There is a very slight theoretical risk
When a cell phone is transmitting it creates an electromagnetic field
in the air nearby. This could contribute to the "focusing" of
electrostatic potentials in the air and thereby "channel" a lightning
strike towards the phone (or possibly away from it -- one would have to
work through the equations or do a computer simulation to tell for sure).
However, I'm confident that any such effect is very slight. A cell
phone broadcasts only a few hundred milliwatts, and I'm guessing that
you'd need power in the range of killowatts to produce a measurable
Probably more significant than the presence of the phone is the
posture of the user. Having one arm elevated, holding the phone, is
likely to make the user a more "attractive" target for lightning, by
creating a less uniform electrostatic gradient around the user.
It's interesting to note that in the other classical case of a golf-
course lightning strike, the lightning strikes the upraised club. In this
case the strike is directed away from the head, so brain injury is less
likely to occur. It may be that holding a club has a protective effect
(for the brain) that holding a phone does not.
Competing interests: No competing interests