At this point in your life, chances are good you’ve mastered basic potty training skills. But there’s one golden rule we don’t all follow in the bathroom: Closing the lid before we flush the toilet.

Maybe you’re a little bit lazy, or maybe you don’t think flushing with the lid down is a big whoop. But have you ever thought about what reallyhappens when you pull that lever with the lid wide open?

“You get a good spray out of the toilet area,” explains Charles P. Gerba, a professor of microbiology at the University of Arizona. “When droplets come out of the toilet, it looks like the Fourth of July.”

It’s something called “the aerosol effect”: Instead of colorful sparklers erupting in the sky above you, you’re showered with fecal bacteria and viruses. (Keep reading. Please.)

The aerosol effect, in disgusting detail

Fun fact: The average person flushes the toilet five to six times each day, adding up to nearly 2,000 flushes per year, says Doyle James, president of Mr. Rooter Plumbing, a Neighborly company. With all that flushing, you might think toilet bowl germs are regularly getting swept to the sewers.

We hate to break it to you, but they’re not. Many different types of germs stay in the toilet bowl after you flush. After all, the average human stoolweighs just under a quarter of a pound (sorry, McDonald’s) and contains a trillion bacteria. You could flush multiple times and never get them all out. They simply glom onto the porcelain interior of the bowl.

So each time you flush, “an aerosol is created due to the rush of water into the bowl,” explains Jason “The Germ Guy” Tetro, a visiting scientist at the University of Guelph and author of “The Germ Files: The Surprising Way Microbes Can Improve Health and Life (and How to Protect Yourself From the Bad Ones).”

“When this happens, any microbes deposited into said toilet may be sent into the surrounding environment.”

What’s grosser? Bacteria can rise up to 10 inches into the air—and will still be there an hour and a half later, according to research from the Leeds Teaching Hospital Trust.

If all this information about irresponsible flushing isn’t making your stomach turn yet, try this on for size: You could actually get physically ill from keeping the lid up.

“Although most of the time the risk is low for becoming sick, if the pathogen happens to be norovirus—which can cause infection in very, very low amounts—then you may have a problem,” Tetro says. (Norovirus is a highly contagious virus whose hallmark unpleasant symptoms include diarrhea, vomiting, and stomach pain that can last up to three days.)

Do low-flow toilets reduce spray?

“Whew,” you may be thinking. “Good thing I have a low-flow toilet.” Because surely less water equals less spray, right?

Nope. Scientists have already asked that question—and answered it. Low-flow or no, the aerosol effect is still firmly in effect. Hundreds to thousands of little droplets are still spewed into the air.

“So, even though you may be saving energy [with a low-flow model], best to expend a little arm energy and cover the toilet with the lid before you flush,” Tetro says.

And in case you were wondering, not even bidets are off the hook.

“There isn’t contamination per se, as you might see in a freshly deposited toilet,” Tetro says. But the inner compartments and spray of a bidet can have what’s known as a biofilm, a community of bacteria. And that can be sent out in the spray.

As you might expect, “this can end up sending bacteria all over the place,” Tetro adds, “including your gluteal cleft, which most people might simply call the ‘bum crack.’”

Bottom line (we won’t bother excusing the pun): No matter if you’re using a regular tank toilet, a low flush, or a bidet, make sure you keep the lid down and/or the nozzle cleaner than clean.

When you’re not down with ‘lid down’

If “lid down” isn’t your normal routine or you fear it’ll take a while before muscle memory kicks in, here are some other actions you can take to spare yourself from germs spewing everywhere.

  • Move your toothbrush. And we mean far, far away. Your toothbrush should be at least 3 feet away, or youll be brushing your teeth with whatever was in the toilet, Gerba says.
  • Clean your bowl. A “drop-in” cleaner that constantly cleans your toilet bowl can cut back on the amount of germs that spew toward you during a flush. The downside? They can damage the rubber flaps and other mechanisms inside the tank that are necessary to flush your toilet when you pull the handle, James says. A better bet is to periodically apply an in-bowl solution. Then, use a porcelain-safe household cleaner to disinfect the tank, seat, and handle.
  • Be extra cautious when you flush in public. “Valve-type” toilets like you often find in public restrooms are the worst offenders when it comes to spray. “I always flush and run,” Gerba admits. And to play it safe, use your foot rather than your hand when you press down the lever.
  • Wash your hands. You hear it all the time, but it bears repeating. Because otherwise, you’re part of the germ-spreading problem.
  • Clean around the toilet. The nasty little germs sent into the air after a toilet flush dont levitate for infinity. They eventually find their way to the floor, where you walk all over them. “There are about 2 million bacteria per square inch on the average public restroom floor,” James says. “You have more control over the bathroom floor in your own house, so clean it often.”
Stephanie Booth’s stories have appeared in magazines such as Real Simple, Cosmopolitan, Glamour, and Psychology Today.