In the past, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards defined air filter efficiency as the percentage of airborne particles larger than 1 micron captured by a filter; for example, 30 percent, 80 percent or 95 percent.

The new ASHRAE Standard 52.2 — which defines the MERV scale — assigns a value for the filter’s minimum efficiency in capturing particles at six progressive stages of dust-loading with particles in the range of 0.3 to 10 microns. Comparing the new standard with the old, for example, a filter rated MERV 8 will capture roughly 30 percent of 1-micron particles at maximum dust-loading; MERV 13, 80 percent; and MERV 15, more than 95 percent.

A filtration system consists of a filter frame, filter pack, separators, filter media and sealants. Media are the filtering materials, usually paper, that is pleated to increase surface area and fit into the desired space. Separators support the filter media and provide channels through which the air can flow to reach the filter. The filter pack contains the media and separators.

Sealants create a leak-proof seal between the filter pack and the filter frame, which contains the whole assembly. Sealants are an important component to the proper functioning of the filtration system. For example, a decision may be made to replace existing low-efficiency filters with high-efficiency filters in existing filter frames. However, the existing sealant may not meet the requirements of the higher-efficiency filters, and a portion of the air may be drawn through the weak seals rather than the filter itself.

Filtration systems are sized in square feet according to the velocity of the air flow, generally 1 square foot of filter per 500 cfm air flow. Consider an office building that averages 1 cfm per square foot of floor space. Therefore, every 1,000 square feet of office space requires 2 square feet of filter, and a 100,000 square feet office building requires a total of 200 square feet of filter area.