I used to refill plastic water bottles like it was my job. That is, until I learned that the seemingly harmless habit is actually, well … pretty gross.

Germs are among us

Plastic water bottles aren’t meant to be used more than once. Rather, they’re meant to be recycled after drinking the original water they contain. A 2007 article in the journal Practical Gastroenterology noted that manufacturers of commercial bottled water don’t recommend consumers reuse the bottles. This is because the plastic on disposable bottles is easily broken down and thinned, making it easy for bacteria to grow in subtle cracks.

The researchers said that “bacteria can harbor in the cracks [of disposable bottles], posing a health risk. Reuse of plastic water bottles can lead to bacterial contamination unless washed regularly.” And who honestly washes their plastic water bottles?

Even if you’re adamant that you do indeed clean your disposable plastic bottles before refilling them, you still might not be preventing bacteria growth. To properly rinse your bottle, you’d have to rinse it thoroughly with soap, check to make sure there’s no thinning or tearing of the plastic, and make sure the water you’re using to wash it isn’t hot.

That’s because hot water increases leakage risk of Bisphenol A (commonly known as BPA), a chemical used to manufacture polycarbonate plastics. BPA consumption has been linked to a wide range of health issues, including endocrine disorders and certain types of cancers.

So what should you do?

Of course, if you refill your disposable plastic bottle every now and then, don’t freak out. It’s not the end of the world. Just try not to make it a habit. Instead, experts say that investing in a reusable plastic or stainless steel water bottle is the way to go when it comes to your hydration and health. You still have to wash that bad boy, though. Here’s how to clean it effectively, because I, for one, was definitely not doing it right.

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Remembering: Katherine Johnson