MERV 13+ filters are an important part of air filtration systems, providing a high level of efficiency in removing particles from the air. These filters are rated to remove at least 90% of aerosols containing viruses, making them ideal for use in schools, universities, and other places where airborne pathogens may be present. In addition, they can provide points towards LEED certification for green buildings. When it comes to air filtration, the Minimum Efficiency Report Value (MERV) is an effectiveness scale developed by ASHRAE in 1987. The MERV classification system ranges from 1 to 20, with the highest rating (20) being the rating used for filters in places such as hospitals.
A filter with a MERV rating of 14 can remove volatile organic compounds, such as formaldehyde, from the air, while another MERV 14 filter may not. The MERV 13 air filter standard is just one of many included in the LEED certification program for green buildings. HEPA filters are the most efficient for residential or commercial use, followed by MERV 13-16 filters. One of their recommendations is to use air filters with at least a MERV 13 rating, or a higher HEPA rating when possible.
Alternatively, you can upgrade the ventilation system itself, making it suitable for at least Merv 13 filters. Air filters of different sizes will have different prices, the same goes for the MERV classification of the filter. The following table summarizes the average arrest and applications of filters along the MERV scale, and the typical particle size for which they are used. Any air filter rated higher than 13 will restrict air flow to the point of damaging the HVAC system, which would entail an additional expense to repair it. In addition to providing protection against airborne pathogens, MERV 13+ filters can also provide surface disinfection benefits when used in conjunction with UV systems.
UV systems are quite effective in maintaining the cleanliness of HVAC coils, drain trays, and other wet surfaces. Properly designed systems can be quite effective at inactivating microorganisms in moving air currents on the fly. These systems generally require more lamps so that 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 air conditioning system and located next to the cooling coils can also provide surface disinfection benefits. Another way to install UV is in a “higher air” configuration.
Specially designed wall-mounted devices create a radiated area above the occupant and disinfect the air in the space as the air circulates naturally, mechanically or through the air conditioning 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 and other spaces. Systems like these are commonly used in unoccupied spaces due to concerns about occupant exposure. All three types of systems may 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 because of the need to determine the dose administered to a moving air stream or to an irradiated region of a room. Inlet systems are further complicated by the configuration of the air treatment unit and ducts and reflections from surfaces, which can help achieve higher levels of irradiation.
Upper air systems require an adequate air mix to function properly and, at the same time, 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 making the necessary calculations and designing specific systems for individual spaces. With more than 57,000 members from more than 132 countries, 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. For even better results, filtering can be combined with an air purification method such as germicidal ultraviolet irradiation (UVGI). Therefore, it is recommended that workers who maintain and replace filters in any ventilation system with potential for viral contamination use a properly fitted respirator (N95 or higher), eye protection and gloves.