We all know what we're supposed to do after using the toilet.

But survey after survey (including one in which scientists secretly camped out in bathrooms) have revealed a dirty truth: people don't always wash their hands before they leave the bathroom. One study suggested that only 67 percent of people wash their hands after they go.

Don Schaffner, a professor of food science at Rutgers, has been studying hand washing for years and says the conventional wisdom shouldn't be ignored.

"It doesn't matter whether you're peeing or you're pooping, you should wash your hands," he told Business Insider.

Here's why.

Germs can hang out in bathrooms for a long time

Each trip to the restroom is its own unique journey into germ land. So some occasions probably require more washing up than others.

"If you've got diarrhoea all over your hands, it's way more important that you wash your hands than if… you didn't get any obvious poop on your fingers," Schaffner said.

"My gosh, if you've got poop on your hands and you have the time, certainly, get in there, lather up real good and do a real good job."

Compared to faeces, urine can be pretty clean when we're not harboring any infections, though it's not totally sterile.

"People who use urinals probably think they don't need to wash their hands," Michael Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy, said to The New York Times. (In studies, women tend to be better about adhering to hand washing than men.)

But it's best to wash your hands after every trip to the toilet because human faeces carry pathogens like E. coli, Shigella, Streptococcus, hepatitis A and E, and more.

You can also easily catch norovirus by touching bathroom surfaces that have been contaminated with a sick person's poo or vomit, then putting your hands into your mouth. The super-contagious illness is the most common food poisoning culprit, and causes diarrhoea, vomiting, nausea and stomach pain.

A wide variety of other microbes and bacteria can be found in bathrooms, too. Some strains of Staphylococcus, or staph, are "found on almost every hand," as a team of hand-washing researchers pointed out in a 2004 study.

Public toilets can house many different drug-resistant strains of that bacteria.

Even if your own hands are clean and poo-free, can you say the same for the last person who touched that toilet handle, used the sink, or opened the bathroom door?

Hand washing is a life-saving routine

Religious traditions have urged cleanliness via ritual hand washing for thousands of years. But it wasn't until the 1800s that health care professionals linked good hand hygiene to lower infection rates.

Dr. Ignaz Semmelweis noticed that when doctors and medical students handled cadavers before touching patients in the maternity ward, more mothers developed fevers and even died. He instituted an unpopular chlorine-washing rule and saw the death rates tumble.

Similarly, during the Crimean War, nurse Florence Nightingale initiated hand-washing rules and other hygiene measures in the British hospital where she worked. Death rates there dropped by two-thirds, providing some of the first hard evidence that proper hygiene saves lives.

We're still far from perfect at preventing infections.

People generally contract diarrhoea – which kills around 525,000 children under five annually around the world – by drinking dirty water, eating contaminated food (often soiled by dirty hands), and from person to person contact "as a result of poor hygiene," according to the World Health Organisation.

Better hand washing could cut diarrhoea death rates in half and save more than a million lives (adults and children) every year, researchers at the London School of Hygiene and Tropical Medicine estimate.

Regular hand washing can also cut your risk of developing a respiratory infection by 16 percent.

'Wash your hands any time you feel that they might be dirty'

We all come in contact with unsavoury substances regularly, especially after touching public doorknobs, our cell phones, and moist kitchen towels – all of which are likely dirtier than a clean toilet seat, which typically houses just 50 bacteria per square inch.

So hand washing at any time of day can help stop the spread of many kinds of bacteria, yeasts, and viruses.

"I think a good general rule of thumb is you should wash your hands any time you feel that they might be dirty," Schaffner said. In other words, seize the opportunity when you're near a sink.

He said he's not "super paranoid" about making sure his own hands are always squeaky clean, but some of his favourite times of day to wash up are after walking the dog, working in the dirt, or handling raw meat.

Even a quick "splash 'n dash," as researchers like to call the practice of rinsing with water but no soap, can help fight off some bacteria that causes infections.

But that shortcut is not advised if you might have raw meat or faeces on your mitts, and a lather with soap and water is more effective at disinfecting hands than any wipe or sanitizer.

Here are Schaffner's best tips for your next journey to the toilet

Follow this simple, three-step hand-washing plan to lower your chances of getting colds, self-inflicted food poisoning, and diarrhoea.

First, don't worry about the temperature of the water; Schaffner's studies have confirmed that doesn't make a difference. He suggests that you "adjust the water temperature so it's a nice comfortable temperature, so you can do a good job."

Second, give yourself enough time to "get some soap in there, lather it up real good, clean under your finger nails," Schaffner said.

Spending even five seconds washing your hands can help reduce the amount of bacteria on them, but 20 seconds is better. The Centres for Disease Control recommends humming the Happy Birthday song to yourself twice as a timer.

Third, dry off before you leave the room. This step is key because wet hands transfer more bacteria than dry ones.

"If your hands are still wet, you go to touch that door of the bathroom, having your wet hand might actually help transfer bacteria," Schaffner said. He'll even dry his palms on his pants if there's no paper towel around.

Despite all the evidence demonstrating the health benefits of regular hand washing, Schaffner knows his advice can only go so far.

"I'm not in charge of you washing your hands, just because I'm a guy who did some science and did some research on hand-washing," he said. "You do what you want."

This article was originally published by Business Insider.

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