Response to McEvoy et al.,2023,Cumulative Blast Exposure Estimate Model for Special Operations Forces Combat Soldiers

To the Editor:

We would like to recognize McEvoy and colleagues’ ¹ efforts to create a practical algorithm for documenting occupational blast-exposure (OBE) during military service. OBE is poorly understood but believed to have significant impacts on long-term health. We appreciate the user-friendly design to reduce excessive burden on service members. However, we noticed potential problems with the explosive weights listed. The net explosive weights (NEW) described by McEvoy and colleagues ¹ appear to be based on total weight, and therefore not representative of PSI-values described. NEW, as defined, is limited to the explosive filler. The differences between 0.92 lbs NEW and 0.05 lbs NEW when operating at close minimum safe distances (MSD) is substantial.

These errors carry many implications. First, study replication with the weights as described would result in higher PSI-values and potentially place people at risk of serious injury. Second, the NEW needed to create the overpressure values outlined was significantly less, indicating how little explosives are needed to generate significant overpressure, and thus, highlighting the small margin of error faced by explosive workers. When considering cognitive decline relative to OBE, the authors offer a cumulative process to create an overall PSI-value. We suggest tracking the number of events contributing to the PSI-value as vital context for understanding vulnerability thresholds and the long-term impact of occupational blast overpressure.

Finally, the authors suggested jelly charges as a safer option for breachers due to lower PSI-values recorded during their study. We strongly caution against suggestions regarding preferred explosive setups without addressing the techniques being used. Jelly charges are tamped, directional charges representing a technique, rather than a guaranteed outcome. Breacher charge selection, like all explosive setups, should be outcome and technique driven rather than based on generalized rules or application derived from PSI-values and perception of safety. Ultimately, safe explosive handling practices rely more on education and application of the techniques to mitigate hazards than on NEW and MSD alone. Continuing education and consultation with explosive ordnance disposal technicians may offer opportunities for breachers to gain a deeper understanding of application of explosive techniques to better mitigate risk over time.

The following NEW corrections are based on the charges as described by the authors, using 7000 gr/lb, and 1.66 relative effectiveness factor (RE-factor) for pentaerythritol tetranitrate (PETN) based on explosive fillers for common breaching charges to calculated trinitrotoluene equivalent (TNTe) NEW quantity-distances (QD). ² Initiation was not addressed but the standard NEW of blasting caps often used by breachers is approximately 1.452 g/cap (approx. 22.41 grains or 0.05 oz) of PTEN. In addition, it appears TNTe was not factored in the original study. Conversion to TNTe is a standard approach to determining NEWQD and often offers increased protection. ² Therefore, we offer corrected NEWs using the recommended RE-factor standards for breaching operations (see Table 1).