Distinguishing realistic military blasts from firecrackers in mitigation studies of blast-induced traumatic brain injury

In their Contributed Article, Nyein et al. (1, 2) presented numerical simulations of blast waves interacting with a helmeted head and concluded that a face shield may significantly mitigate blast-induced traumatic brain injury (TBI). A face shield may indeed be important for future military helmets, but Nyein et al. (1, 2) derived their conclusions from a much smaller explosion than typically experienced on the battlefield.

The blast from the 3.16-g trinitrotoluene (TNT) charge in ref. 1 had the following approximate peak overpressures, positive-phase durations, and incident impulses (3): 10 atm, 0.25 ms, and 3.9 psi-ms at the front of the head (14 cm from charge) and 1.4 atm, 0.32 ms, and 1.7 psi-ms at the back of a typical 20-cm head (34 cm from charge). The peak pressure of the wave decreases by a factor of seven as it traverses the head. The blast conditions are at the threshold for injury at the front of the head but well below threshold at the back of the head (4). The blast traverses the head in 0.3 ms, roughly equal to the positive-phase duration of the blast. Therefore, when the blast reaches the back of the head, near-ambient conditions exist at the front. Because the head form is so close to the charge, it experiences a wave with significant curvature.

By contrast, a realistic blast from a 2.2-kg TNT charge (approximately an uncased 105-mm artillery round) is fatal at an overpressure of 10 atm (4). For an injury level (4) similar to that shown in ref. 1, a 2.2-kg charge has the following approximate peak overpressures, positive-phase durations, and incident impulses (3): 2.1 atm, 2.3 ms, and 18 psi-ms at the front of the head (250 cm from charge) and 1.8 atm, 2.5 ms, and 16.8 psi-ms at the back of the head (270 cm from charge). The peak pressure decreases by only a factor of 1.2 as it traverses the head. Because the 0.36 ms traversal time is much smaller than the positive-phase duration, pressures on the head become relatively uniform when the blast reaches the back of the head. The larger standoff implies that the head form locally experiences a nearly planar blast wave. Also, the positive-phase durations and blast impulses are much larger than those in ref. 1.

Consequently, the blast model used in ref. 1 is spatially and temporally very different from a military blast. It would be useful to repeat the calculations using military blast parameters.

Finally, ref. 1 overlooked a significant part of the information in ref. 5. On pages 1 and 3, Nyein et al. (1) stated that Moss et al. (5) did not consider helmet pads. However, pages 3 and 4 of Moss et al. (5) presented simulations of blast wave propagation across an advanced combat helmet (ACH) helmeted head form with and without pads. Moss et al. (5) stated that, when the pads are present, the underwash of air under the helmet was blocked compared with the case without pads. Nyein et al. (1) reached this same conclusion but reported it as a new result rather than a confirmation of information already found in ref. 5.