Exhalation valve

The exhalation valve acts like a check valve, permitting air to flow out of the respirator facepiece, and preventing reverse flow through the valve on inhalation. ... Dry exhalation valves and valve seats will be subjected to a suction of 25 mm water-column height while in a normal operating position.

A respirator is a device designed to protect the wearer from inhaling hazardous atmospheres, including fumes, vapours, gases and particulate matter such as dusts and airborne microorganisms. There are two main categories: the air-purifying respirator, in which respirable air is obtained by filtering a contaminated atmosphere, and the air-supplied respirator, in which an alternate supply of breathable air is delivered. Within each category, different techniques are employed to reduce or eliminate noxious airborne contaminants.