Why is it so important to capture submicron particles?

In offices and other commercial buildings, employee productivity can suffer due to poor indoor air quality (IAQ). Picture courtesy of Anna Omelchenko/Shutterstock.com
In offices and other commercial buildings, employee productivity can suffer due to poor indoor air quality (IAQ). Picture courtesy of Anna Omelchenko/Shutterstock.com

While some people may still think of air filters as a commodity product to be specified primarily on purchase price, this silent workhorse of the Heating, Ventilation & Air Conditioning (HVAC) system plays an important role in today’s modern buildings. Robert Martin of Kimberly-Clark Professional Filtration looks at how air filtration technology is now capable of capturing submicron particles, helping to reduce illness and improve productivity among building occupants.

Air filters have come a long way since first being introduced in the 1930s. Air filters were developed to protect forced-air heating system elements from dust, and thus reduce the risk of building fires. Air filters of this period consisted of loose mats of fibrous glass, animal and synthetic fibers, metal mesh and various fabrics of cellulose and other porous materials. In the 1940s and 1950s, buildings started to use air filters to protect the cooling coils of the HVAC system from dust, thus preventing fouling. Filtration efficiencies for these early filters were very low. In the 1980s and 90s, the industry started looking at the ability of filters to not only protect HVAC equipment, but also to clean the indoor air for building occupants. Advances in styles and types of filters began to emerge. And more attention began to be paid to the filter media – the material within the filter that removes particles and impurities/pollutants from the air. Today, air filtration is seen as playing a role in Green Building initiatives, energy conservation, sustainable building design, and occupant health and productivity.

Capturing submicron particles

Air filters really began to evolve with the introduction of synthetic, nonwoven filter media and the ability to apply an electret charge to enhance the media’s particle capture performance. It has since been established that filters that provide a good balance of robust mechanical structure and an electret treatment will almost always outperform filters with media that relies solely on mechanical efficiency.

The electrostatic effects created in an electret-charged synthetic media are particularly useful in increasing the capture efficiency for submicron particles. While submicron particles are much smaller than the void spaces present in most commercial electret media, the electrostatic forces within the media allow those particles to be removed with high efficiency.

Why is it so important to capture submicron particles? Lung-damaging dust can be as small as 0.5 microns, and airborne bacteria can be as small as 0.3 microns. On the other hand, many heavier and larger particles will settle out of the air and never even make it to a building’s air filters, which is one reason the EPA is starting to stress PM2.5 or air particles that are 2.5 microns or less in diameter. These small, toxic particles are most likely to travel to the deepest part of the lungs, where they can cause a variety of respiratory problems and other health and productivity issues. For example, in schools, there is evidence that indoor environmental exposure to common allergens such as dust mites, pests, and molds, plays a role in triggering symptoms of asthma, a costly and primary cause of school absenteeism, accounting for 10 million to nearly 13 million missed school days each year. Asthma is not the only problem linked to indoor airborne pollutants. Students, teachers and staff also may be at risk for headaches, dizziness, nausea, allergy attacks and more.

In healthcare facilities, airborne diseases are thought to account for up to 33% of all healthcare-acquired infections (HAIs). HAIs are expensive. In addition to the direct medical costs (estimated to range from US $28-45 billion), there are costs relating to reduced productivity among patients and staff as well as immeasurable costs related to diminished quality of life.

In offices and other commercial buildings, employee productivity can suffer due to poor indoor air quality (IAQ). In fact, in the US alone, adults miss about 14.5 million work days each year because of poor IAQ, and productivity losses are estimated to be anywhere from 3-33%.

MERV under the microscope  

ASHRAE Test Standard 52.2 addresses filter efficiency and indicates the filter’s ability to remove particles of differing sizes, including E1s (very fine particles in the 0.3 to 1.0 micrometer range), E2s (fine particles in the 1.0 to 3.0 micrometer range), and E3s (coarse particles in the 3.0 to 10.0 micrometer range). A MERV, or Minimum Efficiency Reporting Value, is assigned to each filter, with a MERV 1 being least efficient and a MERV 16 being most efficient.

When evaluating filter performance under ASHRAE 52.2, the E1, E2, and E3 efficiencies represent the true measure of filter performance and give users a more complete picture of the filter’s particle capture performance. High E1 and E2 efficiencies are critical for providing for good IAQ, which helps students improve learning and helps commercial and institutional building occupants avoid illness and the many costs associated with employee productivity losses.

Unfortunately, many filters today have low E1 and E2 efficiencies. In fact, under ASHRAE 52.2, filters in the MERV 1-12 range aren’t even measured for E1 efficiencies, and filters in the MERV 1-8 range aren’t measured for E2 efficiencies. It is therefore possible to have a MERV 8 mechano-electret media filter with better E1 particle capture than a MERV 11 mechanical-only filter.

This disconnect is the reason some filter manufacturers and filter media suppliers are suggesting that future revisions to ASHRAE 52.2 should add E1 and E2 efficiency requirements at lower MERV ratings. This would ensure that the standard does an even better job of rating how effective filters are at protecting people from potentially harmful air particles.   

Many more related articles are also published in World Pumps magazine.

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