It is now known that COVID-19 has the ability to spread from one host to the other primarily through droplets of saliva or discharge from the nose when an infected person coughs or sneezes. According to ASHRAE “droplets generally fall to the ground or other surfaces in ranges of 3 - 6ft, while particles (aka aerosols), behave more like gas and can travel through the air for longer distances, where they can transmit to people and also settle on surfaces.”
And since COVID-19 travels through the air via aerosols due to its size, pressure is being put on institutions, universities, K-12 schools, hospitals, research facilities, and commercial businesses to improve the efficiency of the filters in their HVAC systems.
ASHRAE and other leading bodies have recommended upgrading your filters to MERV 13 or higher to help mitigate the transmission of infectious aerosols. Typically, educational buildings use anywhere from MERV 6 - MERV 8 pre-filters.
What does MERV mean?
The MERV rating for HVAC filters is an acronym for Minimum Efficiency Reporting Value. This rating is determined by filter manufactures in accordance with ASHRAE 52.2 - Method of testing general ventilation air-cleaning devices for removal efficiency by particle size. It rates filters on a scale from 1 - 16. The higher the number, the higher efficiency the filter has at catching particulates from the air running in sizes from 0.3-micron diameter up to 10 microns in diameter at standard airflow conditions. Below is a chart that shows you how efficient a filter is, rated from 1 - 16, at removing particulates from the air.
Why MERV 13?
Having a filter that is rated a MERV 13 or higher in your units has a removal efficiency in the 0.30 - 1.0 micron ranging from 50% to 95%. You’ll notice on the chart that having a MERV 10 or less filter has no effect on particulates in the 0.30 - 1.0-micron size range. The reason ASHRAE recommends at least a MERV 13 and not a MERV 11 or MERV 12 as the starting point is because the MERV 13’s removal efficiency is at least 50% in the range 1 category.
Another reason for having MERV 13 or higher rated filters in your units is because the COVID-19 virus attaches itself to droplets and droplet nuclei that are predominantly 1-4 microns in size. And as you can see from the chart, MERV 13 filters are 85% efficient at removing particulates from the air at that 1.0 - 3.0-micron range and 90% efficient in the 3.0 - 10-micron range. And a MERV 14 is 75% in the 0.3 - 1.0-micron range and 90% efficient at capturing particles in the 1.0 - 3.0-micron range. So the higher the MERV rating, the more efficient it is at removing particles in that particle range.
Also, note that having a higher MERV-rated filter in your unit can increase the amount of energy used for that unit to pull air through the filter. Higher efficiency filters require greater air pressures to drive or force air through the filter because of how the filter is designed. You need to be careful when increasing the efficiency of your filter and verify that the capacity of your HVAC systems is sufficient to accommodate the higher MERV-rated filters.
The term efficiency in air filters can relate to energy efficiency and/or particle efficiency. There are a few types of filters that can be efficient at both, but for the majority higher-rated filters will affect your HVAC system's ability to maintain indoor temperature and humidity conditions. You can expect energy costs to increase due to the higher restriction with higher MERV-rated filters.
Another side effect of having higher rated MERV filters in your units is that you may need to change your filters more often. The starting pressure drop for a typical MERV 8 pre-filter is 0.33 vs a 0.36 for a typical MERV 13 pre-filter. Since the pressure drop is higher at the start, the MERV 13 will reach it’s final capacity (1.0 pressure drop) more quickly than a typical MERV 8 pre-filter. This is due to the media being more tightly woven to capture the smaller particulates from the air thus making it clog faster.
New higher Dust Holding Capacity (DHC) media may be able to compensate for this and allow the same if not longer life for your filters.
If you’re in the position that you can not use a MERV 13 or higher filter, then use the highest MERV-rated filter that you can.
How big is a Micron?
Scientists have stated that the COVID-19 virus ranges in size from 0.12 - 0.16 microns in diameter. To give you some perspective on how small that is, the diameter of a human hair ranges from 50 - 500 microns. Below is a chart to help give perspective on different micron sizes of different particles.
What about HEPA filters?
HEPA filters or High-Efficiency Particulate Air filters are even more efficient at removing particulates from the air in the 0.3 - 1.0-micron range. By definition, a HEPA filter must be at least 99.97% efficient at capturing particles 0.3 microns in size. The reason for this criteria is because particles that are 0.3 microns in diameter are considered the most penetrating particle size (MPPS) through a filter. HEPA’s are more efficient at capturing particles both larger and smaller than the MPPS but the standard has been set for a HEPA filter to be 99.97% efficient at 0.3 microns.
The MERV rating for a HEPA filter exceeds the highest MERV rating of 16 and is therefore measured on the removal efficiency of particles in the 0.30 - 1.0-micron range. HEPA filters are rated as such: 99.97% (starting point of a HEPA filter), 99.99%, 99.999%, and 99.9999%.
Because of the design of HEPA filters, they may not be an appropriate option for most HVAC systems. This is due to the extremely high pressure drops that are associated with HEPA filters. If your HVAC system is not designed to handle such a filter, it can overwork your HVAC system.
Other air cleaning options
Portable HEPA Air Cleaners - If you’re absolutely needing HEPA filtration but your equipment can’t handle that type of filtration there are other options such as portable HEPA filtration units. These units are portable and come standard with a 99.99% HEPA filter and have the option to install UV lighting as an extra precaution.
UV-C Lighting - UV lighting or Ultraviolet Energy inactivates viral, bacterial, and fungal organisms so they are unable to replicate and potentially cause diseases. With wavelengths in the range of 100 - 280 nm, this provides the most germicidal effect, with 265 nm being the optimum wavelength. UV lighting can be portable, installed in your air ducts, installed in your HVAC units, or installed on walls or ceilings.
Conclusion
Many universities and K-12 school districts across the country have jumped on the opportunity to transition to MERV 13 or higher filters, portable HEPA cleaners, or UV lighting options. That being said, many other institutions have not made this transition yet. In setting the air filtration goals for your institution it is important to balance the increased filtration and energy cost with the health and safety of your faculty, staff, employees, and students.
If you have any questions about MERV 13 or higher filters, whether your HVAC systems can handle them, HEPA filters, portable HEPA air cleaners, or UV lighting options that can all help mitigate the spread of COVID-19 in your buildings, please contact Tim Deleon at tdeleon@wtfilters.com or call our office at 806-791-2299.