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:

∆p=(m/V)0.72

where:

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 visible and invisible impacts of blast.

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.

'Open' vs. 'closed' space blast exposure.

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.

The Defence and Veterans Brain Injury Center (DVBIC) reports that more than 50% of injuries sustained by soldiers in the Iraq and Afghanistan conflicts are the result of explosive devices, including bombs, grenades, landmines, mortar and artillery shells, and improvised explosive devices (IEDs). Since 2006, blast injuries have been the leading cause of harm among American and allied military personnel.

Brain injuries from such blasts can manifest in various forms, including cognitive impairments, behavioural changes, and emotional disturbances. These injuries often occur alongside mental health conditions like depression, anxiety, and post-traumatic stress disorder (PTSD).

Current diagnostic approaches primarily rely on neurocognitive assessments and advanced imaging techniques. Recently, the Department of Defense (DoD) has explored the use of "body sensor" technology, where soldiers wear sensors to monitor the overpressure exposure from blasts (see image below). However, these sensors currently provide only a "semi-quantitative" measurement, which may not accurately reflect the severity of potential brain injury.

Research groups, such as Glia, are investigating more specific and sensitive biomarkers that could provide clearer insights into brain injuries. These biomarkers aim to enhance triage and clinical decision-making, helping to identify injuries earlier and mitigate the long-term effects of undiagnosed and untreated traumatic brain injury (TBI).

In the meantime, collaborations are underway to integrate multiple technologies, focusing on sensors and biomarkers, to better assess the extent of injury. These efforts aim to support real-time clinical decision-making, both on the battlefield and in immediate medical settings, improving the care and treatment of injured soldiers.

Civilian Populations

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

References

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. [1977]. The effects of nuclear weapons. 3rd ed. U.S. Department of Defense and the Energy Research and Development Administration.

3. Sartori, L. [1983]. The effects of nuclear weapons, Physics Today, March, pp. 32-41.

4. ARMY Manual TM 5-1300 [1990]. 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

References

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