Plastic water bottle left in a hot car? Think twice before sipping from it

But before you take a sip out of one of those, you might want to think twice, experts say.

"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.

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.

Studies in animals have linked microplastic exposure to inflammation, cellular stress and hormonal disruption, though the full impact on human health remains unclear.

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.

To reduce exposure to potentially harmful chemicals and microbes, experts recommend avoiding the reuse of single-use plastic water bottles - especially in hot environments.

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.