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How leaving bottled water in your car could start a fire

On a hot sunny day, light shines through the front windshield, through a full water bottle, focusing the heat onto one point.
How a water bottle could spark a fire
Concentrated light shining through a filled water bottle can actually spark a fire.Facebook / Idaho Power
/ Source: TODAY

August marks the hottest month of the year in many parts of the country, and drinking water to stay hydrated is essential. But while it might seem tempting, your car is one of the worst places to keep bottled water: it can start a fire.

That's exactly what happened in July 2017 to Dioni Amuchastegui, a battery technician with Idaho Power, when he saw smoke from beneath the center console of his truck during his lunch break.

"At first I thought it was dust, but the window was rolled up so there was no wind," he told TODAY. "Then I noticed that light was being refracted through a water bottle and it was actually smoke."

He moved the bottle before it could do more damage, but the fire managed to burn two small holes in the front seat of his truck.

How a water bottle could spark a fire
Idaho Power employee Dioni Amuchastegui displays burn marks on his truck's front seat after a water bottle left there caused a combustion.Facebook / Idaho Power

Amuchastegui decided to safely recreate the combustion with his team at Idaho Power, sharing the video on the company's Facebook page on July 13.

"It’s not something you really expect, having a water bottle catch your chair on fire," Amuchastegui said of the unlikely scenario. "I actually had to do a double take the first time."

The video caught the attention of the Midwest City Fire Department in Oklahoma, who referenced it on their Facebook page as a public safety warning, albeit low-risk, during the sunny summer months.

"The conditions must be just right," David Richardson, a spokesperson for the Midwest City Fire Department, explained to TODAY. "The bottle has to have liquid, the liquid has to be clear, the bottle has to be clear and sunlight has to pass through it at the right angle."

Also key is the rounded shape of the bottle. On a sunny day, the light shines through the front windshield, through the full water bottle and that water focuses the light — and its heat — onto one point. That concentrated beam can exceed 400 degrees on a darker surface within seconds. That's hot enough to spark a fire, according the Richardson.

He stressed these conditions cannot be met in a moving vehicle or when the sun shines on a bottle through a tinted window.

While the risk for a sustainable fire is unlikely (his department has no record of vehicle fires resulting from this method), his team was also able to recreate the same combustion as Amuchastegui with just a small bottle of water and a dark sheet of paper.

So instead of leaving your bottled water baking in the car for hours (which you shouldn't drink anyway) take it with you! Also, consider these reusable water bottles as well as some simple tricks for staying hydrated.