times with no issues at all.

"Everyone should see themselves as equals when it comes to foodborne illness," says Sims. That said, people with weaker immune systems, such as young kids, older adults and people who are pregnant or immunocompromised, may be particularly susceptible to foodborne illness, Amidor notes.

But adhering to the two-hour rule is advisable for everyone, Ambidor says. This guideline is the result of scientific research into the growth rate of pathogenic microorganisms on food, she says.

"Many people may say they have a 'strong stomach' but we should not gamble with our health in this manner," Sims says.

So no, you should not eat food that has been left out overnight — no matter how tempting pizza for breakfast is.