For a filter to obtain the MERV-13 classification, for example, it must filter a minimum of 90% of PM10 and a minimum of 85% of PM2. Average particle size efficiency in microns. What is a MERV 13 filter? MERV stands for “Minimum Efficiency Reporting Value” and a filter receives a MERV rating as determined by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). 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 them inside 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 discarded. While UV systems are quite effective at maintaining the cleanliness of HVAC coils, drain trays, and other damp surfaces, properly designed systems can be quite effective at 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 single-pass inactivation efficiency is 85%, much like that of 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 accessories create an irradiated area above the occupant and disinfect the air in the space as the air circulates naturally, mechanically or through the air conditioning system. The 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 cleaning terminals and disinfecting surfaces in healthcare and other spaces. Systems like these are commonly used in unoccupied spaces because of concerns about occupant exposure. All three types of systems can be relevant, depending on the type of building and the individual spaces within the building. The design and size of effective ultraviolet disinfection systems can be a complex process because of the need to determine the dose delivered to a moving air stream or to an irradiated region of a room.
Internal duct systems are further complicated by the configuration of the air handling unit and ducts and by reflections from surfaces that can help achieve higher levels of irradiance. Overhead air systems require an adequate air mix to function properly, and at the same time, they pay close attention to reflective surfaces that could cause room occupants to be overexposed to UV energy. Accredited system manufacturers and designers can help by performing the necessary calculations and designing specific systems for individual spaces. With more than 57,000 members from more than 130 nations, ASHRAE is a diverse organization dedicated to promoting the arts and sciences of heating, ventilation, air conditioning, and refrigeration to serve humanity and promote a sustainable world. It's also difficult for many existing HVAC (heating, ventilation, and air conditioning) systems to adopt a MERV 13 because of the increased fan load due to the finer filter medium, which can actually cause more harm than good, as well as reducing airflow if the system isn't designed to support that type of filter. In general, the higher the MERV rating, the more particles the filter can trap, but as we will see, this is an oversimplification, as some higher MERV ratings can hinder the performance of an HVAC system if the system is unable to manage restricted airflow and, over time, this can cause repairs to the HVAC system. MERV 13 filters can block up to 98% of air contaminants and can trap microscopic particles between 0.3 and 1.0 microns in size.
A MERV 13 will trap less than 75% of air particles that are 0.3 to 1.0 microns in size (coronavirus measures 0.1 microns). Despite the numerous ratings of MERV 13 filters, high-efficiency filters are not necessary for most homes. A HEPA filter is essentially the ultimate solution in the world of air filters and far exceeds what a MERV 13 can do. Considering the threat posed by COVID-19 and other germs, upgrading a building's air filter to a HEPA is much more effective than simply going up to a MERV 13 since coronavirus measures 0.1 microns; thus, it's important that we choose an efficient filter. In addition to filtering out common dust particles such as dust mites, lint and pollen that other filters trap, MERV 13 filters can also filter smoke, smog, microscopic allergens, viruses, bacteria, mold spores, exhaust fumes and pet dander from various pets. Currently ASHRAE recommends using at least a MERV 13 filter which has a minimum efficiency of 85% when capturing particles between 1 μm - 3 μm in size; however this particle size range does not include COVID-19 which measures 0.1 microns. According to EPA standards MERV 13 filters are 90% efficient when capturing particles between 3 - 10 microns; 85% efficient when capturing particles between 1 - 3 microns; 50% efficient when capturing particles between 0.3 - 1 micron. MERV 13 filters are suitable for homes in densely populated areas with high levels of air pollution; immunosuppressed people; people with severe allergies or asthma.
