Indoor Air Quality and Virus Mitigation: Ventilation versus Filtration

Indoor environments provide plenty of opportunities for people to come in contact with viruses and bacteria — through airflow, from surfaces, and from the way our built spaces cause us to interact with one another. In light of the recent SARS-CoV-2 pandemic, we’ve gathered the latest research and resources that evaluate indoor air quality and disease transmission to clear what we believe are common misconceptions on the effectiveness of ventilation and filtration as health-safety strategies.

Our understanding of how the virus spreads is evolving; however, there are some items we know to be a fact:

  • The virus travels as droplets of saliva when we talk, breathe, cough, and sneeze.
  • The majority of those droplets are larger than air molecules, and therefore, they drop from the air and land on surfaces.

However, recent research has shown that smaller particles carrying the novel coronavirus may linger in the air longer prior to falling. In this case, an air purifier with “High-Efficiency Particulate Air” (HEPA) filtration placed in the vicinity of a potentially infected person may work to minimize potential contamination via air particles. In fact, HEPA filtration does a great job of removing particles that are 0.1 microns, and the COVID-19 virus is 0.12 microns in diameter.

Before we look to stock our indoor spaces with localized air purifiers, we should consider some other strategies that are at play. For example, research-based on other airborne illnesses of the past has also proven that increasing overall ventilation is a highly effective strategy because increased ventilation dilutes potentially contaminated air with fresh air. This same concept is why being outdoors is touted as a very safe practice during this pandemic.

We have seen many office buildings asking about how to retrofit their existing air systems. When considering major HVAC systems, activated carbon, HEPA, and ultraviolet (UV) filtration media are typically located far from the actual breathing zone (within three to six feet above the finished floor). Therefore, any contaminated air that circulates in these systems is likely only carrying non-active virus particles by the time it reaches those filters.

The counterargument to that statement is that good indoor air quality, in general, helps promote healthy lung function, and therefore, both activated carbon and HEPA filtration are great at scrubbing particulate matter 2.5 microns and 10 microns (PM2.5 and PM10), carbon monoxide (CO), carbon dioxide (CO2), volatile organic compounds (VOCs), ozone (O3), and nitrogen dioxide (NO2) from the air. However, our air quality in Atlanta is not bad enough to warrant using a HEPA filter. Our biggest concern when ventilating a space with outdoor air is pollen and dust transfer. For those reasons, activated carbon or MERV filters are the perfect solution to help remove particulate matter from the outdoor air.

A UV filter will not remove these particles. It will only guarantee those particles previously alive are now dead. This is especially helpful when it comes to the transfer of mold and mildew indoors, even though UV light at certain strengths will also kill viruses and bacteria. But remember, the type of UV light which kills bacteria and viruses as quickly as air passes through are so strong that they pose a threat to humans, adding potential risk for the people responsible for upkeep with those systems.

As designers and consultants, we tend to understand concepts better than we can see. So, perhaps we can review why N95 masks work to prevent the spread of the virus:

  • An N95 mask is a mask that can filter 95% of airborne particles that are 0.3 microns or larger. Short for a micrometer or “one-millionth of a meter,” a micron is about 0.00004 inches. A human red blood cell is about 5 microns.
  • A HEPA filter can remove particles as small as 0.1 microns, roughly the same size particle as the SARS-CoV-2 virus, so an N95 mask acts a lot like a HEPA filter.
  • The only confirmed transmission source is through droplets of saliva, which are typically 5 microns or larger.
  • An N95 mask is intended to prevent the transmission of the virus through saliva, but there is still a small chance that the SARS-CoV-2 can transmit through a mask if it is strictly an airborne pathogen. The same can be said for HEPA or UV filtration not located in the breathing zone.

Which brings us to our final point: the CDC suggests that people wear masks to reduce your spread of saliva when you talk, breathe, cough, or sneeze ⁠— not to protect you from those around you. In the same sense, there is not an air filtration system that will 100% safeguard building occupants from the viral transmission. However, increased use of air ventilation to dilute pathogens, face masks to reduce saliva transmission, handwashing to further safeguard saliva transmission, and cleaning of high-touch surfaces will go a long way to reduce the spread of any pathogen compared to extreme filtration practices.

For more specifics behind how to handle building systems to reduce the spread of pathogens, we’re happy to answer any questions you may have.

For our colleagues, we have collected links to some of our trusted resources on the topic:

NAFA, COVID-19 and Air Filtration Frequently Asked Questions (FAQs)
ASHRAE Position Document on Infectious Aerosols
Center for Active Design “5 Ways to Optimize Buildings for COVID-19 Prevention”
IWBI Global Health and COVID-19

UPDATE:
On July 9, 2020, the World Health Organization released a statement confirming that the main mode of transmission for the SARS-Cov-2 virus may be airborne. While there are confirmed cases of airborne transmission, WHO also acknowledges that additional studies are required to indicate whether the airborne virus remains infectious when suspended in air over long distances or long periods of time in sufficient quantities to cause infection within a recipient of the shared air. The organization also reports on two separate studies that confirmed that viable virus particles survived, suspended in air, after three hours and sixteen hours, respectively. In our assessment of the viable options for indoor air quality for virus mitigation through ventilation versus filtration, we suggested that filtration within the breathing zone (three to six feet above the finished floor) and an increase in clean air to dilute the potentially infectious air within a space were the best modes of mitigation. Through the research reported by WHO, our statements remain true. And above all else, the use of a face mask and staying home should be our community’s main focus.

We are committed to continuing to provide information that will keep you and your loved ones healthy. Stay safe, stay home, and mask up!