Old water: drinkable or petri dish?
February 15, 2008 6:03 PM   Subscribe

How long can I leave a cup of tap water sitting out and not worry about what might be growing in it? What about an opened bottle of spring or carbonated water?

I'm not looking for a tap vs. bottled water debate. I want to know how long I can leave a cup of tap water sitting out without worrying about bacteria (or some other nasties)? A few hours, overnight, days, weeks? What if I open a sealed bottle of spring water or carbonated water, take a few sips, re-cap it, and leave it out unrefrigerated for a few days?

Assume tap water that is otherwise considered safe to drink (in the US).
posted by ellenaim to Food & Drink (19 answers total) 1 user marked this as a favorite
its dust that will make you not want to drink it before bacteria etc.
posted by Max Power at 6:04 PM on February 15, 2008

My pardons.
posted by Max Power at 6:05 PM on February 15, 2008

Don't know how long it takes, but it's the few sips that's the problem. Bacteria in the backwash will get into the water and be invited into a (soon) warm environment in which to thrive.
posted by hungrysquirrels at 6:17 PM on February 15, 2008

Seconding the backwash as your major problem. If your pour the water into another glass then the water remaining in the bottle will be good for weeks. Drinking directly will lower that time significantly.
posted by TrashyRambo at 7:02 PM on February 15, 2008

You could leave it for months until it was a putrid slimy stinking mess, drink it, and you would probably be ok. Even if you weren't, you would almost certainly only be ill for a short period, which I wouldn't consider dangerous, just unpleasant. Your digestive tract can take a lot in stride. So I'd consider that if you can't actually see anything growing in it, it's safe.

But I suspect you're talking a more mollycoddled threshold of "safe" :-)

Use the sniff test. Our noses have evolved to be able to detect the smell of common bacterial excretions long before the level of bacteria is dangerous. (Your nose isn't much help against pathogens like, say, bubonic plague, but that kind of thing shouldn't be in your tap-water to begin with). For your garden variety tap-water bacteria, in my experience you'll smell it long before it can have any noticeable effect. The bacteria will mostly be around the edges (ie where the water meets the cup or plastic, or where your mouth touched the cup), so sniff there.

As to actually answering your question, in my experience, you'll be able to smell something after a couple of days IF you've sipped it and thus provided the bacterial backwash that Hungrysquirrels mentions, otherwise, it can be good for much longer.
posted by -harlequin- at 7:04 PM on February 15, 2008

I regularly drink my water from 1 gallon jugs of spring water. I don't refrigerate them, ever. They start tasting bad after about a week or 4 days if they are near bright sunlight. I have never gotten sick from doing this.

A cup of tap water in a clean cup that has not been touched by someone's lips will evaporate long before it goes bad.

The limiting factor here isn't really if there are bacteria in the water, there always will be, it's if those bacteria have an energy source and amino acid source that allows them to reproduce and create a large enough population to effect the taste of the water.
posted by 517 at 8:27 PM on February 15, 2008

Pure water provides nothing for bacteria or other nasties to grow / thrive in. Generally, living things require an energy source -- either a sugar of some sort or sunlight to survive. It's unlikely you're going to start growing algae in there in the short term. Any backwash from drinking it probably won't find it a very hospitable environment. It's not going to kill the bacteria, but they won't reproduce very quickly.

As previous commenters have suggested, the bigger issues is environmental dust and any leachate from the container it's in (e.g. plastics, which make water taste funky quickly)
posted by printdevil at 9:30 PM on February 15, 2008

Best answer: This article is the closest thing that I could find to an answer. Apparently it comes from a science fair project that ended up as a published article. Unfortunately, it doesn't address safety, just bacterial colony counts at various times and storage temperatures after taking a sip from a bottle. There is a bit more detail here.

Also, I feel compelled to refute Harlequin's incorrect, and possibly risky, assertions regarding the ability to smell pathogenic bacteria in food. Smell is not a reliable indicator of food safety. Some of the more common and well-known sources of foodborne infection are Salmonella species, Campylobacter jejuni, Shigella species, Clostridium species, Staphylococcus aureus, and E. coli. Not one of these can be reliably detected by smell.

Salmonella "does not usually affect the taste, smell, or appearance of the food."

"It is impossible to tell from its appearance whether food is contaminated with campylobacter. It will look, smell and taste normal..."

Food contaminated with Shigella "may look and smell normal."

Food contaminated with Clostridum botulinum "may not look or smell spoiled."

Staphylococcus aureus "toxins may be present in dangerous amounts in foods that have no signs of spoilage, such as a bad smell."

"You can't tell whether a food is contaminated with E. coli by the way it looks, smells or tastes."

I hope that at least helps narrow down your methodology if you are planning to test this.
posted by nobodyyouknow at 10:00 PM on February 15, 2008 [2 favorites]

I've drunk water from rivers, lakes, ponds, springs, mountain tarns, open wells and 1000 year old glaciers. All have water sitting around much longer than a few days or weeks with all sorts of little living critters in them.
posted by JackFlash at 10:03 PM on February 15, 2008


I disagree. I stated the limits of the smell test (which you have re-stated), but also noted that such pathogens - including the ones you mention - should not (and in some cases could not) be present, according to the scenario described in the question:
"What if I open a sealed bottle of spring water or carbonated water, take a few sips, re-cap it, and leave it out unrefrigerated for a few days?"
. To successfully cultivate fecal contamination in that bottle involves a few steps missing from the scenario, and some require some pretty imaginative and unlikely scenarios to successfully contaminate that bottle.

Inciting worry about contaminants that do not have a plausible vector unless Asker has also done X, Y, and Z, only muddies things, IMO. That said, those are useful links, and I learned some things from them that I didn't know :)

Also, completely anecdotal (and food poisoning is often hard to trace anyway) but I'm not aware of anyone I know getting ill from bottled water. I also suspect that if I did know someone who got sick from bottled water, removing the bottled water from the scenario would not have prevented the illness - a contaminant would be in their environment and if not the bottled water as the vector to their body, something else would have done the same job just as well.
posted by -harlequin- at 10:38 PM on February 15, 2008

Also, that "two days" I suggested was for pure water without chlorine, so it would best apply to bottled water (though in my case, it was tap-water). Your tap-water will have chlorine, which would further retard growth.
posted by -harlequin- at 10:49 PM on February 15, 2008

In my experience, it takes about a week for the water in my Nalgene bottle to taste musty. It's never hurt me, but it's unpleasant. I assumed it was algae (not bacteria) that caused the change since it smells and tastes like lake water.
posted by cali at 11:01 PM on February 15, 2008

I buy a 500mL bottle of tea, or sometimes a sports drink, and then refill it with tap water and use it next to the computer. It doesn't get much, if any sunlight, and I drink from it quite often, so lots of backwash. My limiting factor is when I can see the threads on the cap or bottle changing, then it's time to pitch it and use another one I've got around. Or wash it. This is usually once a week or less.

Kinda gross, but it works and I've never gotten sick from it (that I know of).

Also, it's winter and my apartment is rather chilly, typically low teens Celsius.
posted by Jhoosier at 11:42 PM on February 15, 2008


I hope my comment wasn't "inciting worry," as you say. I was trying to address what I percieved to be a more general statement on your part about an evolved ability to smell pathogens or their by-products. Sometimes it is possible to smell them and sometimes it isn't, but it is not a reliable indicator of safety. My examples were intended to illustrate that, but, given the constraints of the question, I agree with you that most of those specific pathogens are unlikely to contaminate the water.

The only step missing from the fecal contamination scenario that you expressed doubt about is the sipper touching their perioral area with a dirty hand prior to sipping, or touching the top of the bottle or cup at any time. Given the statistics that I've read about hand-washing after bathroom use, I don't think that scenario is implausible.

Health Canada recommends refrigeration after a water container is opened in case harmful bacteria are introduced. This article seems to have a reasonable perspective, recommending washing, but suggesting that you are not likely to have issues with your own microbes. I can't find anything else that is a useful guide for how long the water is likely to be safe.
posted by nobodyyouknow at 1:03 AM on February 16, 2008

I once drank a glass of dustwater that had been sitting around for about two weeks (by mistake). It was absolutely vile, but after the initial ick factor, I didn't get sick. Just a data point.
posted by tiny crocodile at 7:54 AM on February 16, 2008

You're going to think that I'm making this up, but it's for real. I promise. Really. I actually leave a small glass of water out on the kitchen counter all the time. It's the only way that my cat will drink water. She refuses to drink out of a bowl. Maybe it's because she likes to drink without leaning down, I don't know. Anyhow her water sits out for days on end, and she never seems to get sick. I add water when it evaporates down about a 1/2 inch, or change the water if it looks yucky, and every once in awhile I get a clean glass for her.

She's ten years old, and she's been doing this for at least the last five years. She seems very healthy, although on semi-rare occasions she sneezes. The sneezes, unfortunately, usually seem to happen when she's sitting on my partner's lap pointed up his chest toward his face. I don't think it has anything to do with the water, more likely some mild allergy that she, in her feline mystery, is compelled to express near him. It's not that she's allergic to him. There's something in the environment here in Southeast Texas that didn't do it to her when we lived in California.

Anyhow, all I really wanted to say is that the her water is left out and it doesn't make her sick. If you wanted to get a cat or two and compare the health of those who drink "left-out" water and those who drink fresh water, that would make a great juinior-high science project. Be that as it may, my cat is doing fine.
posted by Robert Angelo at 2:07 PM on February 16, 2008

Everything I have to say is anecdotal here, not *science*. I pretty regularly will fill a glass (not a plastic cup) with water, take a sip out of it, and leave it on my bedside table overnight and then drink more of it in the morning. Occasionally, there will still be water in the glass and I will have a sip when I get home from work. A roommate I had used to find this insane but for fuck's sake people, it's water. I do it with glasses of water from the cooler at work, too.

I've been in situations where we did not have access to fresh water due to a severe winter storm, and also stored tap and bottled water for long stretches in preparation for a boil water alert after any of the Florida hurricanes I went through. Simply put, if you leave it sitting in a container for a day, week, whatever, you're generally not going to get sick. I often have a bottle of water on the go which I refill from the tap or the water fountains at the gym. I also used to keep one in the car, when I was in FL, and other than the water getting warm and gross from being in the hot car or taking on the plastic taste from the bottle, it never did me any harm. I have noticed what Jhoosier mentions with the threads on the mouth of the bottle being the indicator it's time to scrub out the bottle or recycle it and get a new one. I do this because I've lost about four Nalgenes in my life and it's not really worth buying another at this point.
posted by SassHat at 8:33 PM on February 16, 2008

When I was a kid I had a glass of tap water next to my bed in case I woke up in the night thirsty. It would sit for weeks or months. Eventually it would have visible dust in it and I would toss it, although on occasion I would wake and take a sip from water that had sat there for weeks. It didn't taste mountain spring pure, just sort of flat and not particularly palatable.
Considering the quality of water the western world has, and the poor quality many developing world people regularly drink, I would suggest you could leave water forever without health issues. As 517 said up thread, it would likely evaporate before anything bad would grow in it.
Try it, we'll know you died if you stop posting ;-)
posted by bystander at 3:01 AM on February 18, 2008

Last night I found a Nalgene that had been missing for some time. It was full, packed in a bag. I opened it, took a whiff, and drank about 10 ounces. It tasted fine. That was last night and I'm OK so far.

I can't quite recall when I would've packed the Nalgene in that bag. I think it was for an out-of-town meeting December 12.
posted by altcountryman at 2:39 PM on February 18, 2008

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