Lesser-known dangers of hot cars include common items left in vehicles, experts say

The interior of a car is one of the most dangerous places for young children and pets during a heat wave. But everyday items can also be dangerous.

Water bottles, for example, pose unique hazards when temperatures begin to climb, according to experts.

One of the unexpected risks is the ability for water bottles -- specifically clear bottles that contain clear liquid -- to start a fire, David Richardson, administration major for the Midwest City Fire Department in Oklahoma, told ABC News.

Depending on the presence of sunlight, the reflective qualities of the bottle and liquid can heat up nearby objects to the point of fire ignition if hot enough, Richardson said.

When Richardson first learned of the potential danger of water bottles, he didn't believe it. So he tested it out in a controlled environment and found that a water bottle was able to burn a hole through a piece of paper, like a magnifying glass.

Richardson emphasized that the "perfect storm" of conditions would need to be present for this phenomenon to occur. Not only would the bottle and liquid need to be clear, but sunlight would have to enter the car just right. Vehicles with tinted windows probably would not pose a similar risk, he said.

"The chances are pretty small, but they're not impossible," he explained.

Ingesting liquid that has been baking inside of a plastic water bottle in a hot car for a period of time could also impact human health.

A bottle's manufacturing process, chemical composition, the outside temperature and how long the bottle has been exposed to heat means liquids could become contaminated with toxins, toxicologist and board-certified emergency medicine physician Dr. Stephanie Widmer told ABC News.

While research and expert opinions differ, the risk is not zero, Widmer said. It's best to avoid drinking from plastic bottles that have been exposed to high temperatures until more conclusive research is available, she added.

Electronics with lithium batteries, such as a vape, electric scooter or toy, could also become a hazard when left in a hot car, Richardson said.

If the plastic casing surrounding the battery melts and exposes the lithium-ion battery to direct sunlight in addition to the high temperatures, there is a possibility it could explode, Richardson said.

Other everyday items that experts caution against leaving in hot cars due to the potential for them to explode include aerosol cans, canned and bottled soda and lighters. Items commonly left in hot cars like sunscreen, medicine and alcohol all have the potential to spoil, while glasses and sunglasses could melt and warp.

Leaving human beings in hot cars still remains the No.1 danger -- often with fatal consequences.

It's crucial to remember that leaving children or pets in a parked car, even for a short time, is very dangerous and can be fatal, Widmer said.

More than a dozen children have died so far this year after being left in a car during hot temperatures. Nearly 1,100 children have died in hot cars since 1990, according to KidsAndCars.org.

Extreme heat is expected for the rest of the summer in much of the U.S., forecasts show.