Blast from explosives causes a spectrum of blast injuries from the direct overpressure from an explosion, the negative pressure following the blast wave, or effects of these pressure changes. Formally, there are four classes of blast injury:
- Primary blast injury is tissue damage by the pressure changes caused blast wave itself — barotrauma — most common with tissue-gas interfaces such as the lungs, the intestines, and the tympanic membrane.
- "Secondary injury refers to penetrating or blunt injury that results from the acceleration of shrapnel or debris"
- Tertiary injuries result from acceleration-deceleration forces imposed as the blast wind propels the victim. As the body is tumbled on a rigid surface, it suffers from blunt injury, in particular closed head injury, as well as penetrating injuries as it is accelerated over sharp debris.
- A fourth mechanism includes flash and flame burns, inhalational injury, and crush injuries incurred from fires and structural collapse.
Secondary and tertiary injury overlap significantly, and both are more common than primary blast injury. However, primary blast injuries are the most severe."
Primary blast injury
Many victims of primary blast injury will not survive to reach a hospital.
Spalling is the mechanism of the most severe primary lung injuries. In the specific context of lung injury, it takes place when a blast wave moves from higher-density solid tissue (e.g., bronchi) to air-filled areas of lower density, causing increased tension at their interface. Particles are hurled from the more-dense into the less-dense, tearing the wall between them and causing hemorrhage, edema, and the loss of structural integrity.
On examination, signs can vary from bruising of the chest to massive bleeding from the mouth and nose, difficult breathing and poor oxygenation, and air leak, leading rapidly to death. The injury may be asymptomatic at first, and can be diagnosed only with imaging such as a chest X-ray or computed X-ray tomography scan, which may not be available in large disasters. Patients with a ruptured eardrum but no other symptoms should receive priority for imaging. 
Symptomatic injury tends to develop early, and includes "pulmonary contusion, systemic air embolism, and free radical–associated injuries such as thrombosis, lipoxygenation, and disseminated intravascular coagulation (DIC). ARDS may be a result of direct lung injury or of shock from other body injuries." 
Respiratory failure is often due to secondary additive effects such as shock, organ failure, or inhalation of smoke and toxic substances.
Secondary blast injury
While primary injury is the most lethal, secondary injury is often the most prevalent, such as in the Oklahoma City bombing. Nonfatal secondary injury was four times as common as fatal injury in that event. 
Tertiary blast injury
Traumatic brain injury is increasingly common, especially among military personnel whose improved vehicle armor and personal protective equipment lets them survive blasts that previously would have been fatal. 
Quaternary blast injury
Of the four categories, these are the least unique to explosions, with similarity to "routine" trauma such as car accidents. The most significant exceptions are when blast injuries from the first three categories coexist, or if there is poisoning from explosive or incendiary residues. Crush injuries are much more common after explosions than after routine accidents, although they are seen in cave-ins and building collapses.
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- D. Dhar (2007), "Terrorist Blast Injuries", Internet Journal of Rescue and Disaster Medicine 7 (1)
- Leibovici D, Gofrit ON, Shapira SC. Eardrum perforation in explosion survivors : is it a marker for Pulmonary blast injury ? Ann. Emerg. Med 1999; 34: 168-72, cited by Dhar 2007
- Andre Pennardt, Eric Lavonas (Jun 23, 2009), "Blast Injuries", eMedicine
- , Chapter 7. Blast Terrorism, Pediatric Terrorism and Disaster Preparedness, Agency for Healthcare Research and Quality, U.S. Department of Health and Human Services
- Mallonee S, et al. (7 August 1996), "Physical injuries and fatalities resulting from the Oklahoma City bombing", JAMA 276(5): 382-7.
- Eddie Chaloner (2005), "Editorial: Blast injury in enclosed spaces", BMJ 331: 119-120, DOI:10.1136/bmj.331.7509.119
- Katherine H. Taber, Deborah L. Warden, Robin A. Hurley (May 2006), "Blast-Related Traumatic Brain Injury: What Is Known?", J Neuropsychiatry Clin Neurosci 18: 141-145, DOI:10.1176/appi.neuropsych.18.2.141