Overpressure (blast overpressure), is the pressure caused by a shock wave over and above normal atmospheric pressure. The shock wave may be caused by an explosion or sonic boom.
Blast overpressure (BOP), also known as high energy impulse noise, is a damaging outcome of explosive detonations and firing of weapons. Exposure to BOP shock waves alone results in injury predominantly to the hollow organ systems such as auditory, respiratory, and gastrointestinal systems.1, but also can dramatically impact fluid filled organs such as the brain.
Overpressure in an Enclosed Space
The detrimental effects of overpressure are most obviously accentuated in an enclosed space where the pressure cannot readily dissipate and hence the effects are magnified, only partially minimised or at least, period of exposure is maximal.
Overpressure in an enclosed space is determined using Weibull's formula:
2410 is a constant based on 1 bar (100 kPa; 15 psi)
m = net explosive mass calculated using all explosive materials and their relative effectiveness
V = volume of given area
The above table, based on Department of Defense (DoD) data from Glasstone and Dolan (1977)2. and Sartori (1983)3., summarizes the effects of increasing blast pressure on various structures and the human body. This data originates from weapons tests and blast studies to assess the effect of blast overpressure on structures and people.4.
The below graphs show the variation in pressure 'intensity' in an open air explosion as opposed to a closed-space. In short the open air rise quickly to peak and dissipates relatively quickly. The closed-space also rises quickly but the intensity is prolonged even causing 'rebound' type secondary impacts due to reflection, etc.
Defense and Veterans Brain Injury Center (DVBIC) has quoted that more than 50 percent of injuries sustained during the conflicts in Iraq and Afghanistan are the result of explosives including bombs, grenades, land mines, mortar/artillery shells, and improvised explosive devices, or IEDs. Indeed since 2006, blasts have been the most common cause of injury among American and other allied soldiers.
Actual brain related injuries present in may forms ranging form cognitive, performance/behavioural and emotional. These may culminate or present along side depression, anxiety and PTSD.
Current measures are neurocognitive and/or advanced imaging based. Recently the DoD has investigated 'body sensor' type technology whereby the warrior wears a sensor/monitor that physically measures the amount of overpressure they have been exposed to (see image below). The limitation however to this point has been that such sensors only provide a 'semi-quantitative' measure which may or may not equate to level of potential injury sustained.
Research groups such as Glia are investigating biomarkers which are more specific and sensitive and are able to provide a definitive answer so as to improve triage and clinical decision making to ultimately mitigate the long-term effects of undiagnosed and treated TBI.
In the interim, collaborations are being investigated which will combine all technologies with emphasis on the sensors and biomarkers to stratify injury and assist with real time clinical decision making, both on the battlefield and immediate hospital filed settings.
With a changing world health-care providers are increasingly faced with the possibility of needing to care for people injured in explosions, but can often, however, feel undertrained for the unique aspects of the patient's presentation and management. Although most blast-related injuries (eg, fragmentation injuries from improvised explosive devices and standard military explosives), can be managed in a similar manner to typical penetrating or blunt traumatic injuries, injuries caused by the blast pressure wave itself cannot. The blast pressure wave exerts forces mainly at air–tissue interfaces within the body, and the pulmonary, gastrointestinal, and auditory systems are at greatest risk. Arterial air emboli arising from severe pulmonary injury can cause ischaemic complications—especially in the brain, heart, and intestinal tract. Attributable, in part, to the scene chaos that undoubtedly exists, poor triage and missed diagnosis of blast injuries are substantial concerns because injuries can be subtle or their presentation can be delayed. Management of these injuries can be a challenge, compounded by potentially conflicting treatment goals.5
1. Pulmonary Biochemical and Histological Alterations after Repeated Low-Level Blast Overpressure Exposures", Nabil M. Elsayed, and Nikolai V. Gorbunov, Toxicological Sciences, 2007 95(1):289-296
2. Glasstone S, Dolan PJ, eds. . The effects of nuclear weapons. 3rd ed. U.S. Department of Defense and the Energy Research and Development Administration.
3. Sartori, L. . The effects of nuclear weapons, Physics Today, March, pp. 32-41.
4. ARMY Manual TM 5-1300 . Department of the Army, Washington, DC, 1990
5. Stephen F Wolf, et al. Blast Injuries, The Lancet Vol 374, No. 9687, p405–415, 1 August 2009