Minimum Efficiency Report, or MERV, values report a filter's ability to capture particulate matter. Filters with MERV-13 ratings or higher can trap smaller particles, including viruses. Many home HVAC systems will have a MERV-8 filter installed by default. If chemical disinfectants are used, they should only be applied with the HVAC system turned off.
In addition, disinfectants should not be applied to ventilation filters before continuing to use the filters within ventilation systems. The effects of disinfectants on filter performance are unknown. Filters should only be treated with disinfectants if they are to be removed from service and disposed of. While 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 the surface disinfection benefits mentioned above. 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 the air 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 three types of systems can be relevant, depending on the type of building and the individual spaces within the building. 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. With the recommendation of a Merv 13 or higher, does a Merv 13 filter meet your needs? A Merv 13 filter is a step in the right direction and captures more particles than a typical Merv 8 filter. However, it's not as good at capturing small virus-sized particles as a HEPA can. A MERV 13 will trap less than 75% of air particles that are 0.3-1.0 microns in size (coronavirus is 0.1 microns).
It is also difficult for many existing HVAC (HVAC) systems to adopt a MERV 13 because of the greater fan load of finer filter media, which can actually cause more harm than good and reduce airflow if your system is not designed to handle that type of filter. On average, many installations are limited to one type of MERV 8 or MERV 9 filter. MERV 13 and above can be used as a final filter in cleanrooms, operating rooms, inpatient hospital care, and smoking rooms. ASHRAE 52.2 can capture this drop if the test is performed with the optional Appendix J provided by MERV-A.
Therefore, it is recommended that workers performing maintenance and replacing filters in any ventilation system with potential for viral contamination wear a well-fitting respirator (N95 or higher), eye protection, and gloves. A high-efficiency filter will trap at least 85 percent of contaminants that aggravate allergies and cause sneezing. Good Merv efficiency will improve indoor air quality and reduce the spread of virus particles. Some of the common particles that filters are tested for include pollen, dust mites, textile and carpet fibers, mold spores, dust, pet dander, bacteria, and tobacco smoke.
You are more likely to find this type of filter in hospitals and laboratories, sterile environments that need advanced purification. Filters with higher Merv ratings should be changed more frequently (at least every three months) to avoid restricted airflow that can reduce efficiency or even damage the system. Gaskets can be added to most filters, either at the installation location or on the filter frame. MERV 17 to 20 will also capture virus carriers, carbon dust, combustion smoke, radon progeny and microscopic allergens (particles of size 0.3 micron).
Unlike other manufacturers, they also test each individual filter instead of batch testing, which is what other companies do. 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. A HEPA filter is essentially the ultimate solution in the air filter world and far exceeds what a MERV 13 can do. A True HEPA reports that the filter captures at least 99.97% of particles of at least 0.3 microns in size by DOE standards.
The higher the MERV rating on a filter, the less dust particles and other contaminants can pass through it.