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Rapid response to:


Injury from lightning strike while using mobile phone

BMJ 2006; 332 doi: (Published 22 June 2006) Cite this as: BMJ 2006;332:1513

Rapid Response:

Far fetched Scaremongering

It is a real shame the authors did not research better the physics of
lightning strikes or talk to a physicist about conductors, insulators,
cellphones, lightning, or statistics.

Firstly the statistics - 3 mentions of people using a cellphone
whilst being hit by a lightning strike is by no means compelling evidence
for a correlation. I would wager more people have been struck by lightning
whilst picking their nose than whilst on a cellphone, it doesnt mean
there's a link between the two. In fact, using the same straw-grasping
conclusion drawing techniques, we can conclude from one of the pieces of
evidence that use of a cellphone during a lightning strike saved 10 lives
"Last year, 10 tourists seeking shelter from the rain in a beacon tower at
Juyong Pass, another section of the Great Wall, were also struck by
lightning when one of them was using a mobile phone. Fortunately, no one
was seriously injured." [
08/15/content_3355652.htm]. There simply is not enough evidence
presented to argue that use of a cellphone attracted the strike or
worsened her injuries.

Secondly the suggestion that the metal in a cellphone is channelling
the path of the current through the body is completely wrong - in fact if
it was behaving as a lighting conductor, it will aid the flashover effect,
not channel current into your body. Flashover occurs because the current
travels 'easier' over the surface of your thunder-storm-water soaked body
than through it - the current follows the path of least resistance.
Holding a conductor against the side of your head would actually aid the
strike in channelling down that side of your body rather than punching
straight through the top of your head. However the amount of metal in a
cellphone is very small, compared to the amount of plastic containing it,
unless you happen to have an all-metal cased cellphone. Due to this
insulation and the small amount of metal involved, the cellphone is highly
unlikely to be able to build up enough of the static charge from the
ground to trigger the strike and be the point of contact. The likelihood
of the charge flashing over you or going through you is governed heavily
by how wet you are and the clothing you are wearing, not by whether you
have a celly or not. The burst eardrum mentioned in the paper will have
been caused because the phone was creating a seal against the ear,
trapping the air inside. When the strike occured the air around the plasma
channel will have been superheated and rapidly expanded. This rapid
expansion of air inside the ear, sealed off by the cellphone held against
it, will have burst the eardrum. Burst eardrums are a common side effect
of being hit by lightning anyway due to the same mechanism - the rapid
expansion of superheated air and the massive shockwave associated with it
(the thunder clap), regardless of whether the ear was sealed off or not.

Thirdly - why lightning strikes occur. Without going into excessive
detail, a large electrostatic charge is built up in the thundercloud
(various mechanisms exist in the literature - try Wikipedia-ing
'lightning' if you are interested in them) and this large build up of
charge induces a build up of the opposite charge in the ground underneath
the cloud. If there is no mechanism to dissipate the charges gently, then
they can build up until the potential difference is so great that a
discharge occurs between the cloud and the earth, ie a lightning strike.
If some of the charge can move higher up from the surface of the earth,
drawn by the attraction of the opposite charge in the cloud above, then
since the gap between the charges is reduced the strike is more likely to
occur as the charges build up, and will occur once a critical gap size is
reached for a given potential difference. The larger the conductor is that
shortens this gap, the more charge that can build in its tip and the more
likely a strike is - so holding a large metal pole up in the air is asking
for trouble, but holding a small amount of metal inside an insulated
plastic case is unlikely to increase the risk further. The response just
above from a woman who's son was hit by lightning whilst holding a phone
and sitting in a metal chair made me laugh - the metal chair will have
increased the likelihood of the strike much more than his phone - and will
have saved him from injury as it will have acted somewhat as a Faraday
Cage and will have taken the brunt of the current flow. Someone with more
spare time than me should get themselves a Tesla coil and run some
experiments on a transmitting and non-transmitting mobile phone to test
all this out - and this is definitely what should be done before BBC news
or any other band-wagon-jumping news team grabs this 'story' and goes nuts
about how cellphones are going to kill us all in another new and exciting
(and far fetched) way.

Competing interests:
Actually knowing something about physics, cellphones, and lightning

Competing interests: No competing interests

26 June 2006
Ramsey M Faragher
PhD Student at the University of Cambridge
Cavendish Laboratory, Cambridge