How disgusting is your water bottle, really?

It takes a lot of grime for a reusable water bottle to look dirty. After all, it's usually just water in there. Bad news, though: If you're not giving your Hydro Flask a regular scrub with a bottle brush, there might be a bacterial storm a-brewing inside its walls that could potentially get you sick.

Buying a reusable bottle, whether it's made of stainless steel, glass, or a thick plastic, is a good investment. Not only will you save money by never buying bottled water, but you'll also avoid contributing to the nearly 50 million water bottles Americans purchase per year. (Less than 30 percent of these end up recycled.) Still, it's important to make sure you are dumping water into your body from a clean vessel. And yours is probably not that clean.

Is my water bottle gross?

If you're just cleaning the parts of the bottle that your mouth touches, then yes, your water bottle is gross. According to Dr. Philip Tierno, clinical professor of pathology and microbiology at NYU Langone Medical Center, it's imperative to scrub the inside of the bottle, too. Merely rinsing is not going to do the trick.

"Bacteria tend to form a biofilm on the inside of the reusable container over time," he said in an interview. "So you need mechanical action to get rid of that biofilm that coats the inside of the bottle." He compared the accumulation to barnacles on a boat, or the soap grime that builds up on a bathtub.

Ew, what's in a biofilm?

This biofilm likely contains various types of bacteria, from the materials that were already inside your mouth (backwash) to the other stuff that sneaks in when you bring your water bottle out into the world. It's this outside material that presents the highest risk of making you sick, Tierno explained.

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"Some people may [for example] carry strains of staph that other people don’t have. They may pick those up shaking hands with somebody, touching things like countertops, doorknobs, elevator buttons, telephones, computer keyboards," he said. "You’re constantly exchanging flora on your hands, and then you’re touching your water bottle." You're unscrewing it, capping it, scraping lipstick from the mouthpiece — basically, you're ensuring that whatever was on your hands is getting into that water.

Tierno gave a harrowing example: Someone who has norovirus touches a doorknob, then you touch the same doorknob. Then you use your hand, which now has norovirus on it, to flip up the rubber mouthpiece on your CamelBak. "You've now introduced norovirus into that mixture of flora," said Tierno. "The same [can happen] with other fecal-borne organisms, and that's what you really have to worry about."

Oh, god. So how do I make it not-gross?

Here's the good news: You can absolutely salvage your slimy Nalgene. If you clean it correctly, it'll likely still be safe for indefinite use. The key is to clean the entire thing — interior, exterior, and mouthpiece — with hot water, soap, and a bottle brush when applicable.

Bottles with wide, circular mouths tend to be easier to clean — there aren't any nooks and crannies where moisture can build up over time. If you simply must have a water bottle with a rubber mouthpiece (like a CamelBak bite valve), make sure you follow the brand's specific instructions for sanitizing.

It's also important to keep your bottle brush clean, just like you would any other sponge. Experts suggest running it through the dishwasher after every couple of uses and replacing it often. If you don't have a dishwasher, you can clean your bottle brush by soaking it in a large bowl of hot, soapy water and vinegar, then scraping off any residue with a clean fork or comb. In either case, you should replace the whole brush every few months or so.

Finally, it's probably smart to get in the habit of applying hand sanitizer before you touch the top of your water bottle, especially if you're in a germy place like an airplane, a doctor's office, or public transit.

What if I'm one of those people who refills the same Dasani bottle until it smells weird and mildew-y?

You should not do that. Please get a reusable water bottle. It will be cheaper in the end, and there is no good reason to lug around your week-old SmartWater receptacle.

But if you absolutely must fill up your single-use bottle with tap water one extra time, do your thing. Tierno explained that it's technically fine as long as you wash the bottle with soap and water in between uses.

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However, it's important to remember that single-use water bottles, per their name, are not designed to be used more than once. This is because it's easier for them to get dirty. When you touch the bottle continuously over multiple days, Tierno said, the surface becomes littered with "cracks and scratches and fissures that can actually house bacteria." (Single-use bottles have become thinner over the years in an effort to use less material, making them even more susceptible to such nicks.)

It's also worth noting that some types of plastic — including polyethylene terephthalate, which is used to make many one-use consumer plastic products — can leach chemicals if exposed to extreme conditions. "The longer you hold your bottle, the longer it's subjected to heat and light. That's where leaching can occur," Tierno said. "They're really meant for a one-time use and then a recycle."

Tierno also cited a 2003 study from the University of Calgary in which 75 elementary school student reused their own plastic water bottles over the course of several months. When the study concluded, the water bottles contained so much bacteria that a boil-water notice would have been required under normal circumstances. This is an extreme example, Tierno emphasized, but underscores the point that even if no one shares your bottle, outside bacteria are going to find their way in.

"You might say, well, I already have mouth flora and my own skin flora. But in reality, there are additional flora coming in ... and they have a chance to grow because of the nutrients from your backwash," he said.

Mmm, backwash and chemicals. Just what you want to think about as you do your best to stay hydrated.