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The warning signs at gas stations warn that you ought not enter and exit your vehicle while filling up your tank at the gas station. I have also been told by many people that doing so runs a risk of igniting the fuel or making your vehicle explode.

Is that a legitimate concern? Can getting in and out of your car actually cause combustion of the fuel being dispensed or the fuel already present in the tank? Or are the signs and stickers merely legal precautions to deter lawsuits and the widespread beliefs of the layman just an urban legend created from them?

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    There aren't warning signs saying that at petrol stations in the UK as far as I am aware. (But there are signs warning not to use mobiles.) I assume both are worried about a spark igniting vapour. Opening a car door does normally turn the light on, which could ignite vapour from a fuel spill I suppose? Have to go hunting for references! I know there was a case of someone's car exploding from a leaky cylinder of acetylene when the owner used the key to open the central locking. Fortunately no-one was hurt but there was a huge fireball! – Nick Sep 13 '12 at 16:01
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    Wasn't it an episode of Mythbusters that while trying to show that cellphones were a risk actually showed that a bigger risk was in wearing nylon shellsuits, or other clothing that could carry a high charge? Will look for a link. – Rory Alsop Sep 13 '12 at 16:09
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    If anyone's interested in looking into it, this claim is often justified by the possibility of static electricity building up from rubbing on the seats as you enter and exit. The claims may be associated with Oregon and New Jersey (I think that's the other one) which do not legally allow people to pump their own gas. – William Grobman Sep 13 '12 at 16:11
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TLDR: Yes, getting into and out of a car increases your chance of fuel combustion.

What:

The reason there are unusual instructions like :

  • not entering your car
  • not using a CB radio
  • not using a mobile (cell) phone

... while refueling is because of static discharge - you feel it when a car zaps you.

Why:

When you're in your carpeted car and you rub your rubber shoes on that carpet there can be a massive static electicity build up.

That build up is released into the first earthed item you come in contact with. In this case it's a car or petrol bowser.

Most of the time you touch your car, discharging the static before opening your petrol tank. But when uses a mobile phone or re-entering a car you often won't discharge the static before touching the car near petrol fumes, AND BOOM!

Example Refueling Fire:

Have a watch of this video: Refueling Fire Caused by Static Electricity (long version)

Note:

  • The woman re-enters the carpeted car in what looks like rubber-soled shoes.
  • She doesn't touch the metal frame of the car (which would have discharge the static)
  • The first metal object she touches is the bowser

The Remedy:

Aircraft and petrol tankers have a discharge connector to release static from an area on the vehicle not exposed to flamable fumes to the bowser. You can see it here (it's the yellow cable):

Static Discharge reel in use

Maybe you need one of these on your car...

Sources: Snopes Wikipedia

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First of all, gasoline is used because it is very good at being ignited by a spark, and it contains a lot of chemical energy. So on the face of it, a spark could ignite gasoline vapours in the air, and a vehicle with sufficient static charge could produce a spark if suddenly grounded.

It is possible, and extremely rare. Two things need to happen before the gas will ignite.

  1. The vapour pressure of the gasoline needs at the right level. "If the gasoline fumes are between 1.4% (14,000 ppm) and 7.6% (76,000 ppm) then any sort of ignition source, i.e. spark of some kind, could cause the atmosphere to ignite or explode. "source
  2. There needs to be a sufficient static charge on the vehicle, and it needs to be grounded to produce a spark.

Typically, 1. will not be an issue unless there is a spill on the ground that was not properly cleaned. 2 is certainly possible, particularly in dry conditions.

This article summarizes several sources. Of interest is this interesting breakdown of fueling accidents:

  • 50% happen when the person returns to the vehicle during refueling and leaves the vehicle door open

  • 29% happen when the person unscrews the gas cap

  • 21% happen for unknown reasons - theories include new chemical used in tires, more electronics in cars, seat fabrics, and more volatile fuels

  • 78% are women who return to the car to put a credit card into their purse, get money, write a check, get warm, check on the kids, write down odometer reading, apply lipstick, or use cell phone

The first three bullets add to 100%, so I'm not sure what the 4th stat is all about.

I don't know why the vehicle door being open is a factor, but returning to the vehicle is a common factor in fueling accidents. (Apparently being female is a factor as well.)

The above article also states that there are about 100 fueling accidents a year out of 11-12 billion fuelings in the US. So it is rare.

  • Perhaps I'm being stupid, but surely everyone returns to the car after fuelling to drive away? Or do they mean mid-refuelling? – Nick Sep 13 '12 at 20:23
  • I think they mean while the gas is pumping. Moving fuel being more likely to vapourise. – Chris Cudmore Sep 13 '12 at 20:38
  • Ah after watching the video underneath I understand. In the UK petrol only pumps while you hold the trigger on the pump handle; its impossible to keep pumping and leave to get charged. – Nick Sep 14 '12 at 7:15
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    It could also be another person pumping. – Chris Cudmore Sep 14 '12 at 12:37

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