The main application of this membrane filter types is the air / gas filtration. They are made purely of PTFE and are therefore permanently hydrophobic. Unlike other (hydrophilic) filter types, they are not wetted by air humidity, allowing unhindered passage of air also at low differential pressures. PTFE membrane filters have an excellent chemical compatibility, so that they are also used for the filtration of solutions and acids, to which other filter types are resistant. Due to their hydrophobic characteristics, they must be pre-wetted with ethanol or methanol before the filtration of aqueous media.