Primary blast injury is that due to the overpressure of the blast.
Primary blast overpressure can travel through the abdomen, ears, eyes, nose, neck and mouth. The brain, lungs, intestines, eardrum, and peripheral vestibular system are particularly vulnerable to primary blast overpressure.
Study by Sandlin et. al 2018 isolated blast overpressure waves to the ear canal. Blast exposed rats had rapid, shallow breathing, slow heart rate, and low blood pressure (tachypnea, bradycardia, and hypotension).
The authors noted that if this exposure and response were to occur in humans, it would likely result in passing out.
Some rats in this study who were exposed to blasts higher than 50 PSI (345 kPa) stopped breathing immediately and died.
Findings support the idea that blast overpressure energy can be transmitted through the ear and even cause brain injury. While the ear isn’t the only way for blast overpressure to be transmitted to the brain, hearing protection plays an important role in limiting ear-conducted blast-induced neurotrauma.
References:
S Sandlin, D. S., Yu, Y., Huang, J., Zhang, C., Arteaga, A. A., Lippincott, J. K., … & Zhou, W. (2018). Autonomic responses to blast overpressure can be elicited by exclusively exposing the ear in rats. Journal of Otology, 13(2), 44-53.