A Driving Instructor's Blog

Ethanol - molecular modelAs I mentioned some weeks ago – Jeezus, it was the day before we stopped work – it is easy to make your own hand sanitizer.

The best kind contains alcohol, and it has to be at least 60% alcohol to be classed as effectively anti-bacterial. So any holistic fruit extracts people are using are both not as good (if at all), and not affected by what I’m going to say next.

In keeping with the usual social media ethic, a lot of slightly wrong, wholly fake, and completely true information is being posted on virtually every topic imaginable, and many people are unable to decide which is which. The latest one concerns hand sanitizer being kept in the car in hot weather, and the circulation of photoshopped images more or less showing the craters left in roads after a bottle of it spontaneously ignites.

The images are almost certainly faked, but there is a risk with storing any flammable liquid in the car during hot weather, and I’ve mentioned that before in my article on how to make your own screenwash concentrate. The risk is proportional to how much of it you’re storing and the temperature it gets up to.

Alcohol-based sanitizer typically contains 70% ethanol. Pure ethanol has a flash point of 13°C, which means that at that temperature or above, a combustible vapour exists above it which would be ignited in the presence of any spark or flame. In an enclosed space, it would be explosive. A 70% solution of ethanol in water is slightly less dangerous, having a flash point of 21°C. In sunny weather, the inside of a car can easily exceed 30°C – especially if it is parked on a driveway with the windows closed. However, the autoignition temperature (where it just catches fire anyway), even of pure ethanol, is close to 400°C. You’re not going to get craters in the road driving on this planet!

It is therefore a scientific fact that any alcohol in the car will evaporate, and the vapour will be in an enclosed space. Depending on how much alcohol you had in there to start with, any spark – such as static from touching the car chassis (we’ve all felt that), or smoking – could ignite it.

Fair enough, a small 20ml bottle wouldn’t produce much vapour, so the chances of any ignition at all even with a blowtorch would be almost non-existent. But a 500ml one – especially if you were also carrying spare bottles of it – definitely could. As I said in that article about screenwash, my advice would be never to carry neat or nearly-neat alcohol (or any other flammable liquid) in the car when it’s hot, and only a minimal amount at other times, since if there was a spill in a crash it could easily lead to a fire.

In the case of sanitizer, it is not neat alcohol, but it is a high concentration of it nonetheless. It would be safest not to leave it in the car – keep it in your bag, and carry it with you when you park up. Keep it out of the sun, and preferably in the boot (though I can think of reasons why that might not be the perfect place for it, either).

Oh, and if the alcohol evaporates, it means the sanitizer isn’t 70% anymore, so it will stop being effective for its intended purpose.

And one more thing. If your sanitizer contains isopropyl alcohol instead of ethanol, it is still potentially flammable.

The bottom line is that there is a small risk. Be careful.

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