Soda Cans, Asthma Inhalers, And... Water Bottles? All Things That Could Explode In Your Car This Summer

We (hopefully) all know not to leave dogs or children in hot cars, but what about a half-full water bottle? Probably safe, right? Can’t suffocate; can’t get heat exhaustion; can’t catch on fire – no need to worry at all!

Wait – okay, scratch that last one. Turns out, that half-finished Evian in your cup holder is more of a fire risk than you might imagine. So too is your sunscreen, deodorant, and asthma inhaler – plus a whole bunch more innocuous items you probably didn’t expect.

So, with temperatures soaring across the Northern Hemisphere right now, we’re asking: what should we definitely not be leaving in our cars this summer?

Water bottles

It feels cruelly ironic that the very thing you need most when it’s hot – water – can be dangerous to have in your car. Under the right circumstances, though, this otherwise life-saving liquid can become downright dangerous.

“A couple of years ago […] a worker had left a water bottle in his vehicle,” David Richardson, administration major for the Midwest City Fire Department in Oklahoma, told ABC News last year. “And the light had come through and melted something in his vehicle.”

If you’re thinking that sounds unbelievable – well, you’re not alone.

“I didn’t believe it,” Richardson admitted. “I was like, no way could this be possible.”

Rather than simply sit back in his disbelief, however, he set out to prove it one way or the other – and, to his surprise, it worked. “I went out in front of the fire station, took a water bottle and a piece of paper, and sure enough, it acted just like a magnifying glass,” he recalled. “I was able to burn a hole through [the paper.]”

Now, we’re not saying it’s likely – you really need a “perfect storm” of conditions in your car for this to happen, admitted Richardson. Colored bottles or liquid won’t have the same effect, and even then, “the chances are pretty slim that it actually would cause enough of a fire to actually destroy a vehicle,” he said.

Nevertheless, he cautioned, “[it’s] not impossible.”

But perhaps more of an issue is what’s going on inside as the contents and plastic heat up. “Heat helps break down chemical bonds in plastics like plastic bottles,” Julia Taylor, a scientist who researched plastic at the University of Missouri, told National Geographic back in 2019. “And those chemicals can migrate into beverages they contain.”

What that means in practice is toxic chemicals like antimony or BPA entering your body – both substances that have been found seeping out of heated water bottles at dangerous levels in recent years. The former, a metalloid chemically similar to arsenic, can cause toxic effects in multiple organ systems in the body; the latter is a hormonal disruptor that has been linked to breast cancer.

“The bottom line is that glass is better than plastic, wherever possible,” Taylor advised. “Otherwise, the message should be to keep the water bottle in a bag or covered when not in use (not exposed to bright sunlight for long periods of time) and not to leave plastic bottles in a hot car as temperatures rise fast at this time of year.”

Pressurized canisters

There’s a reason your deodorant comes with a warning label: keep away from heat and hot surfaces. Compressed gases – like the propellant in your body spray, or the carbon dioxide in a soda can – will expand outwards when they’re heated, slamming against the walls of their container. 

Without a release, the pressure inside builds and builds, until, eventually, there’s only one way it ends.

“[They’re] little bombs,” Kate Biberdorf, a chemistry professor at the University of Texas at Austin, told CNN last year during a spate of heat-related soda can explosions aboard Southwest flights. “I don’t want to say that to scare people, but that’s really what you should think of them as.”

Unlike water-bottle fires, soda explosions are pretty common, too. Southwest Airlines – the carrier is fairly unique in this regard, as it doesn’t refrigerate its beverages like other airlines do – reports hundreds of can-splosions per year; packages of aerosols left on Arizona porches have been known to blow up, firing shards of metal into security doors; in 2018, a can of compressed air left in a car exploded straight through the vehicle’s windshield.

And watch out for a surprise bang, too. “In the summer, people will store [cans] in the car and it’s usually ok,” Biberdorf explained, “but as soon as you start moving, you’ve added a little bit of extra force – maybe it slams against the side [of the car] – and then it explodes.”

Sunscreen and medication

Far less immediately dangerous – but nevertheless good to know about – is the effect of heat on your various medical items. Unlike your water and sodas, these are unlikely to explode – although it can happen with sunscreen, leaving you with a gross sticky mess all over your car seats.

More likely, though, is it denatures in the heat, becoming little more than expensive face paint. Exposure to temperatures of just 30 to 60°C (86 to 140°F) – more than possible inside a car baking under the Sun – can irreversibly alter the chemical makeup of sunscreen, making it less effective at protecting you. If your brand contains retinol, it may even become irritating to your skin. 

Similarly, medications you may rely on to stay alive can become basically useless in hot temperatures. Insulin, for example, can’t withstand temperatures of above 30°C (86°F) without losing efficacy; anxiety meds such as lorazepam and diazepam lose efficacy of up to 75 and 25 percent, respectively, when stored above 37°C (98°F). Epinephrine – the drug inside an EpiPen – can lose 64 percent of its potency when exposed to cyclical heating, and albuterol inhalers for asthma will deliver less of the chemical at best – and at worst (say it with us) straight up explode in extreme temperatures.

Basically, this summer, you should probably try not to leave anything in your car. And if you have to leave a dog in there, at least stick an unopened soda in there with her – so if it really does get too hot, at least the can will shoot out the windshield and let some air in for the poor mutt.