Answer to Question #10893 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
What are the radiation exposure risks to a person hit directly by lightning? What are the risks to the people in very close proximity to the lightning strike? Is there any danger from the emission of x rays and gamma rays from the lightning that could potentially cause serious health issues, such as cancer?
A person hit by lightning has a lot to worry about, but radiation exposure is pretty close to the bottom of the list. A lightning bolt can have a voltage of up to a million volts (V), it can carry an electrical current of over 100,000 amps (A), and it heats the air it passes through to temperature of 10,000°C—twice as hot as the surface of the sun. Lightning kills about 30 to 40 people annually in the United States (http://www.lightningsafety.noaa.gov/resources/RecentLightningDeaths.pdf).
There is some evidence that lightning can produce x rays and gamma rays (http://www.sciencedaily.com/releases/2013/04/130424210319.htm). X rays are formed when high-energy electrons are slammed into other atoms in a target of some sort. In an x-ray tube the target is made of tungsten, copper, or some other metal, but any time high-energy electrons are shot into a target there can be some x-ray radiation given off. The very high voltages that lightning produces can produce very high-energy electrons; when they slam into atoms in the air they can produce radiation, similar to what happens in x-ray tubes. This is called "dark lightning" and it wasn't even discovered until 1991. Since then it has been picked up by satellites and by ground observations so there seems to be little doubt that it actually exists.
So yes, lightning can produce radiation. The next question is whether or not this radiation is harmful. Here, the answer seems to be "no." Part of the reason is that it lasts only a very short time. Dark lightning pulses last for only a fraction of a second and it's hard to pack a harmful radiation dose into a fraction of a second of exposure. As one example, an industrial x-ray machine generates a radiation dose of about 10,000 gray (Gy) in an hour. This would give a person a fatal dose of radiation in about 3 seconds (s). The longest-lived dark lightning is about 0.3 second. In that fraction of a second, assuming that the lightning bolt produces as much radiation as an industrial x-ray machine, a person who is close to the bolt would receive about a tenth of a fatal dose. Any more than a few steps away and the dose would be lower still.
As far as I know, however, there are no direct radiation dose measurements from this dark lightning so we don't really know exactly what the dose is. That being said, I'm not aware of any reported cases of radiation sickness or radiation injury resulting from dark lightning (or from visible lightning). While this is only anecdotal, at the very least I think it's safe to say that the radiation from dark lightning isn't nearly as high a risk as the high voltage, electrical current, and high temperatures from the lightning bolt.
P. Andrew Karam, PhD, CHP