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Phoenix666 writes:

Tap water that has been left to sit out slowly begins to acquire an off taste. Many assume that this is because of micro-organisms. Treated water's added chlorine will take care of small communities of these little guys, but at room temperature they begin to multiply rapidly and can really get the party started. Even with clean water and a clean glass; one sip introduces a host of germs to the mix, in addition to whatever the water may have picked up by ambient dust.

But that's not what makes old water taste stale. For that we can thank carbon dioxide. After about 12 hours tap water starts to go flat as carbon dioxide in the air starts to mix with the water in the glass, lowering its pH and giving it an off taste. But it's most-likely safe to drink.
...
As for plastic water bottles that have been left out in the sun or the car, step away from the bottle, warns Dr. Kellogg Schwab, director of the Johns Hopkins University Water Institute. "A chemical called bisphenol-A, or BPA, along with other things used to manufacture plastic can leach into your water if the bottle heats up or sits in the sun," he explains. BPA, as you likely know, is a hormone disruptor has tentatively been linked to everything from heart disease to cancer. Schwab also says that plastic used for bottled water isn't meant to be washed or refilled, so use only one time and recycle. Or way better, don't buy them at all; use a refillable water bottle instead.

I like to let mine sit out until it acquires malarial mosquito larvae.

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