Comparison of manufacturers' specifications for 44 types of heat and moisture exchanging filters

Although heat and moisture exchanging filters (HMEF) are recommended for use during anaesthesia, the criteria for choosing a filter are not clearly defined. Manufacturers offer many different types of HMEF with various technical characteristics. We compared the technical specifications provided by the manufacturers for different types of HMEF.

Filter manufacturers were asked to provide technical information. Additional information was obtained from websites. Information about 44 filters (16 mechanical and 28 electrostatic) was collated.

Filter performances were estimated with different sizes of microorganism and durations of challenge. Twenty-eight filters had not been tested by independent laboratories. For 12 of the filters, information obtained from websites and from the manufacturers differed. Most filter specifications claimed high efficiency, particularly for filtration, microbial challenge number and test duration. Electrostatic filters used in anaesthesia were claimed to have high filtration efficiency, similar to the efficiency provided by mechanical filters. Excluding moisture output values did not alter the general conclusions.

Technical aspects of the tests, international standards, and independent validation should be considered when a filter is chosen.