Efficiency Percentage Formula
Efficiency of a filter is calculated by measuring the particle count downstream and upstream from the filter. The downstream count is divided by the upstream count then this number is subtracted from one then multiplied by 100 to create an efficiency percentage3.
Filter Efficiency Comparison Chart
|Filter||Efficiency and Particle Size|
|SAS HEPA||Up to 99.97% on particles as small as 0.3 microns|
|SAS ULPA||Up to 99.9995% on particles as small as 0.12 microns|
|SAS ASHRAE||Up to 95% on particles as small as 0.5 microns|
To illustrate the miniscule size of particles that these filters capture, one micron is equal to one millionth of a meter. The human eye cannot see particles smaller than 10 microns. Sizes of common particles are as follows: Human hair: 100-150 microns in diameter, emissions 1 – 150 microns, dust: < 100 microns, pollen: 10 - 100 microns, spores: 3 - 40 microns, mold 3 - 12 microns, bacteria: 0.3 - 60 microns, and pure oxygen 0.005 microns4. The chart below compares the sizes of particles and shows which can be seen by electron microscopes, microscopes, and the human eye.
HEPA FiltersHigh efficiency particulate air (HEPA) filters must meet the efficiency rating of 99.97%, on particles as small as 0.3 microns, set by the US Department of Energy in order to be a true HEPA filter5. Some HEPA filters marketed for consumers do not meet these standards and are not true HEPA filters with the same capability or efficiency. HEPA filters have a MERV rating of 17. HEPA filters provide excellent filtration for bacteria, lung damaging dust, spores, pollen, pulverized coal, and metallurgical dust and fumes.
HEPA filters were first invented for use in World War II to capture radioactive particles released from an atomic bomb6. However, the certification standard was not implemented until 1983. Today, HEPA filters must undergo strict testing to ensure accurate efficiency rating and classification.
How does a HEPA filter work?
HEPA and other fibrous filters capture particulates using four main methods: sieving, inertial impaction, interception, and diffusion3. The type of filtration method depends on the size of the particle. Particles bigger than 0.3 microns are captured primarily though impaction and interception2. 0.1 micron or smaller particles are primarily caught through diffusion2.
Sieving is when a large particle is trapped by a filter because it is too large to pass through the hole in between the filter fibers3. Sieving is a method that all filters use to capture particles1. Although, it is not the main method of filtration for HEPA filters, HEPA filters do still collect larger particles in this manner.
Inertial impaction happens with larger particles that collide directly with the fibers of the filter4. Larger particles are unable to adjust quickly to the air stream due to inertia and continue along its original path directly into the fiber3. This happens more often with larger particles because more mass equals more inertia or resistance to change paths.
Interception occurs when a particle is following a gas streamline or path and comes within one particle radius away from the fiber and brushes up against it3. The fiber captures the particle and is removed from the gas flow.
Diffusion refers to the filtration method where smaller particles move in a zigzag or random path and ends up sticking to the filter fiber3. Diffusion occurs because of the Brownian theory of motion where gas particles constantly collide with each other moving in a random motion3. Diffusion primarily happens to particles smaller than 0.1 microns in size and with slower air flow3.
Ultra low particulate air (ULPA) filters have efficiency of up to 99.9995% on particles as small as 0.12 microns. ULPA filters have a MERV rating of 20 and are the highest quality filters available. ULPA filters provide better filtration for tobacco smoke, oil smoke, insecticide dust, carbon dust, and even capture some viruses. The chart below contains common particulates captured by HEPA and ULPA filters and outlines the overlap between the two filters.
An ASHRAE filter refers to a fibrous filter that has efficiency up to 95% on particles as small as 0.5 microns. This type of filter is very similar to the HEPA filter but does not have as high of an efficiency rating. The MERV rating is 14 for this filter and can be used to capture bacteria, pollen, spores, and other harmful particulates.
HEPA, ULPA, and ASHRAE filters efficiencies are tested using a set procedure created by the American Society of Heating, Refrigerating, and Air Conditioning Experts (ASHRAE Society). The ASHRAE society set guidelines under the Standard 52.2 for testing filter efficiency in order to help consumers select the filter best suited for a specific contaminant while still offering maximum air flow. The Standard 52.2 created MERV ratings and specified that an efficient filter must also be installed securely and fit tightly to ensure maximum performance.
MERV ratings refer to the minimum efficiency reporting value and helps classify filters. The chart below details different MERV ratings, efficiency and filters at that rating7.
|MERV Rating||Efficiency for Smaller Particles||Efficiency for Larger Particles||Particulates Best Used For||Filter Type|
|1-4||<20%||65-80%||pollen, dust, sanding dust, bigger than 10 microns||residential A/C units, throwaway filters|
|5-8||20-35%||80-95%||mold spores, cement dust||better residential filters, commercial building filters, pleated filters for industrial settings|
|9-12||40-75%||90-95%||milled flour, welding fumes||better commercial building filters, hospital laboratories|
|13-16||89->95%||>98%||bacteria, smoke||ASHRAE filters, hospital inpatient care, smokers lounges, general surgery areas.|
|17-19||–||99.97%||combustion smoke, pharmaceutical manufacturing, viruses or radioactive materials||HEPA filters|
|20||–||99.995%||Clean rooms||ULPA filters|
1. Woodford, Chris. “HEPA Filters,” Explain That Stuff! 2018 Apr 15. https://www.explainthatstuff.com/hepafilters.html.
2. Department of Health and Human Services. Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological or Radiological Attacks. 2003 Apr. https://www.cdc.gov/niosh/docs/2003-136/pdfs/2003-136.pdf.
3. Mechanisms of Filtration for High Efficiency Fibrous Filters. http://www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Application_Notes/ITI-041.pdf.
4. “HEPA Filters: What They Are & How They Work,” Allergy and Air. 2014 Mar 25. https://learn.allergyandair.com/hepa-filters/.
5. “HEPA Filters and ULPA Filters Information,” Engineering 360. https://www.globalspec.com/learnmore/manufacturing_process_equipment/filtration_separation_products/hepa_filters_ulpa_filters.
6. “What is a HEPA Filter.” https://www.cowaymega.com/what-is-a-hepa-filter/.
7. “Merv Rating Chart.” http://www.mechreps.com/PDF/Merv_Rating_Chart.pdf.