Overpressure

... or whether the bronchiectatic cavity is due principally to a yielding of the bronchial walls to the inspiratory and expiratory pressure, to the overpressure of the engorging secretion, ...

The walls facing the explosion will experience a reflected overpressure.

Figure 5-19 shows a breather valve that carries separately a direct-loaded vacuum relief value and an overpressure relief valve.