You Should Never Leave Plastic Water Bottles in a Hot Car

Leaving plastic water bottles in your car during summer is a bad idea—and not just because chemicals from the material can leach into your water when it gets too warm out.

A plastic bottle of water can actually set your car seat on fire if sunlight hits it at just the right angle. If you don’t believe that, just ask Dioni Amuchastegui, a battery technician with the Idaho Power Company.

Amuchastegui was sitting in his truck during his lunch break when he saw some smoke out of the corner of his eye. He looked over “and noticed light was being refracted through a water bottle and was starting to catch the seat on fire,” Amuchastegui said in a Facebook video from 2017. He recorded the clip to warn others about the dangers of leaving plastic water bottles in hot cars.

He tested it again, and a thermometer held up to the bottle registered a temperature of 213°F. The Midwest City Fire Department in Oklahoma conducted its own test and concluded that the dangers are very real.

Why Does It Happen?

“Vinyl generally starts to burn at 455 degrees," David Richardson, of the fire department, told CBS News. “It wouldn’t take very long to start a fire if conditions were right—depends on how focused that beam of light is.”

Many people already avoid drinking from bottles left in cars—especially in the winter—due to a widely held belief that freezing or reusing plastic bottles can cause carcinogenic compounds to be released into the water. As Snopes reports, some of these claims are merely urban legends, but there may be some truth to the claim that heat can cause harmful phthalates (environmental contaminants) to leach into the water. The fact-checking site rated this claim “undetermined.”

Regardless, the potential for kindling a fire should provide some incentive to clean out your car and remove any bottles that were tossed haphazardly into the backseat.

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A version of this story was published in 2018 and has been updated for 2025.