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I know that St. Elmo's Fire is a form of plasma that occurs during thunderstorms that is similar to lightning because it is formed by a difference in charges.

But why isn't it dangerous if St. Elmo's Fire is around you?

For one, the fact that there is a charge difference usually means an electric shock, whether that is a tiny bit of static electricity from touching metal or a lightning strike. Yet St. Elmo's Fire is caused by a charge difference but doesn't give you an electric shock.

Also if there is a charge difference around you, lightning is more likely to strike you, even if you are as low as you can be. This can lead to heart problems such as tachycardia, fibrillation, or asystole. It can also cause uncontrollable muscle movements which itself can be dangerous because if your muscles are out of control you can end up anywhere. Muscles being out of control can also mean things like vomiting, lots of mechanical digestion, high blood pressure and more from involuntary muscles being out of control. In any case it is dangerous to have voluntary or involuntary muscles out of control. It can also cause nervous system problems, in particular excitation toxicity from positive charges continously reaching neurons until they don't send a signal.

So with all the health problems that can result and the difference in charges itself without an electric shock, why is St. Elmo's Fire not dangerous?

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    $\begingroup$ Maybe partly for the same reason that polar bears aren't a threat to penguins? And partly because of scale: the plasma that is close to people, is an absolutely tiny amount of charge. $\endgroup$
    – 410 gone
    Commented Dec 19, 2015 at 8:27

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As you say in your question, ball lightening and and St Elmo's Fire are related to thunderstorm activity (e.g., Griwr'ev et al, 1991). Grigor’ev et al (1999) worked out a formula for the charge on small drops of water (which is a bit complicated for a simple answer), but suffice to say the total voltage involved is high when you scale from one drop to millions. For example ~30kV per cubic centimetre is in the literature for initiating Ball Lightening, and lower for St Elmo's Fire because the coronal discharge nucleates on pointed or sharp objects.

If you troll physics forums about demonstrating St Elmo's fire in a laboratory setting, you'll see that it is actually quite dangerous, due to the high voltage. Most suggestions by physics teachers are to demonstrate the effect in neon gas, rather than air, and within glass bulbs. So one answer to your question is that the effect can be dangerous, e.g., when creating conditions of coronal discharge in an experimental setting.

What about in a natural setting? You can read in Griwr'ev et. al (1991) about several hundred reports of St Elmo's Fire sightings, but it isn't mentioned whether anyone has been 'within' the effect. However, St Elmo's Fire was observed on the Hindenburg shortly before it exploded (Robinson, 1964); not that this was the likely 'spark' that caused the explosion, but part of the prelude.

I can't find any verified reports of people immersed in St Elmo's Fire during my quick review of published literature, or non-peer reviewed sources like Scientific American and Wikipedia. There is an interesting reference in Wikipedia to Tesla managing to light up some butterflies with one of his coils, though!

Griwr'ev, A. I., I. D. Griwr'eva, and S. Shiryaeva. "Ball lightning and St. Elmo's fire as forms of thunderstorm activity." (1991)

Grigor’ev, A. I., and S. O. Shiryaeva. "Critical conditions for instability of a highly charged oblate spheroidal drop." Technical Physics 44.7 (1999): 745-749.

Robinson, Douglas. LZ-129 Hindenburg. New York: Arco Publishing Co, 1964.

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