Is it OK to drink the old water in your water bottle?

If you keep a BPA-free, reusable bottle on your desk to help you drink more water every day without using up a million paper cups, we here at SELF applaud you. If you find yourself drinking water out of it that’s been sitting there since yesterday—or last week, or longer—we here at SELF say, yeah, we do that, too.

Which got us to wondering… Should we NOT be doing that? We called Charles Gerba, Ph.D., a microbiologist at the University of Arizona and one of the world’s foremost authorities on germs, to find out if we’ve been drinking filth—or whether we can keep being kind of gross and lazy and still call ourselves health conscious.

“It’s pretty much just your own germs [on and in your bottle], so there’s not much to worry about,” Gerba said. “Yeah, you’ll get a lot of bacteria because there’s always wash-back, but basically it’s the bacteria that’s in your mouth anyway, so we’ve never really seen it as an issue.”

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Your mouth is veritably teeming with bacteria, but it’s your bacteria, so it’s all good. The potential for problems arises when you’re sharing a water bottle with someone else, Gerba said. Then you get their germs, which could make you sick. (But of course it’s not a given that you’re going to get sick just from swapping a little spit—it’s not like we’re going to tell you to also stop sharing apps at dinner or making out.)

Really, the bigger issue is if you have a water bottle that requires you to touch the drinking surface with your fingers in order to open and close it.

“Those are the only ones you see issues with because people’s fingers are contaminated,” Gerba explained.

In his lab, it’s on this kind of sports bottle—the kind that requires you to pull up on the top to open it and push down with your thumb to close it—where nasty bacteria like E. coli tend to show up.

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If your bottle has a screw cap, you don’t need to worry. And even if things get a little funky in the threading of the cap, Gerba said it’s probably just some encrusted saliva, bacteria from your mouth, mildew and maybe a little mold, which is no big deal in his book. “Unless you’re rolling it in dirt, it’s not much of an issue for most healthy people.”

Nor does it concern him if you leave the cap off completely and let your water sit out uncovered. What’s a little dust when your bottle is crawling with bacteria anyway?

Now, if you have a bottle where the drinking surface is exposed and you keep it rolling around loose in your backpack or gym bag, that could open you up to possible contamination, Gerba allowed, especially if you also carry food in there from time to time. But even that doesn’t seem to bother him too much.

So, basically, don’t put your grimy hands all over where you put your mouth, and don’t let everyone in your office sip from your water bottle and you’ll probably be a-ok. We’ll drink to that.

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