HEPA Filters: Nuisance or Necessity?
November 11, 2006
Q: Can you explain to me why we have to use filters that are HEPA when we are trying to remove stuff from the air that is larger than 0.3 microns? In other words, can we use a filter that is rated for 1 micron and be just as effective?
A: Rather than answer the question directly, I'd like to explain why we use high efficiency particulate air (HEPA) filters in general.
HEPA filters are made with randomly arranged fibers that may range in size from one to 50 microns in diameter. Fibrous filters are designed for various applications using a variety of fiber materials. It is not uncommon that the space between these fibers is larger than 0.3 microns. The idea that the pores in a HEPA filter are smaller than the smallest particulates that they collect is incorrect.
There are several filtration mechanisms interacting simultaneously that govern filter performance and efficiency. These mechanisms include:
Impaction - Impaction occurs when a particle traveling in the air stream and passing around a fiber, deviates from the air stream (due to particle inertia) and collides with a fiber.
Interception - Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing through.
Diffusion - Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber.
Electrostatic attraction - Electrostatic attraction plays a very minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force.
According to the National Institute for Occupational Safety and Health (NIOSH) impaction and interception are the dominant collection mechanisms for particles greater than 0.2 microns, and diffusion is dominant for particles less than 0.2 microns. Filters use an efficiency rating system based upon the most penetrable particle size (MPPS). MPPS is the particle size that at which the filter efficiency is at its minimum. For various reasons, it is generally accepted that the MPPS for HEPA filters is 0.3 microns. Depending upon a number of factors, the MPPS has been stated as being between 0.12 and 0.3 microns.
Aerosolized particles larger than the MPPS will tend to flow within an air stream. They possess a certain mass and velocity which allow them to stay in the air stream. As air flows around a filter fiber, some larger particles, especially those that are larger than one micron, will collide (inertial impact) with or be intercepted by the fiber.
Below the MPPS, Brownian motion of the particles aids the filter in removing these smaller particles. Brownian motion occurs when these particles collide with air molecules and are bounced around. This causes them to deviate from the flow of the air stream and therefore take an erratic path that increases their likelihood of impaction on the filter media.
The MPPS is also dependent upon a number of operating conditions including filter surface area, airflow rate, and differential pressure across the filter. Therefore, HEPA filters are more efficient above and below the MPPS and are least efficient at the MPPS.
There are various rating systems for filters. They are commonly rated based upon their collection efficiency, pressure drop and particulate-hold capacity. In the United States, there are two test methods used: American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 52.1-1992 and 52.2-1999
HEPA is a term that has been used to describe a filter that is efficient at removing 99.97 percent of particles at 0.3 microns. There are classes of HEPA filters that are more efficient than 99.97 percent. A class "C" HEPA filter is efficient at removing 99.99 percent of particles at 0.3 microns. There are also filters that are referred to as Ultra Low Particulate Air (ULPA) filters and have a minimum filtering efficiency of 99.999 percent at 0.12 microns. ASHRAE Standard 52.2 has established the minimum efficiency reporting value (MERV), which quantifies the filters efficiency in different particle size ranges for a clean and incrementally loaded filter to provide a composite value. The MERV rating system grades filters on a scale from 1 to 20. A filter that can remove 99.97 percent of particles from 0.3 to one micron in size is rated as a MERV 17. The best 1-micron filter is a MERV 16 rated at 95 percent efficiency.
As an example, I used a particle counter and determined the following indoor/outdoor particle concentrations per cubic foot of air (Table 1). If a 2,000 cfm air filtration device with a properly installed HEPA filter was set up in a room, it would process ~2,000,000,000 particles at 0.3 microns and 16,000,000 1-micron particles per minute. Based on the inside air concentrations, we can determine the number of particles for each particle size that could pass through the filter and be exhausted from the air filtration device (Table 2).
The HEPA-filtered air would be 167 times cleaner than the air exhausted from the1-micron filter. Keep in mind that these numbers are based upon ambient uncontaminated air. Studies have shown that during remediation that there can be several orders of magnitude difference between the ambient uncontaminated air and the remediation workplace.
It is true that most mold spores of concern to remediators are larger than 1 micron. However, when performing activities that require the use of air-filtration devices, there are particles that are smaller than one micron. You also need to be concerned about mold fragments and some bacteria. Bacteria can approach the 0.3-micron size. One source stated that ~90 percent of all particles in the air are around the 0.3-micron size. The decision to use HEPA filters is also based upon its economics. While the use of ULPA may provide a higher level of filtration, it adds unnecessary cost. The use of a 1-micron filter would be less expensive but would not provide the level of filtration that would best protect workers and the indoor environment. Finally, the manufacturers of air filtration devices were already producing equipment for the asbestos abatement and other industries that used HEPA filtration. These units had already stood the test of time by demonstrating their effectiveness.