A semi-permeable membrane located inside the plug allows air and humidity to leave the enclosure. In the opposite direction, it allows only dry air into the enclosure while humidity and dust from the outside are blocked by the membrane. The stainless steel model is corrosion-resistant and thus suited for the food industry or for industries that use electrical/electronic control panels. This product is RoHS and CE compliant.Omega Engineering Inc.www.omega.com203.359.1660