IAQ FAQ: Indoor Air Quality - Frequently Asked Questions
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In our recent conversations and presentations with clients about improving indoor air quality to reduce exposure to COVID-19, we’ve been asked — and gotten some helpful answers to — some great questions. Here are a few common ones.
Is COVID-19 transmitted via airborne particles?
The short answer is yes, COVID-19 is transmitted via airborne particles, which can spread for hours and perhaps even days. The good news is that certain changes to a building’s heating, ventilating, and air-conditioning systems can reduce airborne exposures.
How does the CDC statement that 50 ACH (air changes) over an 8-minute period will remove 99.9% of the existing indoor air (assumes perfect mixing, with perfectly clean air) impact an office building?
General scientific and medical industries agree that a virus particle can remain airborne and viral for 2–4 hours. Typical office buildings provide 3-6 air changes per hour, recirculating through air handling systems. Therefore, the virus particles, with consideration for filter efficiency, could be returned to an area between 6 and 24 times an hour. The higher the percentage of ventilation (outside air) that is introduced to and exhausted from a building, the lower the percentage of potentially viral virus particles.
While the CDC Guidelines make a good point about the effectiveness of air changes, most office buildings are not clean rooms and cannot provide 50 ACH of 100% outside air or 7.5 CFM/sf in office space with an 9-foot ceiling height.
The solution then is to provide multi-faceted solutions that include higher filtration and more active mitigation such as UVGI, PCOs, and Ionization.
Does humidification impact the transition of airborne viruses?
Medical studies have proven that 40-60% RH provides the most effective reduction in the transmission of airborne viruses.
Summer months in the northeast typically require the HVAC system to be in “air conditioning” mode, which means they are providing passive dehumidification. In this mode, most HVAC systems will be in the range of 40-60% RH.
There are several concerns to keep in mind at 40% RH in an existing building. The main concern is condensation at the window and other thermal brakes that will create housekeeping and maintenance issues. The second is controlling moist migration. When the air is moist inside of the building and is dry outside of the building, the moisture will push through the walls, and when it reaches the dew point, it will condense inside the wall and grow mold. Mold in the wall or inside the space will lead to other IAQ problems that can be very costly.
Applying a year-round 40-60% RH strategy is a realistic goal in new construction when thermal breaks, insulation values, and vapors barriers can be designed into the building exterior.
Desmond Greene PE, CEM, CPMP
Managing Principal | New York
Asset Strategy & Repositioning Leader
Raymond Doyle PE, LEED AP
Managing Principal | Washington, dc
Life Science Practice Leader