You have a much increased risk of a static discharge, through the alternator, where you can get ignition from.
It’s not commonplace, but long time ago, when I was a trainee for a particular UK Oil company I got to watch what happened in that case. It took 11 seconds before the steel beams started to melt. The person pumping didn’t survive.
Same reason you turn your phone off there, and never ever smoke. You might be lucky, but you really don’t want to be the example of what not to do in those cases, you usually won’t get a second chance.
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