Firefighters are warning folks: don’t leave water bottles in your car. Here’s why

Firefighters around the country are issuing a surprising warning that may catch many folks off guard: do not leave water bottles in your car, especially on sunny days. While it might seem harmless and convenient to keep a bottle of water within reach for hydration, it turns out this common practice could potentially pose a significant fire hazard.

At first glance, a plastic water bottle appears to be among the least likely objects to cause a fire. However, firefighters and safety experts have discovered that these seemingly benign items can become unexpected fire starters under the right conditions. The combination of sunlight, a clear plastic bottle, and a car’s interior can create the perfect environment for a fire to ignite.

The danger lies in the ability of a plastic water bottle, filled with liquid, to act like a lens. When sunlight passes through a bottle filled with water, it converges at a single point, similar to how a magnifying glass can focus light. If the focused light reaches a flammable surface—such as upholstery, paper, or plastic within the car—it can generate enough heat to start a fire.

There have been several documented incidents of water bottles causing fires in cars. For example, in 2017, an Idaho power company employee discovered his truck seat smoldering after he left a water bottle on the seat. The bottle had focused sunlight onto the seat fabric, causing it to heat and begin to burn. There are other reports of similar incidents where water bottles left in hot cars have been implicated in fires or near-miss fire situations.

From a scientific perspective, the risk comes from the bottle’s function as a convex lens. Just like a magnifying glass can catch dry leaves on fire by concentrating sunlight, a water-filled plastic bottle can concentrate light at a point on the car interior. The degree of temperature increase depends on several factors, including the bottle’s position, the angle of sunlight, and the duration of exposure. However, under optimal conditions, temperatures can rise high enough to ignite flammable materials.

To avoid the risk of fire, safety experts recommend never leaving plastic water bottles in your car. If you must keep a water bottle in your vehicle, store it in the trunk or a shaded area where it won’t be exposed to direct sunlight. Additionally, use reusable water bottles made of non-transparent materials like stainless steel, which do not pose the same fire risk. Always ensure that car interiors are free of flammable materials that could serve as tinder in case of a concentrated light beam.

If you notice that you’ve left a water bottle in a sunny car, move it immediately to a shaded area or remove it from the vehicle. Check your car regularly for any materials that could potentially catch fire. In the unfortunate event that you do find a fire has started, evacuate the vehicle and call emergency services immediately, prioritizing your safety over attempting to extinguish the fire yourself.

The advice to avoid leaving water bottles in cars may seem surprising, but understanding the science and real-life incidents behind it makes the risk very clear. By taking simple precautions and being aware of the potential hazard, you can prevent a minor decision from turning into a major disaster. Stay safe by keeping water bottles out of direct sunlight and ensuring your car remains a secure environment for you and your passengers.