A new study suggests that time in the sun could release harmful chemicals in plastic bottles.

Emptying a plastic water bottle.
Emptying a plastic water bottle.
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Next time you find a warm water bottle in your car—don't take a sip.

A new University of Florida study looked at 16 brands of bottled water, and the findings might throw a wrench in current FDA recommendations surrounding bisphenol A (or BPA) levels in plastic water bottles. While the current low levels in the plastic have been deemed safe, the researchers for this latest study watched these supposedly harmless levels grow over a four-week period when left in 158-degree heat. Even the FDA has warned to keep hot or boiling liquids out of packaging containing traces of BPA, due to its reactivity with heat.

Although this extreme heat is a "worst case scenario," drivers can certainly relate to finding a lost water bottle on the floor of the car and quenching their thirst, no matter how long the bottle has lived there. If you're parked in the sun on a hot summer day, the car's internal temperature can reach between 131 and 172 degrees Fahrenheit.

Plastic water bottles are made from polyethylene terephthalate, a material that's used for a lot of food and beverage packaging since it's lightweight, durable, and shatterproof. However, when heated, it is known to release the chemical BPA, which some experts say can affect hormone levels by mimicking estrogen, and may trigger health risks if exposed at high levels. While the initial levels of BPA found in the 16 brands—save for one—did not exceed EPA standards for these chemicals, scientists are more concerned about how the chemicals increased over time.

"If you store water long enough, there may be a concern," study leader Lena Ma said in a press release.

So if you see a bottle lying forgotten in your car, use it to water your plants—an alternative that's safe and green.