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When temperatures rise, first responders want to warn people about leaving plastic water bottles in their cars.

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The water bottles pose a fire hazard when left in direct sunlight in a vehicle, Manlius, New York, firefighter Jay Symonds told WSTM-TV.

“It would need to be a full water bottle that is filled to the top of the water,” Symonds said. “The water and the plastic create a magnifying effect and could essentially set upholstery on fire or a piece of plastic on fire,” he said.

Symonds said the danger of fire with plastic and upholstery is more pronounced in certain conditions.

“The ignition temperatures of those items are about 400 degrees, so it would take some time to achieve that, and you need low humidity, dry heat, but it is possible that it would start a fire,” Symonds said.

Experts say it’s better to keep bottled water in a car inside a cooler or the trunk.

Another concern about heating up plastic water bottles in hot places, such as cars, is the possibility of chemical interactions between the plastic and the water.

Bottle-makers contend there is no danger of this happening, but several studies over the years have shown certain chemicals in some plastic bottles can leach into liquids under extremely hot conditions.