A HEPA filter is the top of the line when it comes to air filters, with a rating equivalent to a MERV 17 or higher. A HEPA filter with a MERV 17 rating will capture 99.97% of air particles that are 0.3 microns in size, and even more of particles that are larger or smaller than that size. HEPA filters are widely considered to be the best of the best when it comes to HVAC air filters, as they can trap airborne contaminants as small as 0.3 microns in diameter. For comparison, the average tuft of hair on the head is approximately 45 microns.
The higher efficiency index of the ULPA filter is due to its higher density filter medium, which requires more power to move air through than HEPA filters. An air conditioning system that is not compatible with a HEPA filter will not work efficiently, as it will have to generate more energy to move air through the filter. The following table lists some of the air pollutants that air filters can capture based on their MERV classification. A stand-alone HEPA air purifier, also known as a portable HEPA air purifier, can provide high-power filtration to your home without damaging your air conditioning system. Most residential spaces will benefit from an air filter in the MERV 11-13 range.
In light of the threat posed by COVID-19 and other germs, changing a building's air filter to a HEPA is much more effective than simply using a MERV 13 filter, considering the small size of the virus (0.06 to 0.12 microns). When looking for the best air filter for your home, you can find recommendations for both MERV and HEPA air filters. Unfortunately, many air purifiers are mislabeled as true HEPA or HEPA-like filters in order to make them more affordable. The problem is that a HEPA is a filter medium that is too fine to be implemented in existing HVAC systems; the only way to add one to an installation is through a portable or stand-alone HEPA air filtration system with its own dedicated fan designed to increase the resistance of this type of filter. Many ventilation systems are not designed for HEPA, but these filters are available as air purifiers or portable vacuums.
ASHRAE recommends using an air filter with at least a MERV 13 rating, or a higher HEPA rating when possible. In most cases, HEPA filters require expensive system modifications and additional fans in order to be used effectively. Homeowners often want the filter that traps the smallest particles, such as HEPA filters with the highest MERV ratings. Filters in residential, commercial and hospital HVAC systems generally have a MERV 16 rating or lower. When schools reopen after the coronavirus outbreak, ASHRAE also recommends installing a portable HEPA and UV air filter for each classroom, with at least two air rotations per hour.