TIANA HERTENSTEIN, M.S., CSCS, and TIMOTHY GRIBBIN, Med, ATC, CSCS;
Clinical Research Associates, Henry M. Jackson Foundation for the Advancement of Military Medicine;
in collaboration with the Consortium for Health and Military Performance (CHAMP)
The use of explosives is an essential part of the military. Bombs, grenades, rockets, breaching charges, mortars and other explosives are crucial components of America’s arsenal — and those of our enemies. When an explosion causes a sudden release of heat, light and sound, the energy causes the air around the point of the explosion to expand outward faster than the speed of sound. The blasts produced by these explosives throw shrapnel, extreme heat and piercing sound at their targets. However, perhaps the most dangerous — yet least understood — effect of an explosion is blast overpressure (BOP).
Overpressure overview
Blast overpressure is one of the most damaging effects of an explosion. Measured in pounds per square inch (psi) or kilopascals (kPa), the rapid wave of high pressure blown outward from the explosion delivers a devastating impact on the surrounding environment. A blast producing as little as 1 psi of overpressure can shatter windows within its blast radius. Explosions producing BOP over 5 psi can destroy concrete walls and buildings.
Injuries from a single BOP exposure don’t typically occur until someone experiences BOP at 5 psi or higher. However, low-level blasts that produce overpressure below the 4-psi threshold can still cause serious injuries. Primary blast injuries are caused by the overpressure wave itself. When the high-pressure wave encounters someone, it violently compresses and decompresses tissue.
- The eardrums, lungs, eyes and gastrointestinal tract are most at risk of rupture and serious damage from smaller blasts.
- The brain can be compressed by the BOP wave as it travels through the skull, potentially damaging brain cells, fracturing the skull or causing bleeding. The blast can also push the brain against the inside of the skull, causing it to rebound violently, which may result in traumatic brain injury (TBI).
- Other body tissues can be badly damaged by extremely high-pressure waves, which can result in amputations or death.
Hazardous exposure to BOP isn’t just caused by explosive blasts. High-caliber firearms, shoulder-fired rocket systems, mortars and artillery pieces may produce significant BOP for the service members firing the weapons. For example, the Carl Gustaf shoulder-fired recoilless rifle can project a “peak” BOP (pBOP) of more than 6.5 psi on the operator. In comparison, a 1/4-kg breaching charge, which is used to explosively open doors, typically exposes a service member to less than 4 psi of overpressure.
Relationship between blast exposure and health problems
BOP intensity is measured in two ways. The pBOP is the maximum pressure level someone feels during the blast. "Impulse" is the length of time someone feels the blast’s effects. Generally, higher explosive yields produce higher pBOP and impulse. Higher levels of either are typically more destructive to targets and people within the range of exposure. But people with the same intensity level of BOP exposure can experience different degrees of symptoms and injuries depending on environmental factors, including distance, angle and blast reflection. For instance, even if you’re standing 10 feet from your buddy when you’re hit by BOP, your injuries could be quite different.
The symptoms of BOP can show up immediately or gradually. In general, a single exposure to a low-level blast (LLB) with BOP of less than 4 psi shouldn’t cause symptoms. However, mild TBI-like symptoms have been reported for single exposures to LLBs below this threshold. Repeated exposures to BOP below the 4-psi threshold, across short periods of time, have been linked to symptoms comparable to those typically seen in victims of larger blasts or in patients with mild TBI. Consistent exposure to LLBs over long periods has been associated with long-term cognitive decline and physical health issues in high-risk populations.
For higher BOP exposures, symptoms generally worsen with the pBOP level. For example:
- 12 psi: Neurological changes and upper respiratory injuries are possible.
- 24 psi: Mild neurological damage is possible.
- 30-40 psi: Moderate neurological and lung damage is possible.
- 40 psi: Lung damage is likely.
- Death is possible at almost any level of exposure that causes injuries. Death is unlikely at lower levels of exposure, but the risk increases as pBOP rises, especially at levels above 100 psi.
Symptoms and health problems linked to BOP exposure can be short-term or long-term. Symptoms from the same BOP exposure can vary by person from a few hours to several weeks or longer, depending on how someone experiences the BOP and their BOP exposure history. Repeated BOP exposure can make things worse. After exposure to repeated LLBs, symptoms such as headache, nausea, concentration issues, memory problems, fatigue, sleep disruption and irritability are common. This exposure makes it especially important to report symptoms after a BOP exposure, not only so you can get some relief, but also to track future symptoms in your medical record.
Once someone sustains a BOP injury, they're more likely to have symptoms with each additional BOP exposure. For example, if you sustain a knee injury, even if you rehab it perfectly, you’re at an increased risk of re-injuring that knee. Essentially, your body is never the same as it was before the injury, making you more susceptible to future injuries. Service members who have spent significant time in high-BOP producing jobs have shown higher levels of brain inflammation, depression, anxiety and susceptibility to further BOP impacts.
BOP and TBI
Cumulative BOP exposure can predict TBI and more severe cognitive symptoms from future blasts. A history of TBI, longer time in the service, high-exposure job fields and more time spent in high-exposure job fields have all been linked to more severe symptoms and injuries from BOP exposure. Repeated exposure to BOP also makes at-risk organs more susceptible to BOP-related injuries at lower thresholds with every subsequent blast exposure. These symptoms and injuries from BOP exposure affect the readiness of individual service members, units and the military.
Service members regularly exposed to explosives or high BOP-producing weapons systems, such as mortar and artillery crews, heavy weapons teams, explosive breachers and explosive ordnance disposal personnel, experience BOP as a regular part of their job and are also more likely to experience TBIs.
Reducing symptoms and injuries
In August 2024, the Department of Defense (DoD) released the requirements for managing brain health risks from BOP in service members. The measures in this DoD memorandum include:
- Minimum standoff distances for personnel participating in training and non-participating audiences.
- Personal protective gear for firers, trainers and all other personnel at risk of BOP exposure during training.
- Minimizing personnel in the vicinity of BOP-producing events.
- Using simulation training instead of actual blasts whenever possible.
- Expanding cognitive baseline testing for all personnel entering the service.
- Identifying and tracking personnel at high risk of BOP exposure and recording exposure data in centralized systems.
- Refining education and training campaigns on the hazards and symptoms of exposure and protective measures.
- Assessing legacy weapons systems currently producing BOP exceeding 4 psi, including warnings and updated user manuals.
Although current understanding of BOP effects is limited and there’s no standardized military protocol yet for documenting or managing BOP exposure or reducing its effects, current efforts by the DoD emphasizes safety across the force and seeks to protect personnel while research on more specific solutions continues. As with any injury, if you have symptoms you suspect might be related to BOP exposure, seek medical care immediately. Ensure your symptoms and visit notes are documented in your medical record.
Bottom line
Education can be a force multiplier in helping prevent BOP-related injuries and improve readiness. Arming service members with information about the potentially harmful short- and long-term effects of BOP exposure can help promote safe practices and build awareness of this invisible threat. If you have questions or concerns about BOP, ask your leadership.
Editor’s note: This article was originally published in the winter 2024 issue of Ground Warrior, the Marine Corps Ground and Naval Expeditionary Warfare safety magazine.