An overpressure protection system is one designed to protect an individual or group of individuals in a chemical, biological, radiological and nuclear (CBRN) environment.
This pressure differential means that any flows of air will always be from the safe area to the outside, preventing the ingress of toxins.
The safe area may be as small as a protective hood for an individual, to a full body hazmat suit, to a fallout shelter or warship.
Most modern armoured fighting vehicles will have such a system with the safe area being the crew and passenger compartments, these systems being first adopted to protect against poison gas attack.
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