No, it won't ignite—but you should still take it out on a hot day.

By Claire Gillespie
May 29, 2020
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Hand sanitizer has become one of life’s essentials—especially on the go. Most of us carry travel-sized bottles of it in our purses and our cars, ensuring we can stay clean when we don’t have soap and water at our disposal. But recent social media reports about hand sanitizer “igniting” in hot cars has led to safety concerns.

On May 21, 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," the WLFD wrote. "Keeping it in your car during hot weather, exposing it to sun causing magnification of light through the bottle, and particularly being next to open flame while smoking in vehicles or grilling while enjoying this weekend—can lead to disaster.”

The post linked to an April 17 YouTube video from the National Fire Protection Association (NFPA) titled “Fire safety considerations for hand sanitizer,” which says hand sanitizer can ignite if it comes into contact with a “viable ignition source” due to its high alcohol content, but doesn’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 their Facebook status to “apologize for any confusion” and “clear up some misunderstandings.” They confirmed—as many people had pointed out—that the image used in their 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,” they wrote. 

Wait, 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, tells Health. It's the same type of alcohol that's in wine, beer, and liquors—just at much higher concentration (around 70%, which is the potency required to deactivate the coronavirus).

"Ethanol has a significantly lower boiling point than water (173 degrees Fahrenheit compared to 212 degrees Fahrenheit)," says 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," he adds.

To that end, Schreiber says 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,” Schreiber adds. 

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,” the NFPA wrote. 

Well, can hand sanitizer lose its effectiveness in a hot car?

This scenario may be more likely, says 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. The bigger risk of leaving a bottle of hand sanitizer in a hot car is that it loses potency. “Alcohol evaporates very quickly in air—especially here in Colorado where it is warm and dry,” she tells 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 that’s common in some hand sanitizers: hydrogen peroxide. “UV light reacts with hydrogen peroxide and converts it to water,” Dobos says. “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 recommends storing hand sanitizer at a temperature of 45 to 75 degrees Fahrenheit. As far as storing it in your car goes, it should be fine to keep a small bottle in your center console or glovebox, 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 after it's 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,” Dobos says. 

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