As temperatures warm and passengers wish to remain hydrated, they’re bringing an unexpected fire hazard into their vehicle: water bottles. Firefighters are warning that the practice could bring about dangerous consequences. As bright sunlight passes through a water-filled bottle, it can magnify the intensity of the sunlight passing through it, igniting any flammable surface in your car. Whether this is upholstery or paper, the danger is real, firefighters warn.
In a test conducted by Oklahoma’s Midwest City Fire Department, sunlight magnified by a water bottle reached 250 degrees “The sunlight will come through, when it’s filled with liquid, and act as a magnifying glass as you would with regular optics,” said MCFD’s David Richardson to local news media. “It uses the liquid and the clear material to develop a focused beam and sure enough, it can actually cause a fire, a combustion,” Richardson explained.
A battery technician with the Idaho Power Company discovered this danger first-hand; he shared the results in a video on Facebook that has since gone viral. While he was on a lunch break, he noticed smoke coming from beneath the center console of his truck. “I looked over and noticed light was being refracted through a water bottle and starting to catch the seat on fire,” said Dioni Amuchastegui in a video shared on the company’s Facebook page.
Experts say you can still safely bring a bottle of water into your car though: the trick is to make sure sunlight doesn’t come in contact with it. When stored in a trunk, under a seat, or in a glovebox, the water bottle won’t have a chance to magnify the sunlight entering the car. The risk of a water bottle starting a fire in a vehicle is low, but firefighters want you to reduce the risk to zero.
Sunlight on its own can be dangerous inside a car. The sun’s shortwave radiation heats objects that it strikes. For example, a dark dashboard or seat can easily reach temperatures in the range of 180 to over 200°F. These objects, which can include the dashboard, steering wheel, or child seat heat the adjacent air by conduction and convection and also give off longwave radiation which is very efficient at warming the air trapped inside a vehicle. In a matter of minutes, the combination of shortwave and longwave radiation can heat a car to a deadly hot reading. While experts warn not to leave children or pets behind in cars in the sun, deaths occur every year. In 2017, a mother was charged with intentionally leaving her children in a sunlit car to teach them a lesson; they eventually died.