My kid just ate floor food. Is it safe?

Let's assume it was an old Cheerio.

You know it's old because you the upholstery attachment fell behind the washing machine months ago. Even if you had been able to vacuum the couch since then, you know you tossed that cereal box amid your mid-summer glyphosate panic. Whatever it was—cereal, yogurt-covered raisin, jelly bean—you watched in slow-motion horror as your child grabbed it from under the couch. Before you made it across the room, your child was chewing.

After your child swallowed, you googled "is it safe to eat food from under the couch?" and wound up here. Or maybe you're a regular reader (thank you!) with a home so much cleaner than mine that you've never had to ask such a question, and are reading on for the schadenfreude. Now that we'e all here together, let's ask ourselves what we're really worried about when we're worried about couch snacking.

Welcome to Part One of The Couch Cheerio Series.

When your mom says you need to pour Cheerios on the couch  for research , you don’t ask too many questions.

When your mom says you need to pour Cheerios on the couch for research, you don’t ask too many questions.

For the next four weeks, we’ll use the humble Cheerio as a lens to view what parents worry about when they see their kids eating: choking, food poisoning, allergic reactions, and more. This week, we’ll look at what happens that first fistful of Cheerios gets thrown from the high chair. Can you pick them up and give them back to the now-howling baby? What if they roll under the couch, to be discovered and eaten months or years later? In short: is the five-second rule real?

What’s the best dropped Cheerio?

The first peer-reviewed study of the five-second rule was published in the Journal of Applied Microbiology in 2006. Paul Dawson coated different surfaces with different amounts of Salmonella and left the samples for different amounts of time. He found that floor type matters: food dropped on carpet samples picked up less Salmonella than equally-dosed samples of wood and ceramic tile. But even in the most generous of conditions, food picked up bacteria instantaneously.

Dawson’s results suggest that if bacteria is present, it’ll hop on that Cheerio as soon as it hits the floor. A Cheerio that falls on the carpet will pick up less bacteria than a Cheerio dropped on kitchen tile.

In 2014, Anthony Hilton, with his students at Aston University, performed a similar study to Dawson’s research, testing carpet, tile, and laminate surfaces coated with E. coli and Staphylococcus. They confirmed that length of time increased bacterial contamination, and that carpet transferred less bacteria than other surfaces. They also found that wetter foods were more likely to pick up bacteria than dry foods.

Hilton’s research also suggests that a dropped Cheerio is going to pick up bacteria way faster than five seconds. Furthermore, a wet Cheerio will pick up more bacteria than a dry one.

Two years later, in the Journal of Applied and Environmental Microbiology, Robyn Miranda and Donald Schaffner confirmed the findings of the previous two studies. They tested the same three flooring types as Dawson—carpet, wood, and ceramic tile—as well as stainless steel, all of which were coated in Enterobacter. Miranda and Schaffner tested these samples with four different foods (watermelon, bread, bread with butter, gummy candy) for differing lengths of time. They found that the longer the food was in contact with the surface, the more bacteria transferred to it. Although their study did not test for pressure, the researchers speculate that pressure might also contribute to increased bacterial contamination.

Miranda and Schaffner’s research confirms everything we know so far: dry Cheerios dropped on carpet will pick up less bacteria than wet Cheerios dropped on other surfaces. The research also allows us to speculate that a Cheerio thrown at or mashed into the carpet might harbor more bacteria than one simply dropped there.

These three studies suggest your kid’s best bet is a dry Cheerio gently tossed onto the carpet. But they also demonstrate that food picks up bacteria the moment it hits the floor, skewering the five-second rule in the process.

Or do they?

Three scientists on the five-second rule

If you’re looking for Hilton’s study, you won’t find it. His results are unpublished, a criticism raised by both Dawson and Schaffner. Usually, that would be disqualifying for me, too: if you don’t publish your results, you avoid the scrutiny of peer review, which is a necessary part of scientific legitimacy.

But this case is more interesting, because all three studies produced incredibly similar results. They all found that dropped food picks up bacteria nearly simultaneously. They all found that different surfaces transferred different amounts of bacteria. Two of them found that wet foods pick up more bacteria than dry foods.

The only major difference in the studies is the conclusions reached by the authors. Sara Whitman-Salkin interviewed Dawson and Hilton about their conclusions for National Geographic in 2014. Hilton’s confident in the relative safety of the five-second rule: any individual piece of floor food will pick up bacteria right away, but it’s probably not harboring deadly bacteria.

Dawson’s study, confirmed that while bacterial transfer was happening, it was unlikely that any individual piece of food was going to kill him. And yet, the possibility of that danger was enough for Dawson to institute a “zero-second rule.”

Schaffner occupies a middle-ground about floor-eating, admitting that while he’s eaten floor food before, he’ll always compost a dropped piece of watermelon.

Dawson’s, Hilton’s, and Schaffner’s divergent responses to the same data are a beautiful illustration of why there’s no right answer to be found around the five-second rule, because risk is individually determined. No study can confirm or deny the five-second rule, because the five-second rule is a personalized risk assessment.

So…can the five-second rule hurt?

If these scientists disagree on the five-second rule, what should parents without degrees in microbiology do? It helps to remember that both the harmless and harmful types of Salmonella and E. coli live in the intestines of humans and other animals. Some scenarios might make you more concerned about E.coli contamination than others. If you have a free-range pet turtle, it’s best to avoid any floor food. If you just dropped a raw chicken on the floor, of if you have pets or children who might be tracking fecal matter across it, you might want to halt all floor feeding until you’ve mopped up.

Remember where we started: a kid across the room, eating something before you can stop it. We might yell “Five-second rule!” when the thing we’ve dropped—a perfectly orange, evenly salted Cheez-it or the last of our favorite cookies—is something we really want to eat. At these moments, the five-second rule is a calculation: we’re willing to accept a little risk for the reward of a favorite food.

But when we’re the parent watching food floor consumption, the five-second rule becomes something else, a sort of metaphorical shrug to the things that we cannot avoid. Our kids are going to eat the couch Cheerio, lick the shopping cart handle, and chew gum they found on the street. The five-second rule allows us to, at least temporarily, turn off fear of all the invisible dangers we imagine lurking and reassure ourselves that our kid will probably survive this most recent gross endeavor.

The dropped Cheerio probably won’t hurt. But of course, that’s just one of many Cheerios could theoretically kill you. We’ll tackle another in Part Two.