Indoor Air Resources

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Managing Mold-Part1: Select the Right Air Filter for Line of Defense

March 14 Mold small photo

By: Linda Barlow, freelance writer  |   03.14.14

It is impossible to eliminate all mold and mold spores in the indoor environment. To reduce mold growth, public health experts recommend strategies like reducing indoor humidity and increasing ventilation. However, it may be possible to trap and eliminate airborne mold spores via the proper HVAC air filtration strategy. Just match the filter’s particle capture efficiencies to the size of the mold spores.


When evaluating filtration efficiency, most people turn to MERV, a filter’s Minimum Efficiency Reporting Value, which is assigned to filters based on the ASHRAE 52.2 Standard. A MERV 1 is considered least efficient, while a MERV 16 is most efficient.


A filter’s MERV provides the efficiency of the filter over three particle size ranges:

  • E1: very fine particles in the 0.3 to 1.0 micrometer range.
  • E2: fine particles including smaller mold spores in the 1.0 to 3.0 micrometer range.
  • E3: coarse particles including larger mold spores in the 3.0 to 10.0 micrometer range.


Remember to review the Fractional Particle Size vs. Particle Diameter Curve included with the 52.2 test report. The curve will provide the efficiency of the filter for the specific particle size of interest (E1, E2 and E3). High E1 and E2 efficiencies are useful for removing mold spores and are critical to providing for good IAQ overall.


Beware: many pleated filters today (especially at commonly used MERV 8) have very low E1 and E2 efficiencies. In fact, under the ASHRAE 52.2 Standard, there is no minimum requirement for E1 particle capture below a MERV 13 and no minimum requirement for E2 particle capture below a MERV 9.


When selecting filters to trap mold spores, consider those with a good balance of mechanical efficiency and electret efficiency (via an electrostatic charge). These filters will almost always outperform a filter that relies solely on mechanical efficiency:

  • A well-designed charged filter media can be manufactured to provide high initial and high sustained filtration efficiency over its filter lifecycle.
  • The electrostatic effects created in an electret-charged media are particularly useful in increasing the capture efficiency for submicron particles.


Remember that even the most efficient filter cannot remove existing mold growth, but selecting a filter with high efficiency in capturing mold-sized particles may help buildings avoid the spread of mold through airborne spores.




  • Humidity control
  • Regular facility cleaning with bleach or other fungicide
  • Use building materials that do not support mold growth
  • Air filtration
  • We do not have an active mold management strategy



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