Blast, burns, asphyxia and hypoxia: how do pyroclastic surges damage humans?


P J Baxter, Medical effects of volcanic eruptions: 1. Main causes of death and injury. Bulletin of Volcanology (1990) 52: 532-544.


P J Baxter, A Neri and M Todesco, Physical modelling and human survival in pyroclastic flows. Natural Hazards (1998) 17: 163-176.




The history of how pyroclastic surges have affected humans is dominated by two catastrophes: the destruction of the cities of Pompeii and Herculaneum, Italy, and their inhabitants by Vesuvius in 79 AD, and the obliteration of St Pierre, Martinique, and its population by Mt Pelée in 1902. These and several other similar disasters have convinced most people that the only sensible attitude towards a risk of a pyroclastic surge is total evacuation from any areas where they might strike. Unfortunately the reality of living with volcanoes is different from the ideal concept of separating people from surges. Although the damage and fatalities around Mt Merapi, Indonesia, in 2010 was a new tragedy, it has also provided us with a very detailed set of digital images illustrating both this topic and the previous one (see Topic 16 below).


The 1980 eruption of Mt St Helens was the first of several incidents in recent years where people were engulfed in surges and a fraction of those near their margins lived to tell the tale. The medical records of both survivors and those who were unlucky have built up a more detailed picture than previously of how surges damage people. In 1990 Baxter summarised what was known then about the medical aspects of this topic, publishing in an Earth Sciences journal (Bulletin of Volcanology, 52: 532-544), where it was easily accessible to those concerned with volcanic hazards. He updated this information in the report in the journal Natural Hazards that is the main source for this summary. In addition, the Natural Hazards report contains results of fluid dynamic modelling by Neri and Todesco which considers several surge-forming eruption scenarios and the sorts of injuries that they might cause to humans.



How do surges damage humans?




If a pyroclastic surge hits a community with sufficient force to destroy houses and topple trees, there is clearly little chance for survival of a human caught outside in such conditions. As with hurricanes, a less violent surge may knock a person down without doing serious damage but the situation becomes much more dangerous if debris is flying about.





The heat of a surge is carried by both the gas (air plus gases released by the lava dome collapse or explosion) and the volcanic ash whirling around in it. The time a person might normally be exposed to a passing surge is set at about 2-5 minutes by Baxter, Neri and Todesco. The eyewitness accounts of survivors from the 25 June 1997 Montserrat surges suggest a wider range of times, between about 1 and 10 minutes (Report by Loughlin and others in the 2002 Geological Society of London book about the Montserrat eruption). Humans in ordinary clothes can remain unharmed in dry and still air at 200-250oC for up to about 5 minutes. Damp air is much more damaging to skin than dry air. This is why the air must be dry in very hot saunas at 80-120oC.


Surges move at hurricane speeds, except when they run out of gravitational energy, and they contain volcanic steam. If the surge temperature is above about 200oC, a person caught in the open will receive very extensive skin burns, both on exposed skin and under any light clothing. Remember from the section on surge damage to buildings that the 1997 Montserrat surges reached 300-400oC. Temperatures around 500oC, causing instant death, have been deduced for parts of Herculaneum in 79 AD, according to Mastrolorenzo and others, Nature (2001) 410:769-770.


If air is hot enough to burn skin, it takes no imagination to see how damaging such air will be when breathed in through the nose, mouth and windpipe into the lungs. Severe scorching of the linings of the throat and windpipe may cause them to swell so much that air can no longer pass through them. In some cases people in this condition have been saved by emergency tracheotomy; inserting a tube into the lower windpipe – hopefully bypassing the obstruction. But deeper damage and swelling (oedema) inside the lung can cause this essential part of the body to stop functioning.


All these effects of hot air on humans are made far worse inside a surge by the hot volcanic dust there. If the surge moves very fast, the coarsest ash fragments are gravel. At any speed, the smallest dust particles in a surge are so small (100 microns or less; micron = micrometre = one thousandth of a millimetre) that they can pass straight into the lungs if inhaled. The situation is even worse if the air is water saturated because this will also burn tissue at temperatures above only about 60oC.


In addition the ash plays another nasty trick when it settles to form a thin layer on the ground. This layer remains extremely hot for up to days after it forms. The result during the 25 June 1997 Montserrat surges was that several people who escaped severe injury inside buildings and cars then suffered terrible burns to their feet while running to safety.





Dust flying around in a cloud is extremely dangerous to people, if its concentration is more than 0.1 kg (100 grams) per cubic metre. The dust clogs the nose, throat and windpipe and its smallest grains clog the lungs. The result is asphyxia (choking, suffocation). This is a frequent cause of death when earthquakes reduce buildings to rubble (Hogan and Burstein, Disaster Medicine, 2006, Lippincott Williams & Wilkins) and some people died this way in the dust cloud that was generated in 2001 by the collapse of the Twin Towers in New York. Natural dust storms have also been known to asphyxiate people.


The problem with pyroclastic surges is that dust asphyxia can remain a deadly threat at the margins of a surge event – even when both air and ash are cooled to temperatures where they cannot cause serious burns. This was how many people were killed at the margins of the 18 May 1980 lateral blast at Mt St Helens. In the second part of their 1998 report Baxter, Neri and Todesco showed the results of mathematical 2-D simulations of relatively small explosive eruptions of Vesuvius where the resulting surge (pyroclastic density current) ran outwards for up to 15 minutes across the gently inclined lower slopes of the volcano. By then the front of the surge was cooled well below 200oC but it still contained lethal concentrations of dust.



Hypoxia and poisonous volcanic gases


Humans have evolved to breathe a mixture of about 78% nitrogen, 21% oxygen and very little of anything else. The gas mixture within the dusty cloud of a pyroclastic surge is radically different from “normal air”. Flying dust will dilute, and thus reduce, the oxygen concentration and water vapour and carbon dioxide may do the same in some instances. If the surge is hot enough, much of the remaining oxygen may be consumed by burning fabric and wood etc. All these factors leave too little oxygen around for humans and this has had fatal consequences in some surge events.


There are also suspicions that some of the poisonous gases emitted from volcanoes, such as sulphur dioxide and hydrogen sulphide, may have contributed to the deaths of people in surges. But this hazard is hard to disentangle from all the others in accounts of lethal surges.



What to do?


There is only one thing to do when warnings and evacuation plans have failed and you are faced with an approaching surge. There are so many ways in which a surge cloud can harm a human that the only sensible action must be to try to hide for the few crucial minutes in a place where there is enough oxygen and the heat and dust are kept out. Obviously a strongly built room with secure doors and shuttered windows is best (as at MVO). Failing this, maybe immersion in a deep (or even shallow!) swimming pool might be better than nothing; remember to pull your top over your head each time you pop your head up to breathe. See more about this at the end of Topic 16.


Within communities such as the Salem area, the traditional hurricane shelters in churches, schools, police stations etc are potential surge shelters (especially if strengthened in the light of surge damage to similar buildings in the south of the island). The problem is that planned use of hurricane shelters to escape an approaching surge is almost certainly not going to happen because, if such a hazard is thought to be likely, government-enforced evacuation will be sure to take place (following the 1997 tragedies).


The risk to Salem and its surrounding settlements is the tiny but non-zero one of a surge sweeping through the area without enough warning to trigger evacuation (as hinted at by the SAC14 preliminary statement on the MVO website in March 2010). This is the same situation as has been faced by people in some villages below the volcano Merapi, Indonesia, for many years. Because the Salem region is much further from the Soufrière Hills summit lava dome than Streatham village, Dyer’s, Tuitt’s and Harris etc, the temperature at the margins of a surge reaching Salem should be a bit cooler than the 25 June 1997 events, as shown by the 3-D modelling results summarised in the Scientific Advisory Committee Main Report 8 in March 2007. Further updates to this paragraph continue at the end of Topic 10.