Risk of fire when leaving water bottles in the car under the sun

The Idaho State Department of Electricity shared a video of a water bottle burning two holes in a car seat last summer. During this year's World Cup, the Russian company Holy Water sold a ball-shaped water bottle that could act as a perfect lens for focusing light. In a video shared by Fontanka Ru on YouTube in late May, the ball-shaped water bottle lit a matchbox and burned a hole in a wooden floor, according toLive Science.

Light consists of countless photons, subatomic particles that travel in straight lines, says Odile Madden, a materials scientist at the Getty Conservation Institute in Los Angeles. Eyeglasses and microscopes guide photons so they converge at a single point. “You can use that property to see more clearly or to focus. Or you can concentrate a lot of light into a very small point with all its energy to melt and burn,” Madden explains.

Madden witnessed the incredible power of focused laser light years ago when she tried using a laser to remove clear adhesive from artworks and artifacts. She found that the adhesive focused the laser beam, generating energy and burning holes in a glass slide under a microscope. Madden published her research in 2005 in the journal Laser Applications in Art Conservation.

What surprised Madden about the plastic water bottle incident was that the light that passed through the car window still contained enough heat to penetrate the bottle and cause a fire. "The accident could have been worse on a hot day or with the windows down, because the windows filter out some of the light," Madden speculated.

“This is a good example of solar energy,” said Michael Doutre, a chemist at the Getty Conservation Institute and a colleague of Madden’s. “We thought it was just a cheap water bottle, but we accidentally created an almost optically perfect shape for the lens.”

According to Doutre, even when shining through a car window, sunlight hits the car seat with 600 watts/m², equivalent to the energy from a small room heater, but focused on a tiny spot less than a millimeter. Just two seconds of focused light is enough to heat a vinyl seat cover to breaking temperature and start a fire, Doutre explained.

But both scientists said the likelihood of a water bottle causing a car seat to catch fire is low because the materials in cars, including the seats, are often made of flame-retardant and self-extinguishing materials. The plastic materials in many car seats are made of polyvinyl chloride, or PVC, which is a poor conductor of fire.