## 3 Results  
### Overpressure Test  
From this test, reflected pressure (\(P_{R}\)) measured by the transducer is obtained. From these data, it is possible to calculate the incident pressure (\(P_{i}\)), which is the actual shockwave pressure of the pyrotechnic articles, by using the following equation [24]:  
\[P_{R}=2P_{i}.\frac{7P_{o}+4P_{i}}{7P_{o}+P_{i}}, \tag{1}\]  
where \(P_{o}\) represents the atmospheric pressure. It has to be remarked that both the reflected pressure and the incident pressure do not take into account the shockwave generated by the electric igniter, which is known due to the previous tests carried out. The reflected-pressure-obtained data are consistent with the predictions obtained using the mathematical model of the manual TM 5-1300 [25] for black powder. Figure 5: Diagram of the layout of the tests inside the shed. Incident pressure refers to the initial shockwave generated by the explosion. This wave is created by the rapid expansion of gases within the confined space, which creates a sudden and powerful burst of pressure that travels outward from the point of ignition. Reflected pressure, on the other hand, is a secondary wave that is generated when the initial shockwave encounters a boundary, such as a wall or floor, within the confined space. This reflected wave travels back towards the point of origin and can interact with the initial incident wave, amplifying or altering its effects. The probability that stored pyrotechnic products explode altogether will depend on many factors, including packaging type, fuse, arrangement of articles in the container or packaging, presence of products that ignite and can spread fire or the type of product. Previous tests carried out in the open air showed that the products known as flash bangers with green fuse and fountains propagated the explosion when an article is ignited; however, no mass explosion was observed. This behaviour is different when the tests are carried out in enclosed spaces, as in the case study, where it is observed that the behavior of the flash bangers with green fuse differs and generates a mass explosion in any studied case. This behavior is predicted by the tests developed by the NATO working group AC/258 [17], which proved that some pyrotechnic products classified as 1.4 G might lead to a mass explosion when confined in enclosed areas. The different mass explosions that occurred during the tests are represented in Figure 6, which collects the data for reflected pressure and incident pressure (kPa) vs. charge density (kg/m\({}^{3}\)). These data are fitted to potential pressure curves. Nevertheless, more detailed information can be found in the Supplementary Materials. Due to the non-existence of pressure spillways in the stores for pyrotechnic products sale, the ignition of certain articles produces the explosion and rupture of the warehouse structure, mainly due to the effect of the pressure reflected on the internal walls of the structure and the pressure of the gases generated and contained within it. Pressure spillways are structures designed to relieve pressure within a confined space. Without these spillways, the pressure generated by an explosion or ignition can cause significant damage to the structure and even lead to its complete rupture. From the data plotted in Figure 6, it can be noticed that incident pressure is significantly greater than reflected pressure, and, the greater the mass density, the greater the difference between both. The incident pressure wave can cause damage to nearby structures and objects and can also be harmful to human beings if they are within close proximity of the explosion. Nevertheless, reflected pressure can be particularly dangerous in confined  
Figure 6: Effect of the charge density (kg/m\({}^{3}\)) on the reflected and incident pressure of the shockwave generated by the mass explosion of pyrotechnic articles. spaces as it can cause additional damage to the surrounding environment and increase the risk of injury or death.