The blast wave progresses from the source of the explosion as a sphere of compressed and rapidly expanding gases, which displaces an equal volume of air at a high velocity (Rossle, 1950). The velocity of the blast wave in air may be extremely high, depending on the type and amount of the explosive used. The blast wave is the main determinant of the primary blast injury and consists of the front of high pressure that compresses the surrounding air and falls rapidly to negative pressure. It travels faster than sound and in a few milliseconds damages the surrounding structures. The blast wind following the wave is generated by the mass displacement of air by expanding gases; it may accelerate to hurricane proportions and is responsible for disintegration, evisceration, and traumatic amputation of body parts. Thus, a person exposed to an explosion will be subjected not only to a blast wave but to the high-velocity wind traveling directly behind the shock front of the blast wave (Rossle, 1950). A hurricane-force wind traveling about 200 km/h exerts overpressure of only 1.72 kPa (0.25 psi), but a blast-induced overpressure of 690 kPa (100 psi) that causes substantial lung damage and might be lethal travels at about 2,414 km/h (Owen-Smith, 1981).

The magnitude of damage due to the blast wave depends on the peak of the initial positive-pressure wave (an overpressure of 414–552 kPa or 60–80 psi is considered potentially lethal), the duration of the overpressure, the medium of the explosion, the distance from the incident blast wave, and the degree of focusing due to a confined area or walls. For example, explosions near or within hard solid surfaces become amplified two to nine times because of shock-wave reflection (Rice and Heck, 2000). Moreover, victims positioned between the blast and a building often suffers 2–3 times the degree of injury of a person in an open space. Indeed, people exposed to explosion rarely experience the idealized pressure-wave form, known as the Friedländer wave. Even in open-field conditions, the blast wave reflects from the ground, generating reflective waves that interact with the primary wave and thus changing its characteristics. In a closed environment (such as a building, an urban setting, or a vehicle), the blast wave interacts with surrounding structures and creates multiple wave reflections, which, interacting with the primary wave and between each other, generate a complex wave (Mainiero and Sapko, 1996; Ben-Dor et al., 2001) (Figure 2.4). Table 2.2 summarizes the effects of different levels of overpressure on material surrounding the explosion and unprotected persons exposed to blast.

FIGURE 2.4 Explosion-induced shock waves: (a) idealized representation of pressure-time history of an explosion in air; (b) shock wave in open air; (c) complex shock-wave features in closed or urban environment.

SOURCE: Mayorga, 1997. Reprinted with permission from Elsevier Science, Ltd. 2008.