The Minimum Efficiency Report Value (MERV) is an effectiveness scale for air filters developed by ASHRAE in 1987. It measures the filter's ability to capture particulate matter, with higher ratings indicating greater efficiency. Filters with a MERV rating of 11 can capture some coronavirus particles (65%), but MERV 13 are more effective, removing 85% of particles at 0.3 microns. The higher you climb, the more efficient the filter is. MERV 14 can capture 90% of identical particles (1 to 3 micron range).Of course, getting a filter higher than 13, such as a MERV 17+, will be even more effective.
However, MERV 17 filters don't always work for the air filtration systems you're using. In cases where the ventilation system cannot handle a high-efficiency filter, you can use a portable air purifier while upgrading to the highest possible MERV rating with existing capacity. Alternatively, you can upgrade the ventilation system itself, making it suitable for at least MERV 13 filters. Any air filter rated higher than 13 will restrict airflow to the point that it could damage your HVAC system, creating an additional expense to repair it. A HEPA filter is essentially the ultimate solution in the air filter world and far exceeds what a MERV 13 can do.
It brings expert knowledge of a wide range of home air filtration topics, including HVAC filters, filtration efficiency, and indoor air quality. Considering the threat posed by the spread of COVID-19 and other germs, upgrading a building's air filter to a HEPA is a much more effective step than just a MERV 13 considering the small size of a virus (0.06-0.12 microns); the more efficient the filter, the better. There are no published laboratory studies demonstrating whether the SARS-CoV-2 virus can be re-aerosolized from ventilation air filters or other HVAC surfaces. School districts can implement air purifiers to mitigate COVID-19 (SARS-CoV) air, allowing them to reopen schools safely and sooner. Many ventilation systems are not designed for HEPA, but these filters are available as portable air purifiers or vacuums. In addition to using high-efficiency filters, chemical disinfectants should only be applied with the HVAC system turned off. The effects of disinfectants on filter performance are unknown; therefore, they should only be treated with disinfectants if they are to be removed from service and disposed of. UV systems are quite effective at maintaining the cleanliness of HVAC coils, drain pans, and other damp surfaces.
Properly designed systems can be quite effective in inactivating microorganisms in moving air streams on the fly. These systems generally require more lamps, so they can provide significant UV doses in a short period of time. A typical one-pass inactivation efficiency is 85%, just like a good particulate filter, but systems can also be designed for inactivation greater than 99.9%. In addition, a well-designed UV air disinfection system within an HVAC system, and located adjacent to the cooling coils, can also provide surface disinfection benefits. Another way to install UV is in a “top air” configuration. Specially designed wall-mounted fixtures create an irradiated area above the occupant and disinfect the air in the space as it circulates naturally, mechanically or through the HVAC system. CDC has approved this type of system for use in tuberculosis control for nearly 20 years and there is guidance from NIOSH on how to design them.
Finally, mobile UV systems are frequently used for terminal cleaning and surface disinfection in healthcare facilities and other spaces. Systems such as these are commonly used in unoccupied spaces due to occupant exposure concerns. The design and sizing of effective ultraviolet disinfection systems can be a complex process due to the need to determine the dose delivered to a moving air stream or to an irradiated region of a room. In-duct systems are further complicated by the configuration of the air handling unit and ducts and surface reflections that can help achieve higher irradiation levels. Overhead air systems require proper air mixing to function properly while paying close attention to reflective surfaces that could cause room occupants to be overexposed to UV energy. Accredited manufacturers and system designers can help by making the necessary calculations and designing specific systems for individual spaces.
