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Acad Forensic Pathol. 2016 Mar; 6(1): 89–95.
Published online 2016 Mar 1. doi:10.23907/2016.008
PMCID: PMC6474511
PMID: 31239875
Ryan Blumenthal, MBChB (Pret) MMed (Med Forens) Pret FC For Path (SA) PhD
Author information Article notes Copyright and License information PMC Disclaimer
This article has been corrected. See Acad Forensic Pathol. 2017 December 01; 7(4): 667.
Abstract
The explosive effects of lightning have been known to exist for some time;however the precise risks associated with it have been generally unknown. Thiscurious injury phenomenon has existed historically under many different names inthe literature: “lightning's pressure blast wave,” “arc blast,” “shatteringeffects of lightning,” “pressures developed by arcs,” “thunder generation ofshock waves,” and “the sixth mechanism of lightning injury” are but a few of themany divergent and disparate terminologies used in the past to describe thisinvisible blast phenomenon. Blunt force trauma injuries and barotrauma injuriesare often identified on lightning strike victims. Lightning's pressure blastwave and its associated overpressure does appear to have significant injuryimplications associated with it. This paper takes an in-depth look at theexplosive effects of lightning and the main blast-related pathologies seen onlightning strike victims. Knowledge and insight into this phenomenon may helpforensic pathologists and those working in the fields of lightning injury andlightning protection. A general literature search of the medical, the electricalengineering, and the mechanical engineering literature was conducted. By lookingexclusively at the pathology of barotrauma in the human body, forensicpathologists may now get a relatively good idea as to the possible overpressuresand distances involved with regards to lightning's explosive effects.
Keywords: Forensic pathology, Arc blast, Barotrauma, Explosive barotrauma, Explosive effect, Lightning, Pressure blast wave, Shock wave, Thermobaric effect, Risks
Introduction
Lightning may be defined as a transient, high-current electric discharge whose pathlength is generally measured in kilometers. The electric current involved inlightning strikes is direct current (DC) on the order of 30 000 to 50 000 Amps(1).
Lightning's pressure blast wave has been known to exist since the time of GaiusPlinius Secundus, better known as Pliny the Elder (AD 23 – August 25, AD 79).Pliny's dictum was that “the man who sees the lightning flash and hears thethunder, is not the one to be struck” (2).
One can hear thunder from as far away as 25 km, which means that there is atremendous amount of energy involved in the generation of thunder (3). However, before thunder exists,there is a pressure blast wave. This pressure blast wave is caused by thesuperheating of the air around the lightning channel, which travels at supersonicspeeds. It is this supersonic blast wave which decays, within meters, and transformsinto thunder. Many people think that lightning injures humans chiefly due to itselectricity and heat. While this is true for the vast majority of lightning-relateddeaths and injuries, the accompanying pressure blast wave (overpressure) can also doserious harm. Lightning causes an instantaneous superheating and expansion of theair close to the victim's body, followed almost immediately by an implosion as theair rapidly cools.
The lightning channel is a narrow channel containing ionized air molecules. Thepreviously neutral air molecules split into positive ions (molecules missingelectrons) and negative free electrons. The presence of the positive ions andnegative free electrons makes the channel conductive and current can flow along thechannel and the charged cloud is able to discharge through the channel to the earth,which is what we see as lightning.
During a lightning strike, the channel temperature will be raised to about 25 000 Kin a few microseconds and the pressure in and around the channel may increase aboveatmospheric pressure as a result of Charles' Law (Figure 1). The resulting rapid expansionof the air creates a shock wave. This shock wave may injure a human being located inthe close vicinity of the lightning flash. The overpressure generated by thunder atthe source may approach 1470 pounds of force per square inch (psi) (4).
Figure 1:
Charles Law is an experimental gas law which describes how gases tend toexpand when heated. When the pressure on a sample of a dry gas is heldconstant, the temperature and the volume will be directly related.
Hill's work for a 30 000 Amp lightning strike shows the following: 588 psi at 0.75 cmradius from the stroke channel, 426 psi at 1.1 cm radius from the stroke channel,279 psi at 2 cm radius from the stroke channel, and 132 psi at 4.1 cm radius fromthe channel (5). This shows a nearinverse relationship between the pressure and the distance. Calculations by Hilltherefore show that the overpressure within a few centimeters of the lightningchannel may reach about 147 psi to 294 psi.
Cooray et al. reported that injuries can be caused by shock waves created by thelightning channel, although they did not commit to a specific distance within whicha victim would be at risk from blast wave injury. In addition to causing damage inthe ears and eyes, this shock wave may also cause damage to other internal organssuch as the spleen, liver, the lungs, and the gastrointestinal tract. Moreover, itmay displace the victim suddenly from one place to another, causing head and othertraumatic injuries (6).
Evidence of barotrauma (Figures2 and and33) doesexist on lightning strike victims. Lightning's pressure blast wave has been known totear and tatter clothing, fracture long bones, rupture eardrums and damage lungs.The blast causes a pocket of air behind the sternum (pneumomediastinum) and it maycause injury to the chest wall and lungs. These findings are similar to what onewould expect to find in bomb explosion victims. Lightning has even been known tocause secondary missile injury. One victim had multiple small fragments of explodedconcrete embedded within her skin, after the lightning struck the pavement (7).
Figure 2:
Lightning explosive barotrauma, imagined as a “pressure shockwave”immediately surrounding lightning's luminous channel.
Figure 3:
Proposed appearance of lightning's “pressure shockwave.” Please keep in mindthat the lightning channel is three-dimensional.
As forensic pathologists we would need to know within what range a human would be atrisk and the risk of lightning's pressure blast wave. A general literature search ofthe medical, electrical engineering, and mechanical engineering literature wasconducted.
Keywords were difficult to locate as this curious injury phenomenon has existedhistorically under many different names in the literature: “lightning's pressureblast wave,” “arc blast,” “shattering effects of lightning,” “pressures developed byarcs,” “thunder generation of shock waves,” and “the sixth mechanism of lightninginjury” are but a few of the many divergent and disparate terminologies used in thepast to describe this invisible blast phenomenon (3, 8-10).
Discussion
Overpressure (or blast overpressure) is the pressure caused by a shockwave over andabove normal atmospheric pressure. Overpressures in this paper have been presentedin the same format as they have been presented in previous papers. None of theprevious literature took into consideration the “rate of change” of overpressure.Using standard conversion tables, all findings have been expressed in pound-forceper square inch (psi).
One psi is the equivalent of 0.068046 atmospheres, which is the equivalent of 6.89476kilopascals.
There exists a relatively large literature which addresses damage approximations fromhigh explosive overpressures, human injury in proximity to a small bomb, and thepredicted injuries and fatalities from direct blast effect of explosions (11,12).
The Otorhinological Medical Literature
There is evidence in the literature supporting the fact that lightning canrupture eardrums (13-22). By looking exclusively at the pathology of trauma of tympanicmembrane rupture, forensic pathologists may now get a relatively good idea as tothe possible overpressures and distances involved with regards to lightningexplosive barotrauma in the field (8,23). Themedical ear, nose and throat (otorhinological) literature sometimes describesthe tympanic membrane following lightning strike as “a large tympanicmembrane perforation with ossicular chain disruption” (19). Proposed mechanisms ofinjury have included concussive “blast” effect on the ear, “direct” effect ofelectrical conduction, “splash” effect, “cylindrical shock wave of electrons,”and/or direct “thermal burn” (13-21, 24, 25). Thehuman tympanic membrane is able to withstand a limited amount of overpressurebefore failure. Overpressures are required to produce minor, moderate, and majoreardrum ruptures.
Rupture of the normal eardrum is a function of age as well as of the effectiveblast pressure. Tympanic membranes may, for example, rupture more easily in theyoung. Failures have been reported at overpressures as low as 5 psi ranging upto 40 or 50 psi. As mentioned earlier, calculations by Hill showed that theoverpressure within a few centimeters of the lightning channel can reach about147 psi to 294 psi (5). Astudy by Richmond suggests a minimum threshold of about 29 psi to produce minoreardrum ruptures (Table1) (26).
Table 1:
A Comparison of Scenarios of Overpressure (or Blast Overpressure) VersusPredicted Injuries (11,12)
| Overpressure (or blast overpressure) | Predicted Injuries |
|---|---|
| 29 psi | Minimum threshold required to produce minor eardrumruptures |
| 40 to 50 psi | Rupture of the normal eardrum |
| 29 psi to 72.5 psi | Chest and/or lung trauma |
| 40 psi (30 psi – 50 psi) | Minimum threshold for lethality |
| 62 psi (50 psi – 75 psi) | Fifty percent lethality |
| 92 psi (75 psi – 115 psi) | One-hundred percent lethality |
| 100 psi | Possible disruption and/or disfigurement |
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One can therefore deduce from human victims with tympanic membrane rupture, thatlightning's blast wave must have had a minimum overpressure of approximately 29psi.
The Chest and the Lung Medical Literature
There appears to be relatively good data for chest and lung damage by blast wavesin the literature. Lightning has also been known to cause damage to the chestand lungs. Pneumomediastinum and hemorrhage from a lung in a tracheostomizedpatient have both been reported (24, 27-29). The bomb blast literature shows that thethreshold for lung damage is in the range of 12 psi (8 to 15 psi) and the rangefor severe lung damage is in the range of 25 psi (20 to 30 psi) (11). Posttraumaticpneumomediastinum is not a cause for alarm amongst clinicians (28). Pneumomediastinum istherefore generally not considered a fatal injury. The literature suggests thatlung damage requires approximately 29.0 to 72.5 psi barotrauma.
Other Medical Literature
Lightning has also been implicated in the fractures of bones and rupture ofinternal organs (30,31). Bowel contusion has beenreported in the direct transmission of a detonation Shockwave (32). No associated overpressuredata could be found in the bomb blast and explosive literature in thisregard.
In addition to causing damage in the ear and eyes, this shock wave can also causedamage to other internal organs such as the spleen, liver, and the bowel tract(33). Moreover, it maydisplace the victim suddenly from one place to another causing head and othertraumatic injuries (6). Noassociated overpressure data could be found in the literature in regard to theseinjuries.
The human body can survive relatively high blast overpressures withoutexperiencing barotrauma (32).There is no evidence to suggest that lightning victims suffer severeblast-related disfigurement. There is no evidence to suggest that lightningwould rip a cavity in human flesh, for example.
When looking at tentative criteria for direct (primary) blast effects in man fromfast-rising, long-duration pressure pulses (such as a bomb explosion), thethreshold for lethality would be in the 40 psi (30 to 50 psi) range. Fiftypercent lethality would be in the 62 psi (50 to 75 psi) range. One-hundredpercent lethality would be in the 92 psi (75 to 115 psi) range. About 100 psi isthe minimum threshold for serious damage (with possible disruption) to humans(11, 12, 32, 34).
In summary, a 35 to 45 psi overpressure may cause 1% fatalities, and 75 to 115psi overpressure may cause 99% fatalities in bomb blasts (11).
Should a forensic pathologist find a human barotrauma victim with disfiguringbarotrauma (in other words, disruption), he/she could theoretically deduce thatthe blast wave must have had a minimum overpressure of approximately 100 psi.Halldorsson et al. believed that this explosion/implosion phenomenon surroundingthe lightning channel may cause trauma which may mimic the patterns of blastinjuries seen in bomb blast victims (29).
Conclusion
Review articles of tympanic membrane rupture in humans in relation to various blastpressure-time patterns suggest that the tympanic membrane can withstand a limitedamount of pressure. Overpressures are required to produce minor, moderate and majoreardrum ruptures. Tympanic membranes tend to rupture more easily in the young forexample. The eardrum may rupture at pressures above 29.0 psi. We can thereforededuce from the otolaryngological literature that lightning's blast wave must have aminimum overpressure of approximately 29.0 psi.
The literature shows that posttraumatic pneumomediastinum is not a cause for alarmamongst our clinical colleagues (28). Pneumomediastinum is not generally considered a fatal injury. Itwould appear that lung damage requires approximately 29.0 to 72.5 psi to induce.
We know that lightning victims do not suffer severe blast-related disfigurement. Tobe fatally injured by a bomb blast, one has to be in the immediate vicinity of theexplosion, within about a meter or so (the “kill zone”). About 100 psi is theminimum estimated threshold for serious damage (disruption) to humans (34).
From the aforementioned results of the literature review, it is now possible toestimate at what range a human would possibly be at risk from lightning's pressureblast wave. A 10 lb (4.5 kg) trinitrotoluene or TNT equivalent bomb would rupture aneardrum (of a 70 kg male) within about 10 meters; lung damage would occur at about 5meters, and the body would be injured at about 3 meters (11).
If one knew the initial conditions of the lightning strike (the thermodynamics andflow parameters as a function of radius at selected instants of time) then one couldpossibly calculate a numerical solution to this thermobaric phenomenon; howeverthere are always varying initial conditions, for example, the magnitude and strengthof the lightning discharge.
In conclusion, it is therefore possible to estimate, based exclusively on thevictim's pathology of barotrauma, the minimum overpressure to which a lightningvictim was exposed and the possible distance from the lightning channel.
Footnotes
Financial Disclosure
The author has indicated that he does not have financial relationships todisclose that are relevant to this manuscript
ETHICAL APPROVAL
As per Journal Policies, ethical approval was not required for thismanuscript
STATEMENT OF HUMAN AND ANIMAL RIGHTS
This article does not contain any studies conducted with animals or on livinghuman subjects
STATEMENT OF INFORMED CONSENT
No identifiable personal data were presented in this manuscsript
DISCLOSURES & DECLARATION OF CONFLICTS OF INTEREST
This work was presented at the 2015 NAME Annual Meeting. The author, reviewers,editors, and publicationstaff do not report any relevant conflicts ofinterest
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