The question: If it's just filled with water, what's so bad about not washing my water bottle?

The answer: If you have a bottle that you use every day for water-drinking purposes, congratulations! We're all about hydration here at Healthy Living. But here's a question for you: When's the last time you actually washed that water bottle? After all, if it's just filled with water, it's not actually dirty, right?

Not exactly, especially if you're using a disposable water bottle that isn't really meant to be used more than once. In an article in a 2007 issue of the journal Practical Gastroenterology, experts pointed out that commercial bottled water manufacturers don't recommend that consumers reuse their disposable bottles. That's because "everyday wear and tear from repeated washings and reuse can lead to physical breakdown of the plastic, such as visible thinning or cracks. Bacteria can harbor in the cracks, posing a health risk," they wrote. In addition, "reuse of plastic water bottles can lead to bacterial contamination unless washed regularly," which entails washing the bottle with mild soap, rinsing it well (but not with extremely hot water) and making sure there is no “physical breakdown prior to use.”

Even reusable plastic water bottles could hold bacterial contamination risks, if you don't wash them or reuse them despite “visual evidence of wear and tear,” according to the article. “Bacteria that may settle in the cracks and scratches of the bottle appear to pose a greater health risk than the possibility of chemicals leaching from the plastic during daily risk."

And water bottles sure can be a haven for those bacteria. In a 2002 study published in the Canadian Journal of Public Health, researchers from the University of Calgary took 76 samples of water from water bottles of elementary school students; some of the bottles were reused for months on end without being washed. They found that nearly two-thirds of the samples had bacterial levels that exceeded that of drinking water guidelines, which may have been the result of "the effect of bacterial regrowth in bottles that have remained at room temperature for an extended period," researchers wrote in the study.

While the researchers did not examine the exact source of the contamination, "the most likely source of enteric bacteria found in the students' water bottles is the hands of the students themselves," according to the study. "Inadequate and improper hand washing after students have used the bathroom facilities could result in fecal coliforms in the classroom area."

Plus, unwashed bottles provide the perfect breeding ground for bacteria, notes Cathy Ryan, one of the researchers on the study and a professor of geoscience at the University of Calgary. She told HuffPost that "bacteria will grow if they have the right conditions," such as the nutrients from "backwash," moisture and the right temperature. "Unwashed bottles have all of these things," she says.

In a more casual (and not peer-reviewed) test, news station KLTV examined bacteria levels in water bottles that were used for a week without being washed. Bacterial cultures were taken from the necks of the bottles and the part that goes in a person's mouth. The results? "All of those grew lots and lots of bacteria that could make you very sick almost like having food poisoning," Richard Wallace, M.D., of the University of Texas Health Center, told KLTV. "That can cause nausea, vomiting, diarrhea. Basically the worst vomiting you have ever had in your life."

Surely your inner germaphobe is thinking, "No problem, I'll just pop all my water bottles in the dishwasher and that'll take care of that." While the "impact of dishwashing or washing in hot (say 120-degree household hot water) should be minor on the chemical structure of most plastics intended as being 'dishwasher safe,'" disposable bottles "are intended to be used [one] time and then disposed, not reused," says Scott Belcher, Ph.D., a professor of pharmacology at the University of Cincinnati, who has conducted research on the release of endocrine disruptor bisphenol A (BPA) from different kinds of water bottles. "Heating will certainly increase the rate at which chemicals can migrate from the plastic," he says.

Of course, we're not saying that you should never reuse a water bottle (after all, we only have one Earth, and we need to take care of it). But you can be strategic about what kinds of water bottles you buy and reuse, Belcher notes. He recommends glass bottles with protective frames, as well as stainless steel bottles. "If you need a plastic bottle, I would recommend a polypropylene bottle, typically a white plastic," he tells HuffPost. "These are the types of non-reactive plastic bottles we often use in the lab," though he notes it's impossible to be sure what plasticizers or other additives may have been used in the manufacturing process. And even if you do opt to use one of these kinds of bottles, remember that it's still important to keep them clean to minimize bacterial contamination (including washing them and letting them dry before using them again and again).

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