How long is it safe to leave food out? The 2-hour rule, explained.

The two-hour rule seems straightforward — perishable food should not be left at room temperature for more than two hours. But a video I posted about this topic over the summer blew up on social media with more than 2 million views on Facebook.

While it got thousands of “likes,” I was startled by the pushback arguing against this simple rule of thumb. I had no idea there was so much confusion and passion regarding food safety.

Heading into this season of feasts, potlucks and buffets, there is no better time to clear up misconceptions about this important guideline, and offer practical tips for keeping your food safe to eat. Most of the comments on my post arguing against the two-hour rule fell into three main categories:

Misconception: It’s not a two-hour rule, it’s a four-hour rule.

First, let’s dig into the reasoning behind the two-hour rule, a long-standing food safety guideline put forth by the U.S. Department of Agriculture, the Food and Drug Administration and the Centers for Disease Control and Prevention, to prevent foodborne illnesses.

Bacteria thrive in temperatures between 40 and 140 degrees and can double every 20 minutes in that “danger zone.” Based on that growth, scientists pinpointed two hours to be the maximum amount of time consumers can safely leave perishable food at room temperature.

Complicating things somewhat, since bacteria grow faster on the warmer end of that range, if food is outside in temperatures 90 degrees or higher, it becomes a one-hour rule.

Many folks argued in the comments on my post that it is not a two-hour rule, it’s a four-hour rule. They aren’t entirely wrong. There is a four-hour rule, too, but that pertains to retail establishments, such as restaurants and grocery stores.

The time window is wider for professional kitchens because they are generally more controlled environments, with multiple layers of strict food safety practices in place to minimize bacterial contamination and growth.

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“It’s not like there is that moment where food instantly goes from being safe to being unsafe,” Jennifer McEntire, founder of Food Safety Strategy, a consulting firm, explained. There are so many variables, but if a food contains a pathogen, the more time it sits out and the warmer the temperatures, the more likely it is to make people sick. It’s a matter of risk.

For the record, perishable food is anything you would ordinarily keep in the refrigerator, including meat, poultry, fish, eggs, dairy products, cut vegetables and fruit, and cooked dishes.

And the two-hour rule pertains to the entire life of the food. So if a guest brings a dip to your holiday party without putting it in a cooler, and their drive to your home takes one hour, you have one hour remaining to leave that food on the buffet unchilled.

Misconception: That rule can’t be valid. I always leave food out much longer and have never gotten sick.

Have you ever had a “stomach flu?” I thought I had, too, until I learned there is technically no such thing. While there are contagious viruses that cause gastrointestinal distress, that illness you experienced was most likely foodborne illness, and it’s harder than you think to pinpoint the meal that caused it. Most people blame it on the last meal they ate, but it’s typically not.

“Incubation periods [for pathogens] are typically 1 to 10 days out,” says Barbara Kowalcyk, who is an associate professor at the Milken Institute School of Public Health at George Washington University. You could have gotten sick from the potato salad left out all day at your family gathering without ever realizing it.

Possibly, you noticed your stomach was off after eating something left out too long and dismissed it as a no-big-deal, 24-hour inconvenience. While that’s often the case, it’s not uncommon to have much worse outcomes.

Approximately 128,000 people are hospitalized and 3,000 die of foodborne illness annually in the United States, according to the CDC. “A significant number of people develop long-term health outcomes, such as reactive arthritis and irritable bowel syndrome, which greatly impact your quality of life,” Kowalcyk says.

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Not everyone’s risk is the same. Children, pregnant women, senior citizens and those who are immune compromised are much more vulnerable to pathogens. No one knows this better than Kowalcyk, who switched career paths to become a food safety expert after losing a child to foodborne illness in 2001. The toxin produced by E. coli led to the death of her 2½-year-old son, Kevin.

Her husband and 5-year-old daughter also tested positive for E. coli at the time but had only mild symptoms. (Microbial contamination can occur at any point during food production, but microbes such as E. coli and salmonella can grow rapidly at the “danger zone” temperatures, increasing the risk of serious illness.)

Abiding by the two-hour rule and other food safety measures at home won’t eliminate the risk of illness entirely — there are many other factors at play — but it is an element in our control that can make it much less likely.

If you are hosting a party, why not take the basic preventive steps to protect your more vulnerable guests?

Misconception: If you avoid bacteria you’ll be more susceptible to illness in the long run.

Many comments on my post implied that following the two-hour rule over time somehow makes a person more susceptible to getting sick, as if lack of exposure to harmful bacteria weakened one’s resistance.

Each expert I spoke with agreed this is misguided and dangerous. It’s important to differentiate between the good bacteria in fermented foods, such as yogurt and kimchi, which may help you maintain a healthy gut microbiome, and bad bacteria that are a result of contamination, such as salmonella and campylobacter.

“The classic foodborne pathogens are not the ones that become part of your microbiome, they just make you sick,” says Cindy Liu, a microbiologist in the Department of Environmental and Occupational Health at the Milken Institute School of Public Health. “Exposing yourself to them is not safe and it doesn’t work to prevent future illness.”

Easy measures to take as the host this holiday season:

“We know that food can be a sign of love. Preparing and storing food safely is also a sign of love,” said Shauna Henley, senior extension agent in family and consumer sciences at the University of Maryland. That may mean doing things a little differently than your great-grandma did, but that is not an affront to her or your family traditions.

The science for this was probably not available in her time. Now that we know more about bacteria and foodborne illnesses, why not use that knowledge to keep yourself and your household safer?

If you are hosting, keep in mind the nonprofit Partnership for Food Safety Education’s “Core Four”:

  • Clean: Wash hands and surfaces often.
  • Separate: Avoid cross-contamination: Keep raw meat, poultry, seafood and eggs separate from other foods.
  • Cook: Use a food thermometer and cook dishes to the proper internal temperature.
  • Chill: Follow the two-hour rule. Never thaw perishable food at room temperature.

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Some helpful strategies for hosts:

If a guest offers to help in the kitchen, guide them to where they can wash up before they start.

Stagger putting out dishes instead of setting them out all at once. If possible, prepare two smaller batches of a dish so one can stay chilled longer. Besides being safer to eat this way, it will be fresher-tasting and more appealing, too.

Add ice to a cooler to expand chilled storage space. Refrigerators get filled up fast at big parties, and air does not circulate properly in an overstuffed refrigerator. Cool foods in shallow (two-inch) containers so they chill quickly.

When in doubt, throw it out (or compost it). Nobody wants to waste food, but it’s better to do that than get you or your guests sick.

Polite measures to take as a guest:

The video I posted on social media was at a cookout I attended. While I was careful to frame the shot so the location and people were not identifiable, I admit that shooting it in the first place was not the most polite thing to do. However, I’m glad that it started this important conversation.

Certainly, though, there are more immediately helpful, considerate approaches than posting videos of questionable food safety on social media.

One thoughtful suggestion from Thomas P. Farley, the etiquette expert and columnist known as Mister Manners, is to head the issue off at the pass before the event by offering to bring an extra cooler filled with ice, or trays with cans of Sterno to help keep foods hot.

If you are bringing a dish, make sure it is well-chilled before packing it in an insulated tote with ice packs for its journey. At the gathering, if you notice the food has been out a while, do what many commenters on the post suggested and offer to help get things into the refrigerator.

Whether you are host or a guest, I hope you’ll stay attentive to the two-hour rule this season and beyond, and take the other basic measures to keep food safe for you and your loved ones. That will help ensure that everyone has a truly happy and healthy holiday.

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