As far as rules go, the five-second rule is a controversial one.

Some people are totally fine with eating food that's only briefly been on the floor. A quick inspection, maybe a gentle blow of air to remove any dust or germs (that works, right??), and it's good as new.

Other people stand firm behind a trash can — not a mouth — being the most suitable place for food that's spent any time on the floor.

And even for those who are proponents of the rule, it can still be one that's riddled with contradiction.

You feel comfortable eating food that you yourself dropped, but you'd likely take issue with someone else serving you food that's been on the floor. You're probably also quick to invoke the five-second rule with something like chips or cookies, but conveniently forget the rule exists when it comes to broccoli or peas.

Regardless of where you stand, though, it's time to determine whether the five-second rule is true or not.

What is the five-second rule for food?

It's a loose (and questionable) definition, but the five-second rule goes as follows: Food that's spent five seconds* or less on the floor is "safe" to eat.

*This time frame is sometimes reduced to three seconds, or stretched to 10 — at the user's discretion, of course

Essentially, the five-second rule is your window of opportunity before food dropped on the floor becomes contaminated with any potentially harmful germs that may be present.

In addition to being a rule, it's also a phrase we feel compelled to announce when we invoke it around other people — a social cue, of sorts, used to acknowledge that we know what we're doing is questionable, perhaps?

Is food dropped on the floor really safe to eat?

Believe it or not, scientists have examined the five-second rule.

To understand whether it's true or not, it helps to break it down into its three main variables:

1. The time frame
2. The food
3. The floor
   
   

Can food really get contaminated in just five seconds of being on the floor?

Unfortunately, we're off to a bad start.

A 2006 study found that a particular diarrhea-causing bacteria (Salmonella) can transfer from floor to food (bologna, in this case) after just five seconds of exposure. The contamination happened almost immediately, in fact.

This means that five seconds is plenty of time for food that falls on the floor to become contaminated — essentially disproving the rule.

Busted on the first variable. And the rule's namesake, to boot! Probably not a good sign...

Still, there are two more variables to go. Might there be more at play with the five-second rule than this false window of opportunity?

Do some foods get contaminated more easily than others?

Bologna can get contaminated in just five seconds. But what about other types of food? Do certain features of food give you extra leeway when it comes to picking it up and safely eating it?

The evidence is mixed.

A 2016 study looking at how contaminated different types of food (watermelon, bread, buttered bread and gummy candy) were after being dropped found that watermelon had the most germs on it when it came back from the floor — implying that perhaps the flatter and wetter a food item, the easier it gets contaminated. Gummy candy was the least contaminated of the four food items, and the researchers hypothesized this was due to the less uniform surface of the candy. Bread and buttered bread were contaminated at about the same rate.

But, looking back to the 2006 study mentioned previously, which examined bread in addition to bologna, the researchers all but discount the importance of the food item variable in the five-second rule. Instead, they place much more emphasis on the source of contamination (aspects of the floor's surface), rather than the features of the food it will eventually cling to.

In the end, the food variable isn't really busted, but it doesn't bolster the five-second rule, either. There doesn't seem to be a food type impenetrable to germs.

Still, does any of this really prove that all food that's fallen on the floor is contaminated to an extent that makes it unsafe to eat?

Does the floor surface matter?

What we know from both of the laboratory studies mentioned above is that common floor surfaces — wood, tile and carpet — are not only able to harbor harmful germs, but any germs present on these surfaces are readily able to transfer to food. Interestingly, though, carpet seemed to transfer germs least efficiently.

In addition, the 2006 study claimed that the amount of bacteria present on a surface was the most influential factor contributing to "how contaminated" a dropped food item was by the time it was picked up again — not how long it was down there or how the features of the food item affect germ transfer.

But are similarly harmful germs (and amounts of these germs) present on the floors you encounter every day?

In reality, that's hard for any of us to answer on our own.

For the floors in your home, the factors to consider may range from how often you mop to whether you have pets. For floors outside of your home, many more factors likely need to be considered.

And herein lies the primary issue with the five-second rule: We can see dirt, we can see dust, but we can't see germs — they're invisible to the naked eye! So a floor that doesn't look dirty isn't necessarily "clean" either.

Plus, even if we could see the germs on a piece of food we just picked up from the floor, we wouldn't know whether they're harmful, like Salmonella, or harmless. And when harmful is described by causing foodborne illness, which can come with symptoms like diarrhea, fever and vomiting, as well as the potential for even life-threatening complications — maybe it's best not to risk it with the five-second rule.

At face value, consider the rule debunked.

The decision to invoke this food safety no-no remains yours, but the science says that the safest choice is to throw it out.