/ 15 August 2014

Is lightning’s ‘blast’ as bad as its strike?


Injury from lightning is capricious and unpredictable: clothing may be torn off, sometimes raising the suspicion of foul play, especially if no one witnesses the lightning. The victim’s clothes are is typically ripped as if by some internal explosion; similarly belts and boots may be split open.

South Africa has one of the highest lightning strike rates per square kilometre in the world, with approximately 26 flashes per square kilometre annually reported on the Drakensberg escarpment. In Gauteng, about 10 people are killed each year by lightning and about 30 are struck and survive, sometimes with severe medical and neurological disabilities.

On the ground, we see lightning as a momentary flash, whereas in actuality it is a transient, high-current electrical discharge whose path is generally measured in kilometres. Unlike the electricity in your plug socket, which is alternating current, a lightning strike is direct current of about 30 000 to 50 000 Amps, which is massive if one considers that only about 50 to 80 milliAmps (a milliAmp is one thousandth of an Amp) across the human heart for more than a few seconds is likely to cause death.

We know that there is a pressure blast wave around lightning’s channel of light. We have known about it since the time of Gaius Plinius Secundus, better known as Pliny the Elder (23 AD to 79 AD). He was a Roman author, naturalist, and natural philosopher, as well as naval and army commander, and his dictum was: “The man who sees the lightning flash and hears the thunder is not the one to be struck”. 

You can hear thunder from as far away as 14km, which means that there is a tremendous amount of energy involved in the generation of thunder. However, before thunder exists, there is a pressure blast wave. This pressure blast wave is caused by the super-heating of the air around the lightning bolt, which travels at super-sonic speeds. It is this super-sonic blast wave that decays, within metres, and transforms into thunder. 

Many people think that lightning injures humans chiefly due to its electricity and heat. While this is true for the vast majority of lightning-related deaths and injuries, the accompanying pressure blast wave can also do serious harm. 

South African law classifies a lightning-related death as an “unnatural death”, which means that it requires a full medico-legal examination, involving a full autopsy and inquest. Every aspect of the lightning-related death needs to be documented, so it can be included in the international guidelines and warnings to prevent more deaths. Our duties as scientists are simple: to protect ourselves, the other person and all of mankind.

During a lightning strike, the temperature of the lightning bolt channel is raised to about 24 700°C, in a few microseconds. This causes the temperature around the channel to rise suddenly, meaning that the pressure in the channel suddenly increases to several atmospheres, as does the volume of space it occupies.

This sudden rise in volume causes a sudden cylindrical pressure shock wave, which may reach pressures of more than 10 to 20 atmospheres (1013,25 kPa to 2026,5 kPa) in the vicinity of the lightning bolt channel. This is enough pressure to form a crater in a concrete pavement.

Lightning’s pressure blast wave has been known to tear and tatter clothing, fracture long bones (such as those in the upper leg), rupture a person’s eardrums and damage their lungs. The blast can cause a pocket of air behind the sternum (pneumomediastinum), which may cause injury to the chest wall and lungs. These findings are similar what one would expect to find in bomb explosion victims. In addition to causing damage in the ears and eyes, this shock wave may also cause damage to other internal organs such as the spleen, liver and bowel tract. The force may cause a victim to fall, causing head and other traumas. It has even been known to cause shrapnel injury. One victim had multiple small fragments of shattered concrete pavement embedded in their skin.

Paradoxically, the human body is both robust and fragile. Humans can survive relatively high blast overpressures without experiencing blast-related injuries. Our collaborative lightning research at the department of forensic medicine at the University of Pretoria and the School of Electrical and Information Engineering at the University of Witwatersrand is therefore all about risk management.

So how close does a person have to be to a lightning strike to be at risk? How far does this pressure blast wave extend? For the past 10 years I have been researching lightning’s pressure blast wave effect on humans. This newly emerging field is known internationally as keraunomedicine or keraunopathology (“kerauno” is Ancient Greek for “lightning” or “thunderbolt”). This field includes aspects of electrical engineering, mechanical engineering, meteorology and climatology, as well as physiology.

In South Africa, approximately 80 to 100 people will die because lightning strikes each year and approximately seven times as many people will be struck and survive. 

These lightning-strike survivors may suffer severe disabilities, many as a result of lightning’s invisible blast wave, and therefore we need to study this phenomenon, understand it and find out how to protect ourselves from it. 

Ryan Blumenthal is a PhD candidate at the University of the Witwatersrand.

This publication is the culmination of a six-month-long Mail & Guardian project, called Science Voices, to teach postgraduate science students how to turn their academic writing into something the public can read and enjoy.