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American scientists writing in The Lancet say the magnetic force generated by lightning could provide an electric jolt so powerful it could stop someone's heart.
The researchers from the Lightning Data Center in Denver, Colorado, came up with the theory while investigating the death of a 32-year-old golfer who died after sheltering under a tree in a thunderstorm.
No marks
The man's heart stopped, he went into a coma and died in hospital 18 days later. But no one could explain why he had no marks on his body.
Two of his companions lost consciousness briefly and another suffered burns to his neck, head and abdomen.
Previous lightning deaths have been linked to direct strikes, side flashes or ground currents. But the Denver researchers believe the golfer died after suffering the ripple effect of a massive magnetic force.
Magnetic fields
They say lightning bolts can generate currents of more than 100,000 amps, which can produce one metre-wide magnetic fields. These could be powerful enough to stop the human heart, even though the jolt may only last one millisecond.
The theory could explain some of the unexplained deaths of hikers in exposed places.
The researchers suggest it should be tested by exposing mice or rats to strong magnetic pulses.
Traditional advice for people caught in a thunderstorm is to avoid sheltering under trees, as they act as lightning conductors, and to make sure you are not the tallest object on the ground.
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