David Richardson, of the Midwest Fire Department in Oklahoma, told KFOR the sunlight “uses the liquid and the clear material to develop a focused beam, and sure enough, it can actually cause a fire.” Richardson goes on to say: “The sunlight will come through (the bottle) when it’s filled with liquid and act as a magnifying glass as you would with regular optics.” A test at the fire department, showed sunlight streaming through a water bottle that was inside of a car raised the temperature of a piece of paper to 250 degrees.
Representatives from Idaho Power showed the same potential problem in a viral Facebook post last year. In their video, sunlight shining through a water bottle left smoke and burn marks in the car seats before the bottle was removed and a true fire was started. There are a number of factors that come into play here. Many people have tinted windows and use the cup holders in the door – both of these combined would lower the risk of fire. Firefighters also say the risk is minimal but there is still potential for a fire to start so it’s better to not leave water bottles in your car for hours.