The ASHRAE 52.2, "Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size," corrects many of the shortcomings of the previous standard, ASHRAE 52.1, which provided an overall air filter efficiency percentage, but did not take into account the size of the particles being filtered. The revised standard provides specific efficiencies relative to particle size as a Minimum Efficiency Reporting Value (MERV) at a given air-flow rate.

To measure the MERV of an air filter, it is mounted in a test duct and run at the air-flow rate specified by the manufacturer. Solid particulates from 0.3 to 10 microns are introduced into the test system; and as air circulates through the test system, the filter catches some of the solid particles. A laser particle counter alternately samples the air upstream and downstream from the filter. When sampling is completed, the two sets of laser particle counts are compared to calculate the Particle Size Efficiency (PSE) of the filter.

 

Chemist’s Note: I welcome the improved particulate standards, however it is well to remember that:    - most indoor air problems are caused by unfilterable molecules, not particles    - manufactured MERV 8 to 16 filters are expensive, the media is not molecules are roughly 0.001 to 0.01 microns viruses are approximately 0.2 microns bacteria range from 0.3 to 4 microns allergens such as pollen and fungal spores are at least 3 to 10 microns plant or animal cells are 10 to 30 microns human hair is approximately 80 microns