Can You Leave Hand Sanitizer in Your Car on a Hot Day?

At the start of the pandemic, hand sanitizer became one of life's essentials—especially on the go. Most of us carried travel-sized bottles of it throughout the day, ensuring we could stay clean when we didn't have soap and water. But social media reports about hand sanitizer "igniting" in hot cars led to safety concerns.

On May 21, 2020, Western Lakes Fire District (WLFD) in Oconomowoc, Wisconsin, posted a (now-deleted) photo on its Facebook page of the melted interior of a car door with a warning:

"By its nature, most hand sanitizer is alcohol-based and therefore flammable," wrote the WLFD. "Keeping it in your car during hot weather, exposing it to [the] sun causing magnification of light through the bottle, and particularly being next to [an] open flame while smoking in vehicles or grilling while enjoying this weekend—can lead to disaster."

The post linked to an April 17, 2020, YouTube video from the National Fire Protection Association (NFPA) titled "Fire Safety Considerations for Hand Sanitizer," which said hand sanitizer could ignite if it came into contact with a "viable ignition source" due to its high alcohol content, though didn't mention the risk of car fires.

Following backlash from Facebook users, the WLFD post was removed, but not before it was shared widely across the internet. The WLFD later updated its Facebook status to "apologize for any confusion" and "clear up some misunderstandings."

As many people pointed out, the WLFD confirmed that the image used in its initial warning post wasn't from their district or of an exploding container of hand sanitizer. "Our message was intended to center on preventing fire or injury from the use of hand sanitizer," wrote the WLFD.

Can Hand Sanitizer 'Ignite' in a Hot Car?

It's extremely unlikely and would take more than just a hot car. "The active ingredient in most hand sanitizers is ethanol," William L. Schreiber, PhD, chair of the department of chemistry and physics and coordinator of medical laboratory science at Monmouth University, told Health. It's the same type of alcohol in wine, beer, and liquors—just at a much higher concentration (at least 60% to deactivate the coronavirus).

"Ethanol has a significantly lower boiling point than water (173 degrees Fahrenheit compared to 212 degrees Fahrenheit)," said Schreiber. That means it heats up to a boiling point much faster than water. "It also has a lower vapor pressure than water, which means it evaporates much faster at any temperature," added Schreiber.

To that end, Schreiber said that, on a very hot day, significant pressure could build up inside a bottle of hand sanitizer, causing it to rupture—but it wouldn't result in the combustion-type scenario the WLFD warned against. "If there was a source of ignition, like a naked flame—which isn't likely—and the expelled material caught fire, that would cause a good deal more damage," added Schreiber.

If you need more reassurance, the NFPA further addressed the issue in the comments section of their YouTube video, stating that spontaneous combustion—which involves self-heating—is not possible with hand sanitizer. "Hand sanitizer is not subject to self-heating and would require temperatures to reach over 700 degrees Fahrenheit to spontaneously combust," wrote the NFPA.

Can Hand Sanitizer Lose Its Effectiveness in a Hot Car?

This scenario may be more likely, said Karen Dobos, PhD, a professor in the department of microbiology, immunology, and pathology at Colorado State University, who helped create hand sanitizer for CSU's James L. Voss Veterinary Teaching Hospital in March 2020. The bigger risk of leaving a bottle of hand sanitizer in a hot car is that it loses potency.

"Alcohol evaporates very quickly in [the] air—especially here in Colorado where it is warm and dry," Dobos told Health. "As it evaporates, the relative concentration of the alcohol goes down, as does its germ-killing ability."

The situation is further complicated by a second ingredient common in some hand sanitizers: hydrogen peroxide. "UV light reacts with hydrogen peroxide and converts it to water," said Dobos. "As a result, both active ingredients are lacking from your hand sanitizer, which completely removes any benefit. For this reason, hand sanitizers should not be stored in direct sunlight, nor left out—especially in warm temperatures."

Luckily, there are things you can do to maintain your hand sanitizer's effectiveness. Dobos recommended storing hand sanitizer at a 45–75 degrees Fahrenheit temperature. As far as storing it in your car goes, keeping a small bottle in your center console or glovebox should be fine, provided you don't leave it there all day long on a warm day.

You can also use a simple trick to check that your hand sanitizer is still effective: Look at the viscosity of the sanitizer in the bottle. It should be pretty thin, move around the bottle easily, and dry relatively quickly on your hands. But "if it becomes thick or takes longer to dry when you use it than it used to, it's time to replace it," said Dobos.