It's not a good idea to leave plastic water bottles in a vehicle that is parked in the hot sun. Heat and bright light can cause plastic residues to leach into the water, which becomes yet another source of toxicity that our bodies have to deal with if imbibed. But now firefighters are warning of another potential danger from the clear plastic bottles. Unless empty, they can act as a magnifying glass that could leave you with a smoldering wreck, instead of just a slightly higher plastic load in your body.
According to news channel KFOR, the Midwest City Fire Department in Oklahoma ran some tests that showed sunlight magnified by a water bottle can reach up to 250 degrees Fahrenheit. The light comes through the bottle and the water inside creates a focused beam of heat, just like the classic trope of a kid burning ants on the sidewalk with a magnifying glass. Though the risk is pretty low, there can be enough heat produced to cause combustion of materials in the vehicle.
In 2017, a technician employed by the Idaho Power Company noticed smoke coming from his truck while on lunch break. A bottle of water left in the truck was refracting the intense sunlight from beneath the center console. The smoke was from the seat, which was beginning to smolder. Luckily, he caught it before a real fire broke out, but he was left with two burn marks on the seat.
This phenomenon could actually save your life, however, in an emergency situation. If you are stranded in the woods with a some bottled water, this technique can be used to start a fire for warmth or as a signal, as demonstrated in the following video.