Filter Factors

by Katie Lee

Helping retailers breathe easier with the right HVAC air filter technology, which can improve IAQ while reducing energy costs.

By Robert Martin

Air filters perform an important function in retail facilities as a primary defense for building occupants and HVAC equipment against airborne pollutants.

Indoor air pollution is a problem in public buildings. In fact, indoor air can be two to five times as polluted as outside air. That is one reason why 50% of all illnesses are thought to be either caused by or aggravated by poor indoor air quality (IAQ). Poor IAQ is more than just a nuisance and a health hazard: Total costs to the U.S. economy from poor IAQ are estimated to be as high as $160 billion each year in medical costs and reduced productivity.

Most of the dust and particles people breathe into their lungs is approximately 0.5 microns or smaller — a fraction of the size of a grain of sand. Not all air filters are capable of efficiently capturing such small particles. That is why it’s important to take a closer look at the media inside the filter. Choosing a filter with the right filter media can provide higher efficiency removal of respirable particles associated with poor IAQ. It can also help to reduce the energy costs associated with operating an HVAC system.

How Air Filters Works

HVACrf pleat inst2HR FAir filters capture particulates on the filter media, which is the material within the filter that removes particles and impurities/pollutants from the air. This capture requires two things to happen. The first is that the particle will collide with or be removed by the “fibers” that make up the filter media. The second is the probability that the particle will continue to adhere to the media fiber. The extent to which filters remove airborne particles is referred to as filtration efficiency.

There are many styles of HVAC air filters on the market today, though only a few types of commonly used filter media. Today, retail facility professionals are increasingly aware of issues associated with poor IAQ. In their efforts to improve IAQ for employees and shoppers and minimize energy expenditures, they are realizing the added value and benefits of pleated HVAC air filters with synthetic filter media utilizing a combination of a robust mechanical structure and an electret charge.

Improving Air Filtration Efficiency

Filters that provide a good balance of a robust mechanical structure and an electret treatment will almost always outperform a filter media that relies solely on mechanical efficiency. For example, a well-designed electret-treated media can provide high initial and high sustained efficiency over the filter lifecycle. Filters that provide only mechanical efficiency begin their life at their lowest particle removal efficiency and rely on the building of the dust cake in the filter to increase efficiency.      

In addition, the electrostatic effects created in an electret-charged media are particularly useful in increasing the capture efficiency for submicron particles. This is because, while submicron particles are much smaller than the void spaces present in most commercial electret media, the electrostatic forces within the media structure allow those particles to be removed with high efficiency.

Also, electro-mechanical media almost always delivers lower airflow resistance in the same filter construction as a mechanical-only filter. This translates into reductions in energy consumption and costs. Mechanical-only filters, on the other hand, tend to create significant drag or resistance, because their filtration mechanisms cause disruption of the particles in the air stream. 

The more resistance there is, the more energy is needed to push the air through the filters. (See Figure 1.) Lower airflow resistance and reduced energy consumption also means electro-mechanical media filters can help reduce greenhouse gas generation — a wise sustainability strategy.

How to Select the Right Air Filter

The ASHRAE 52.2 Standard addresses filtration efficiency and measures the filter’s ability to remove particles of differing sizes. A MERV, or Minimum Efficiency Reporting Value, is assigned to a filter based on its filtration efficiency. A MERV 1 is least efficient, while a MERV 16 is most efficient. Recent studies suggest that an appropriate minimum efficiency for commercial buildings is MERV 7 to 11 or better to provide good HVAC system cleanliness and efficient operation.

It’s important to note that two similar filters of the same MERV rating (one using mechanical only media and the other using electro-mechanical media) can have different filtration efficiencies and initial airflow resistance. To conduct a more thorough filter selection review, it’s important to examine the filter’s efficiency in all particle size ranges: E1 (very fine particles in the 0.3 to 1.0 micrometer range), E2 (fine particles in the 1.0 to 3.0 micrometer range), and E3 (coarse particles in the 3.0 to 10.0 micrometer range).

High E1 and E2 efficiencies are critical for providing for good IAQ and helping building occupants avoid illness due to poor IAQ. Unfortunately, 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 threshold for E1 particulate capture below a MERV 13 rating and no minimum requirement for E2 particle capture below a MERV 9.

Additional Performance Factors

In addition to the performance factors measured under the ASHRAE 52.2 Test Standard, consider these variables when selecting a filter:

• Moisture resistance and temperature limitations: How high humidity/moisture and temperature affect the filter. For example, studies have shown that filtration efficiency of electret-treated media is unaffected by relative humidity and by long term warehousing at high temperatures (130˚F). 

• Sustainability: Consider the entire product lifecycle when selecting filters — from raw material sourcing to manufacturing, from packaging to transport, and from design and usage to final disposal. For example, some filters provide superior performance while using less media than other filters. In addition, choosing high-capacity pleated filters can extend filter life and reduce change-outs, which can reduce waste streams. 

• Filter style: Compared with wire-backed pleated filters, self-supported pleated filters have some drawbacks, including air flow restriction (due to binders and glue that are needed for stiffness but that can clog the “pores” of the filter media), lower dust holding capacity (which leads to more frequent filter change-outs, increased maintenance and sometimes filter blow-outs), and less efficient particle removal over all particle size ranges, especially E1 and E2 particles.


A pleated filter with nonwoven filter media that uses a combination of mechanical structure and electret treatment provides a means of achieving high initial efficiency and sustained high efficiency in HVAC air filters.

From high filtration efficiencies to reduced energy requirements, there are many reasons to select air filters utilizing media that has a good balance of a robust mechanical structure and an electret treatment.

— Robert Martin is an associate category manager with Kimberly-Clark Professional Partnership Products, where he manages the company’s product portfolio for the filtration and wet wipes categories. He is a member of ASHRAE and NAFA. Email the author at [email protected].

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