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Last edited 14 Jan 2022
High efficiency particulate arrestance HEPA
High efficiency particulate arrestance, or HEPA, is the term used to describe the method of air filtration that is incorporated into a specific type of pleated mechanical particulate absorbing air filter.
HEPA filters became commercially available in the 1950s. Due to the mechanical nature of their operations, HEPA purifiers are effective in environments where medical conditions - such as asthma or allergies - may be prevalent.
To qualify as a HEPA filter, the device must remove 99.95% (EU standard) and at least 99.97% (ASME, US DOE) of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm) from the air that passes through it. However, the term is sometimes used to market products that may not comply with this HEPA standard.
This value is used for filter classification because it is close to the most penetrating particle size (MPPS) of approximately 0.2-0.3 μm (NASA). Particles that are larger or smaller than the MPPS are filtered more efficiently than the rated efficiency, although in some circumstances particles below the MPPS may act as nucleation sites and form particles close to the MPPS.
The effectiveness of air filters was initially based on minimum efficiency reporting value (MERV). The rating system was developed in the 1980s by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).
Using a range from 1 to 20 - the MERV rating system reflects the measurement of the filter’s arrestance - or its ability to remove dust from the air - relative to how many of a specific size dust particle (within the range of 0.3 to 10 µm) need to be removed to keep HVAC equipment clean. The higher the MERV rating, the better the filter is at trapping specific types of particles.
In January 2009, ASHRAE stopped recognising MERV values from 17 to 20 in January 2009. This range is instead represented by HEPA filters, which are capable of removing extremely high levels of dust particles as well as some viruses and other dangerous airborne particulates.
 Proper care and operations of HEPA filters
Appropriate operating conditions are needed in order for HEPA filters to perform as designed. These parameters include velocity and direction of airflow, particle concentration, particle size distribution and appropriate temperature and humidity levels. Design factors, such as nonuniformity of filter pleats or improper filter installation, can also have an impact on their efficiency.
HEPA filters also need to be inspected and changed on a regular basis. The period between changes may vary on the setting and the demands of the environment. If the filters are not maintained properly, they will no longer operate efficiently and may even have a negative impact on both the people and the equipment that they are meant to protect.
Since COVID-19 is 50 to 200 nanometres in size, it is smaller than the MPPS of HEPA filters, and thus the filter should be able to remove it with at least the rated efficiency. Contrary to common misconceptions there is no need to produce special filters for blocking smaller viruses like COVID.
HEPA filters are not designed to kill viruses and bacteria, which can stay in the filter as long as they survive, which can be up to nine days for COVID-19 in ideal conditions. However, COVID-19 survives best on hard surfaces and tends to die significantly faster on soft, porous materials such as filter media.
- Air filtration.
- Air filtration and clean indoor air quality standards.
- American Society of Heating and Air-conditioning Engineers.
- Building ventilation and COVID-19 transmission risk.
- Clean indoor air for healthy living - New air filter standards.
- Designing HVAC to resist harmful microorganisms.
- Indoor air quality.
- Minimum efficiency reporting value MERV.
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