IED Awareness: The Blast Zone
In this companion piece to my previous article, IED Awareness for First Responders, I cover the basics of what every first responder should know about the area around a bomb which is affected by the blast. This area is known as the blast zone.
Many are only mistakenly concerned about shrapnel, those small, solid bits and pieces of a bomb, which radiate out from the epicenter of a blast, that can kill and maim. However, there is far more to be concerned with. Many injuries and deaths associated with bombings are a result of other forces within the blast zone. Certainly, shrapnel is of great concern, as it can travel great distances at very high velocity. But one must also consider other forces within the blast zone, namely heat and over-pressure.
One characteristic behind explosives is that they instantly release great amounts of energy. Physics tells us any time that happens; a vast amount of heat is generated and radiates outward from the blast. This is why detonation is always accompanied by a flash of light, the telltale sign of rapid energy release. The heat from this energy release can reach over one thousand degrees, and can cause severe injury and death to anybody within the blast zone. A secondary effect from such blasts is the ignition of flammable material in the area. So, first responders are not only immediately concerned with the injured, they must often contend with controlling resultant fires.
Over-pressure is an invisible force which radiates out from a blast. The laws of physics also tell us that when something is heated up, it expands. The same holds true of air in the blast zone. This expansion of air radiates out from the blast at great speed. The leading edge of this movement of air is called the shock wave. As the shock wave passes by, it creates an extremely rapid increase in air pressure. If large enough, this over-pressure can knock down walls or buildings not built to resist such pressure. According to the Department of Defense, even 1 psi of over-pressure can shatter windows. When people are hit by the shock wave, the sudden increase in air pressure can not only knock them down, but can cause massive internal injuries. As little at 5 psi over-pressure can rupture eardrums, and a 15 psi increase will begin to cause lung damage. An over-pressure of 35–45 psi has been shown to cause death in 1% of people, where an over-pressure between 55–65 psi will result in fatalities in 99% of people.
Another phenomenon associated with over-pressure is what happens when normal air pressure returns to the blast zone. After the blast wave propagates out from the blast, it leaves a void in the air pressure. As the air pressure rapidly returns to normal, it causes a reverse wave of pressure which can cause additional secondary blast effects. As the pressure wave moves back, it can drag objects, such as broken glass and other debris toward the blast epicenter, causing further injuries and damage. This process of over-pressure moving out, and the subsequent return to normal air pressure in the blast zone, often takes only fractions of a second to occur.
It should be noted that blast effects occurring in enclosed areas, such as a building or bus, are greatly enhanced due to the effects of containment. Objects such as walls or shaped charges, which direct blast waves, can also increase the lethality and damage potential of IEDs.
How close one can safely be to an explosion depends on many factors. The amount and type of explosive charge used, composition of shrapnel, and the effects of over-pressure all factor into the equation. The chart below from the Department of Homeland Security provides some general guidelines on evacuation distances for various types of IEDs.
The distances in the chart above are minimum distance recommendations. It can be reasonably stated to get as far away as practicable from any potential IED. A good rule of thumb is that if you can see the bomb, the bomb can see you. Don’t stick around thinking you may get to witness a cool explosion and then duck out of the way just in time to avoid injury. Explosion velocities of some compounds are measured in thousands of feet per second. There is no way you could react fast enough. If shrapnel doesn’t maim or kill you, the over-pressure may.
This article presents only very basic information regarding blast zone awareness. There are a number of resources available on the internet and through your local library that provide an awareness of IEDs and blast zones. The Department of Homeland Security is an excellent on-line resource to learn more. You can follow this link to their Office of Bombing Prevention web site: http://www.dhs.gov/obp for more information.
The best resource I have encountered for first responders is the Incident Response to Terrorist Bombings (IRTB) course, offered by the Department of Homeland Security, through New Mexico Institute for Mining and Technology in Socorro, New Mexico. The IRTB course is offered free of charge, inclusive of travel, by the federal government. It offers intensive classroom instruction and live demonstrations of various explosives. For more information, follow this link http://www.emrtc.nmt.edu/training/irtb.php. You can also contact the state point of contact (SPOC) in your respective state who can provide you with information and the application necessary to register and attend the training. You can locate your respective SPOC by following this link http://www.emrtc.nmt.edu/training/statepoc.php.
Robert Leverone is a contributor to the Homeland Security (HS) Vortex, which is a platform where insiders from the policy, law enforcement, fire service and emergency management fields converge to discuss issues related to Homeland Security.
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