Do Merv Filters Stop Covid? - An Expert's Perspective

As we all socially distance and stay at home amid the COVID-19 pandemic, some may wonder what else they can do to “flatten the curve”. While hand washing and staying at home remain the most effective means of limiting the spread of the virus, conflicting information is emerging about the role and effectiveness of air filters. Can building air filtration protect us from contracting COVID-19? Leakage in building heating, ventilation and air conditioning (HVAC) systems may be part of a general risk mitigation approach, but it is generally not considered a solution on its own. There is no direct scientific evidence of benefit, but reduced exposure can reasonably be inferred based on the ability of some filters to remove particles containing a SARS-CoV-2 virus. For filters to have any impact on the transmission of infectious diseases, transmission must occur through the air route, filters must be properly installed and maintained in systems suitable for treating recirculated air, and filters must be properly designed for the building in which they are used.

More importantly, in most buildings and in most situations, filters can be significantly less effective than other infection control measures, such as social distancing, isolation of known cases, and hand washing.

What Filter Should I Use to Protect People in My Building from COVID-19?

Hospitals (and many health centers) have specially designed mechanical systems that can adapt to the levels of filtration they need. They are often based on other control systems and strategies (e.g. Most importantly, they have dedicated staff who operate and maintain this equipment to provide maximum benefit.

What About Ultraviolet (UV) Lamps? Do They Work?

What about ionizers, ozone generators, plasma and other air cleaning technologies? None of these technologies have been shown to reduce infection in real buildings, even if they are promising based on tests conducted in a laboratory or in an idealized environment. Some of them have substantial concerns about secondary issues (such as ozone production).

What About Portable Air Cleaners?

Yes, most public health guidelines suggest that transmission of COVID-19 is predominantly associated with large droplets.

This is why air filtration is only a small part of a solution, since it generally does not address transmission by contact with the surface or by close contact between people. However, the distinction between droplets and in the air is the size of the particles. We know that (a) droplets can remain in the air for long periods of time (for example,. In addition, DNA and RNA from other viruses, which are generally associated with droplets, have been found in the filters used.

What Precautions Should I Take When Changing Filters?

In general, it is prudent to assume that filters have microbiological active material on them. It is not known if this represents a significant risk of infectious disease from viruses, but the precautionary principle would suggest that care should be taken. This becomes particularly important in any building (including a home) where there are known or probable cases of an infectious disease, including COVID-19, and it also extends to portable air purifier filters and vehicle cabin air filters. Filters should be replaced with the system turned off, with gloves, with respiratory protection if available, outdoors if possible and discarded in a sealed bag.

Where Can I Go for More Information?

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 17 to 20 will also capture virus carriers, carbon dust, combustion smoke, radon progeny and microscopic allergens (particles of size 0.3 micron). Because of their high efficiency, HEPA filters are recommended for critical healthcare applications, such as anterooms, isolation rooms, and rooms of COVID-19 patients. Good Merv efficiency will improve indoor air quality and reduce the spread of virus particles.

The following table summarizes the average stop and applications of filters along the MERV scale, and the typical particle size for which they are used.

Hospitals have specially designed mechanical systems that can adapt to their needs; however these systems require dedicated staff who operate and maintain this equipment to provide maximum benefit. Portable air cleaners may help reduce transmission by contact with surfaces or close contact between people; however they cannot capture small virus-sized particles as well as HEPA filters can. When changing filters it is important to take precautions such as wearing gloves or respiratory protection if available; discarding them in sealed bags outdoors if possible. Ultimately upgrading your building's air filter to a HEPA may be worth considering due to its high efficiency at capturing virus carriers.

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