I’ll admit, I’m a hypocrite. I get grossed out by the glasses of water on people’s bedsides (how do they drink it after leaving it there overnight?), but have no problem sipping on a bottle whose contents I haven’t changed for a day. 

I tell myself that I’m alright to do so, because my stainless steel container is lidded and promised not to release microplastics into my drink. 

But am I right? What’s the exact limit on drinking old water ― and does the container really matter? 

Here’s what the experts say...

Speaking to Self, microbiologist Dr Charles Gerba said that though old water is teeming with bacteria, you’re probably safe to sip it as the germs are all yours. 

“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,” he told the publication (oh, yum!). 

The FDA says that technically, bottled water can be stored forever if it’s made to industry standards and sealed properly ― though that may not be the case if you’re using a flask with, say, a straw attachment. 

Nonetheless, water reacts to carbon dioxide if left uncovered for about eight hours. That changes its pH and can leave the “off” taste so many of us hate.

What about plastic?

Water that’s bottled in plastic containers may also face leaching of plastics over time.

But studies suggest that those stay under the recommended limit as long as the water is kept cool ― hot water leaches more plastic, as anyone who’s taken a sip from a dashboard-bound bottle on a hot day has had the displeasure of learning.

Still, different brands set different shelf lives for their plastic-bound water depending on how they think the taste will change over time. 

Buxton says that while technically “water cannot go off because it does not contain proteins or sugars (which are broken down by microbes in foods which can go off)”, they add that opened bottles will take in too much CO2 to be enjoyable after about three days.

Given that most reusable water bottles aren’t sealed, the three-day limit might be a good guide for your plastic flask.

In other words, you’re safe to drink water from your bottle for a very, very long time so long as you don’t let any other germs from your hands or the outside world into the bottle ― but expect the taste from uncovered containers to turn after about eight hours, and don’t expect much from the flavour after about three days.