During a road trip, you take a big swig of a water bottle you bought at the last gas station. Despite the label's promise of a natural taste, a chemical flavor bathes your tongue. You examine the bottle and learn its sell-by date was a month ago.

You read that right: bottled water can have an expiration date. This date warns you when leached chemicals will begin to affect the water's taste. It doesn't tell you anything about the level of toxic microplastics in your drink.

The exact shelf life depends on the brand. A Fiji bottle can last two years, while those bulk packages of Nestle Pure Life bottles have a shelf life of only three months.

Before you turn the car around and confront the gas station clerk, you should know it's perfectly legal in America to sell water bottles past their best-by date. For the last 21 years, the FDA's stance has been that bottled water can last indefinitely if you store it properly.

However, improperly stored water can not only taste funky, but can also grow toxic or contaminated over time. It's important to know the difference between water that tastes "off" and water that could potentially make you sick.

Chemicals leach into bottled water over time

When bottled water grows stale, it's often because the bottle has begun to deteriorate, shedding microscopic chemicals into it.

These compounds may make your water taste like medicine, chlorine, or ozone, says Bryan Quoc Le, food chemist and author of 150 Food Science Questions Answered.

Every bottle sheds chemicals a little bit, just by virtue of being in contact with water. Some of these chemicals are more toxic than others, such as antimony, a chemical that can damage your stomach and intestines, and phthalate esters, which can disrupt your endocrine system.

Glass bottles release less antimony and phthalate esters than plastic ones.

But most water bottles are made from polyethylene terephthalate (PET) plastic, which tends to release the most antimony out of all common bottle materials.

But even PET bottles shouldn't release enough chemicals to make the water inside toxic. Over time, the water and the bottle reach a chemical equilibrium, so only trace amounts of the compounds are present in your drink at any one moment.

Where you store your water can also impact your water's taste. According to the International Bottled Water Association, plastic containers are slightly permeable, meaning air molecules may travel in and out of the bottle.

If you store bottled water next to, say, a bucket of house paint or drain cleaner, vapors from those household solvents could sneak into the bottle and alter the flavor inside.

Studies so far say the amount of chemicals in expired water is typically below limits set by the FDA. At least, so long as you've stored the bottle in a cool, dark place.

Hot water, you may remember from high school chemistry class, can dissolve larger amounts of chemicals than room-temperature water.

Research shows PET bottles heated past 86 degrees Fahrenheit can release an excessive amount of phthalate esters. Bottles heated past 140 degrees Fahrenheit can release dangerous levels of antimony.

So while an expired water bottle from the fridge may be safe to drink, the bottle you forgot in your hot car trunk or on a sunny shelf probably isn't.

Harmful microbes can contaminate bottled water in rare cases

On rare occasions, the stale taste of expired water could come from microorganisms. While bottled water companies go through rigorous processes to keep their products sanitary, accidents can happen.

For example, in April 2016, some business offices in Spain received bottled spring water contaminated with norovirus. They put the tainted spring water in their office coolers, and over 4,000 people developed gastroenteritis symptoms like vomiting and fever.

In addition to viruses, yeasts, molds, and bacteria can also slip into the bottle, often during the packaging process or transport.

If you drink the bottle right away, the microbes probably won't have time to reproduce to any great extent.

But if you leave a bottle in sunlight for an extended period, you'll create a warm, stable environment for them to multiply.

Quoc Le says contaminated water can have musty, moldy, marshy, sour, or rancid flavors. You may also see a light, slimy film near the bottle's rim, since that's the main entry point for microbes.

If your water tastes like swamp, it could make you sick. How sick depends on your immune system and which microbes are in the water.

While some microbes may give you fever or upset stomach, others could give you severe diarrhea that's enough to send you to the hospital.

What to do with an expired water bottle

If you're a frugal or environmentally-conscious person, you may wonder how to salvage your expired bottled water.

While you may be tempted to give expired water to your plants or pets, your idea may backfire, as the same contaminants that put your health at risk could also make them ill. They need clean water, too.

Quoc Le says you can filter your water through an activated charcoal filter, which should remove many of the contaminants.

If you think microbes have infested your water, boiling it for one minute should kill off almost all pathogens.

Even if your bottle is fresh out of the factory, experts don't recommend reusing it unless the packaging explicitly gives the okay.

Germs from your fingers and mouth can colonize the container, growing into a health hazard by the time you decide to refill the bottle.

Instead, you can simply throw the bottle in the recycling bin when you're done – no need to rinse it out or scrape off the label.

The IWBA says glass and plastic bottles are both 100 percent recyclable. In fact, your bottle will likely be used to help create new bottles, which you can then purchase all over again – hopefully before the expiration date this time.

This article was originally published by Business Insider.

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