Most of us don’t think twice about refilling our plastic water bottles. After all, it’s all in the name of personal hydration and it’s eco-friendly! And there’s absolutely nothing harmful about a simple bottle of water, right?

Wrong! That plastic water bottle could actually do your body more harm than good, experts say. Why? You can thank Bisphenol A (commonly known as BPA), a chemical used to manufacture plastics, for your water woes. This harmful chemical can leach into the water and quickly grow dangerous bacteria in the bottle’s cracks—that’s one of the reasons you should stay away from straws, too—and the health consequences are pretty serious.

We spoke to Kent Atherton, CEO of PuriBloc technology, about the risks of reusing plastics. “Sadly, many people buying plastic water bottles do so because they believe they are making a healthy choice when the opposite is more likely to be true,” said Atherton. “Even BPA free products are not safe since manufacturers are now substituting other estrogenic chemicals, not as widely known, which may pose the same danger to human health. ”

These estrogenic chemicals can have a negative effect on human beings’ hormonal balances, but the potential dangers of plastic water bottles don’t stop there. In a study of 259 plastic water bottles at the State University of New York at Fredonia, scientists found that 93% of the surveyed bottles had some form of microplastic contamination. Additionally, single-use plastic bottles are mostly made of polyethylene terephthalate, or PET, which is safe to use, but not reuse; these plastics can leach chemicals into your water if heated or scratched.

There’s also the bacterial factor to consider. “The thing about water bottles is that, like all beverage containers, they come into contact with our mouth and hands—which are home to a lot of germs,” says Professor Stephanie Liberatore in the academic journal The Science Teacher. “Their openings are small, which makes them difficult to clean. This, combined with their moist environment, can make water bottles a bacterial breeding ground.”

To hydrate without harm, smart drinkers should avoid re-using disposable bottles. Instead, you should recycle them after drinking up once; or, better yet, invest in a BPA-free plastic bottle or one made from glass or stainless steel. Not only will doing so benefit your health, but you can help the environment, too. If you need another reason to pass on the plastic, here’s the scary reason why water bottles have an expiration date.