Carrying a reusable water bottle is an economical and eco-friendly choice. By not lugging around one-time-use plastic bottles, you're reducing waste headed to landfills and saving money on water that flows freely from your tap. (Unless, of course, you own a home and have to pay the water bill.) Whether they're made of glass, have a filter, or come insulated, the bottles carry clear advantages over crates of packaged water.

As beneficial as these bottles are, they tend to lull consumers into a sense of false security. Since only water goes in them, there's a pervasive feeling that they don't need to be washed often. Some users may not even wash them at all. As Reader's Digest contributor Lisa Marie Conklin points out, that's not a good idea.

Conklin spoke with microbiologist Miryam Z. Wahrman, Ph.D., a professor of biology at William Paterson University, to get some insight on why washing reusable bottles should be a habit. For one thing, Wahrman notes, people sipping from the nozzle frequently have traces of food in their mouth that can migrate to the bottle and the remaining water inside. Any germs in or around your mouth can also find their way into the supply. The next time you drink—an hour or a day later—you're consuming that potentially unfriendly bacteria.

Those microbes can begin to proliferate in stagnant water, especially if it's left inside a hot car, in front of a sun-exposed window, or other places where a warm environment can contribute to their growth. Bottles can also pick up all the same germs transmitted by your hands during a typical day.

To avoid contamination, it's best to wash your bottle daily with soap and water. Michigan State University advises filling the bottle with water and dish soap and letting it soak for several minutes. It's also a good idea to empty the bottle if you're not going to be using it for extended periods.

[h/t Reader's Digest]