It's the height of summer, and for many Americans, that means scorching cars and forgotten plastic water bottles rolling around under the seats.
But before you take a sip out of one of those, you might want to think twice, experts say.
High temperatures and prolonged sun exposure can create conditions that lead to both chemical leaching from plastic bottles and bacterial growth, especially if the bottle has already been opened. Experts warn this dual threat could pose a risk to your health.
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"The combination of microbial growth and chemical leaching from the plastic makes it a double-edged sword," said Dr. Paul Savage, a toxin expert and chief medical officer at MDLifespan in Chicago.
Many single-use plastic water bottles are made from a type of plastic called polyethylene terephthalate (PET).
When PET-containing bottles sit in the heat, the polymer chains in the plastic can start to break down and cause the release of various chemicals, including trace amounts of phenol, like bisphenol A (BPA) and phthalates, into the water.
"Phenols and phthalates are endocrine disruptors, meaning they interfere with the natural hormones in our body," said Nicole Deziel, an environmental epidemiologist at the Yale School of Public Health.
Although the U.S. Food and Drug Administration (FDA) has stated that the current levels of BPA and phthalates used in food packaging are safe, many of these substances have already been banned or voluntarily phased out.
Plastics can start leaching chemicals when temperatures reach 85 degrees.
And the process accelerates significantly at higher temperatures, especially above 100 degrees, Savage said.
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Even BPA-free bottles may not be a safer alternative.
Many of these products use chemical substitutes that, according to Savage, are structurally similar to BPA and may have similar endocrine-disrupting effects.
These alternatives haven't been studied extensively, leaving scientists cautious.
Another growing concern is the presence of microplastics - tiny plastic particles that can shed from the bottle over time, especially under heat.
Studies in animals have linked microplastic exposure to inflammation, cellular stress and hormonal disruption, though the full impact on human health remains unclear.
Plastic bottles reused after opening - especially those left in warm environments - can also become a breeding ground for bacteria, including E. coli and Staphylococcus aureus (staph infection), introduced from the mouth, hands or air.
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"While many of these bacteria are harmless in small amounts, heat can amplify their growth, potentially leading to harmful levels that cause gastrointestinal issues or infections," Savage told Fox News Digital.
Bacteria can thrive in microscopic grooves and scratches in plastic bottles, Savage said.
Studies have found that bacteria can grow in open bottles within 48 hours, even at room temperature.
Some concerns about heated plastic bottles have been exaggerated or misunderstood.
For example, claims that dioxins - known carcinogens - are released when plastic bottles are left in hot cars have been debunked by scientists.
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These rumors trace back in part to a viral 2006 email attributed to musician Sheryl Crow, a breast cancer survivor, who warned about cancer-causing chemicals from heated bottles. However, experts affirm that PET does not contain or produce dioxins.
To reduce exposure to potentially harmful chemicals and microbes, experts recommend avoiding the reuse of single-use plastic water bottles - especially in hot environments.
"Glass and stainless steel can withstand repeated heat and sun exposure," said Bryan Quoc Le, a California food scientist and author of the book "150 Food Science Questions Answered."
While bacteria can also grow in reusable containers, glass and stainless steel bottles are easier to clean thoroughly and can withstand high-temperature washing - "which can reduce microbial growth," Le added.
