A quick scan of the office returning to work after the holidays yielded some interesting findings: enough nifty new lunch containers to inspire some serious meal-planning, and a smattering of shiny new water bottles on many a co-worker's clean and decluttered desks.

Everyone was looking so well-rested and hydrated, so high on being both environmentally-friendly and budget-minded. It got me thinking (as I waited in line to refill my own water glass): How often are these beautiful new reusable water bottles being washed?

I'm not here to point any fingers, but I did see those same bottles sit on desks for days on end, making me wonder whether any potential deleterious side effects could be lurking. I took my curiosity to the professionals to find out the proper cleaning frequency.

"Ideally once per day," according to Dr. Brian Chow, an infectious disease physician at Tufts Medical Center. "Or have a few bottles that you rotate through, and clean them all at once." He goes on to say that most of the bacteria found on the bottles come from us. "They are bacteria that live in our mouth and throat that our bodies know, and they don’t make us sick. However, if you share bottles with someone else, they may not be used to your bacteria or viruses. The germs that cause strep throat, mononucleosis, colds and the flu, and even bacterial meningitis can be spread by sharing bottles."

Sharing definitely does not mean caring in this case. Got it! But how bad is it really, not to wash these vessels on the daily? "In most cases, people with healthy immune systems will be okay going a day or two between washing bottles," says Dr. Chow, quickly alleviating my concerns. "However, all year round there are germs that can be transferred from other things we touch—door handles, light switches, faucets—to our hands, and then to the bottle, which we then drink out of. A quick wash and scrub at the end of the day and letting the bottle dry overnight is an easy step to keep you healthy while you stay hydrated."

Sounds like completely sound and reasonable advice. Knowing what I know about our readership's preferences for specific water temperatures when washing dishes, you can bet there was a follow-up question. "Water that is hot to the touch is best," explains Dr. Chow. "While it may not kill all the germs directly, it does help dissolve the residue that allow germs to live on plastic and metal. Using soapy water and a scrub brush are as important (this removes the dirt and grime), and also dislodges the germs if they are stuck to the bottle."

It goes without saying, too, to check your bottle manufacturer's specified instructions as to proper cleaning methods. (Psst! Make sure it is, indeed, dishwasher-safe if you've been throwing yours in for a cycle.) I'd also suggest more frequent or thorough cleanings if your water bottle has a special spout with more intricate crevices.