Can hand sanitiser catch fire in a hot car?

We have seen a number of posts on Facebook suggesting they show the aftermath of alcohol hand sanitiser having caught fire in vehicles. The photo in question has been shared over 20,000 times.

One post has accompanying text claiming to explain the science behind it:

“Hand sanitizer has a fairly low temperature flash point (Ignition/Flame).

Hand sanitizer with 70% alcohol has a flash point of 21 deg C.

The photo below is from hand sanitizer igniting in a hot vehicle that reached 35 deg C. inside the vehicle.”

Facebook user, 19 May 2020

Although alcoholic hand sanitisers are potentially flammable, there would need to be a spark to start a fire at temperatures this low. The flammable ingredients in hand sanitiser would need to be at much higher temperatures, over 350°C, to combust without a spark.

To be safe, you should store them out of the heat, and away from open flames, sparks and hot surfaces though.

What is the flash point?

The flash point is the lowest temperature at which a substance can give off vapour into the air around it, which can be ignited by a spark or other source of ignition.

It’s true that the flash point of antibacterial gels is around 21°C. In the UK, Dettol antibacterial hand gel has a flash point of 24°C, and Carex antibacterial hand gels have flash points of between 22°C and 23°C.

At these temperatures hand gel can release vapour that is flammable. So it’s true that alcoholic hand sanitisers are potentially flammable at these temperatures, but they’ll need an external source of ignition to catch fire.

But that doesn’t mean hand sanitiser left in a car that gets warmer than this will ignite on its own, unless there is a spark. 

The auto-ignition temperature is much higher than this

The temperature needed to make a substance ignite spontaneously, so without a spark or source of ignition, is the auto-ignition temperature. 

We spoke to Dettol, which told us the auto-ignition temperature for ethanol, the main ingredient in its alcohol hand sanitiser product, was 363°C, although that can vary by a few degrees either side.

So that’s much higher than 35°C, which is the temperature one post says the car in the image got to.

Is the picture what it says it is?

We don’t know if the car door in the image was the result of a fire caused by hand sanitiser.

The picture was first investigated in Brazil by fact checker Estadão in April, and the image does seem to have some Portuguese writing in the background, but they were not able to verify whether the picture really showed the aftermath of a fire caused by hand sanitiser.