If you were not familiar with the N95 respiratory mask before the COVID-19 pandemic, you certainly are now. The fabrics used to make the N95 mask are easy to breathe through and able to filter dangerous airborne pathogens too small for our eyes to see. While the N95 is widely known, the meaning behind the letter N and the significance of the number 95 may be a mystery.
What Does N95 Stand For?
N95 masks are regulated by NIOSH (The National Institute for Occupational Safety and Health) and should not be confused with FFP masks whose standards are maintained by CEN (European Committee for Standardization). NIOSH classifies the respiratory masks they regulate into one of three categories based upon the intended use. One of the categories is the “N” class which stands for not-for-oil. This mask is designed for use in applications where the particles encountered consist mostly of dry, ultra-fine non-oil based aerosols such as general dirt and those particles generated from grinding, sawing, and thankfully in today’s world, biological particles. The number “95” stands for 95% particle capture efficiency. Higher efficiencies are available, but the N95 mask is the one most commonly used by medical personnel and sought out by others during the pandemic.
Once these masks have been worn, they are not laundered with other hospital fabrics. Despite the high demand for these masks during the peak of the COVID-19 pandemic, manufacturers continue to recommend disposal after a single use and not worn day after day. The CDC has issued guidelines when masks may be reused, but the guidelines are limited and recommended to be overseen by professionals. The demand has driven companies to research effective methods to sanitize and recycle masks. If masks could be easily cleaned and safely reused, demand could be met; however, it’s not a simple task to remove the dirt particles and maintain the 95% particle capture efficiency.
How the N95 Masks Capture Small Particles
The fabrics (known as media) used to construct many of these masks are nonwoven, which is to say the fibers are randomly arranged, not woven from yarn in a repeating pattern like the fabric commonly used in clothing. Nonwoven fabrics are not unique; they are used in countless applications we encounter every day. However, the next step in the process is unique and the secret to the mask’s performance. The nonwoven media used in many of these masks is imparted with an electrostatic charge creating an effect, analogous to a magnet, known as an electret. The electrostatic charge greatly enhances the media’s filtration efficiency because the charge attracts small diameter particles to the relatively large diameter fibers. Those larger fibers (and in smaller numbers) allow the user to more easily draw breaths through the low resistance media. Without that electrostatic charge, however, many dirt particles would easily pass straight through the media.
This media is typically sandwiched between two outer layers of fabric which acts as a prefilter to capture large particles and to protect the media from being damaged. Safely protected within the mask, the highly charged fabric makes an ideal filter for a respiratory mask. Unfortunately, an electrostatically charged media may turn out to be a very poor choice for an HVAC system intake air filter.
The most critical performance characteristic of an air filter is particle removal efficiency. The efficiency of an air filter is based on a test standard known as the MERV (Minimum Efficiency Reporting Values) scale developed by the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) 52.2 Committee. Air filters are tested in a laboratory setting inside a specially constructed air handling ductwork. Carefully selected test dirt with known particle sizes is introduced upstream and particles that pass through downstream are compared to upstream counts. The percentage of particles the filter captured is used to compute a MERV value.
The MERV value is critical because individuals responsible for air quality inside a commercial office building, retail space, manufacturing or healthcare facility, for example, use the scale to select the MERV value that delivers the air quality needed for the safety of the people, products or equipment within their unique facility. An air filter made from nonwoven media containing a strong electrostatic charge, would seem to be an ideal fabric in an air filter to achieve that goal. However, one of the reasons N95 masks are not recommended for long-term use is also why an everyday intake air filter made with electrostatic media and expected to remain in service up to one year, is not recommended.
The Effect Accumulating Ultra-fine Dirt Particles Has on Charged Media
It was stated earlier that the “N” in N95 means the mask is intended for non-oil aerosols and the “95” indicates the percentage of ultra-fine particles the mask will capture. The outer layers of the mask act as a prefilter to capture large particles and heavy moisture when the user inhales or exhales. The prefilter ensures the highly-charged media in the center generally only encounters dryer, ultra-fine particles that escape capture in the outer layers. The magnetic-like effect of the charged media attracts these small particles out of the airstream and onto the fibers.
As ultra-fine particles accumulate on the charged media, an interesting phenomenon occurs that many may recall from a grammar school science class. A magnet will attract a steel coin and if the coin is put on a piece of paper, it can be moved across the paper with a magnet held underneath. Turn the paper sideways and the coin will not fall; appearing to defy gravity. However, add more sheets of paper, repeat the experiment and eventually, a point will be reached where the magnet’s influence is unable to penetrate the paper and the coin falls to the floor.
An air filter made with highly charged media will do the same. Much like the magnet and a single piece of paper, the filter may achieve a very high initial particle removal efficiency (MERV) when brand new. When the electret is performing at peak capacity, dirty particles passing by in the airstream are powerfully attracted to the fibers. However, as particles accumulate on the fibers, they begin to act as an insulator much in the same way adding paper reduced the magnet’s ability to hold the coin in place.
As the insulation effect continues, dirty particles begin slipping past in greater numbers. There is an inverse relationship between increasing this insulation effect and particle capture efficiency. If it were possible to visibly display the MERV value of an installed filter undergoing this process, the numbers would continuously drop as time passed; 14 to 13 to 12 to 11 for example. Eventually the filter may bottom out three or four levels lower than what its value was when brand new. If a healthcare facility manager installed a filter with a high MERV value (MERV 14) only to discover later it was actually performing at a much lower level ( MERV 11), the air quality in the space the system was designed to protect would not match the level expected.
An Addition Added to the MERV 52.2 Test Standard to Account for the Efficiency Drop-off Phenomenon
Members of ASHRAE’s 52.2 committee were aware of this phenomenon. Appendix J was added to the Standard so consumers would have a clearer understanding of how the filter performed while in use, not just when brand new. The Appendix J conditioning step addresses the electrostatic charge from the media by neutralizing the electret. The second test resultis listed as a filter’s MERV-A value which is the particle capture efficiency without the benefit of any electret that may be present.
For example, an initial MERV test conducted on a filter with an electrostatically charged media may result in a published value of MERV 14. However, the same filter tested under the Appendix J conditioning steps may perform as a MERV 11. Putting the two test results together would result in the filter being labeled as a MERV 14 / MERV-A-11A, often abbreviated as MERV 14/11A. This indicates once the electret is no longer effective, the filter performs as if it were a MERV 11.
In another example, a filter constructed without electrostatically charged media performs as a MERV 14 under the 52.2 standard. The Appendix J conditioning steps are then conducted, but since there is no electret to begin with, that filter still performs as a MERV 14. The labeling therefore would show the filter as a MERV 14/ MERV-A-14A, often abbreviated as MERV 14A. This indicates the filter’s minimum lifetime particle capture efficiency is MERV 14. A consumer who determined a MERV 14 is required to supply air to a particular space can expect the MERV 14A filter will do so for the duration of its service life.
Charged media is an acceptable option for N95 masks because it delivers a high capture efficiency, but for sanitation reasons, is not recommended for long-term use. If however those sanitation concerns were solved and the same N95 mask could be worn for weeks at a time, the drop-off in efficiency as the mask’s charged media collected ultrafine dirt particles, it would mean the mask could effectively degrade to an N90, N85, N80 and possibly lower.
Air filters are expected to remain in service for long periods of time. The reliance on charged media to achieve a higher MERV value than what will actually be delivered while in-service is a performance value that should be considered when selecting an air filter. It is recommended to base such decisions on the filter’s MERV-A value. If that value is not prominently displayed on the filter or its packaging, ask the manufacturer to provide test reports.
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