Tissues (textiles, skin, mucous membrane) and materials in contact with casutic media undergo a change which can extend to their destruction. The corrosive action can become even more intense in the presence of atmospheric oxygen or moisture. Caustic media with vapour pressure or in the gaseous state usually require double mechanical seals pressurized with buffer fluid. With liquid caustic media it is also essential to prevent leakage into the environment. The seal is thus made either by a single mechanical seal with quench (tandem arrangement) or by a double mechanical seal; for some machines (acid pumps) single, external mechanical seals are also used occasionally. The medium's other properties, the operating conditions and the available stuffing box are further key factors in the choice of a particular mechanical seal type, arrangement and material.