Abstract: A pressurization and filter installation for shelters, notably of the substantially horizontal tubular tunnel type, comprises at least one entry-exit section communicating with a respective entry-exit airlock. This is adapted to be pressurized, in use, to a pressure higher than the normal external pressure. There is an armored door on the outside of the airlock. There is a sealed door on the inside of the airlock, controlling access to the shelter interior. The shelter interior is pressurized to a pressure higher than that in the airlock. Filter units are disposed in the airlock. Each filter unit comprises at least one absolute filter and at least one activated carbon filter. The shelter provides protection against the effects of nuclear, biological and chemical weapons.