What does Indoor Air Quality mean to us?
Indoor Air Quality refers to the air quality within the space or surround-built environment. It is vital while facing a public health disease that is transmitted through the air and respiratory systems.
Challenges for IAQ strategies implementation
Since the emergence of the Covid-19 pandemic, Indoor Air Quality (IAQ) control strategies have been applied to current ventilation standards. However, not many recommended methods are easy to follow. For example, a recent report from the Center for Green Schools and ASHRAE pointed out the most challenging to implement IAQ strategies was that buildings were not designed to support the recommended strategies. Many people also face cost constraints while choosing among those protective measurements. I am sure a similar challenge is not only for schools but also for other businesses and operations.
What the research tells us
I recently came across this interview ("integrating IAO control strategies to reduce the risk of Asymptomatic SARs-CoV-2 Infection") and found Mr. Zhang's recent research paper worth sharing.
This research paper categorized the strategies into three main streams: Source Control, Ventilation Method, and Air Cleaning Method. Those methods can be applied from personal scale to building scale. The author compared various IAQ strategies by a risk reduction factor (RRF) and its estimated cost. Buildings that integrate IAQ measures such as ventilation and filtration to maximize indoor air quality can reduce the SAR-CoV-2 transmission by factors ranging from 8.5 to over 500. And some of the integrated strategies have higher than average RRF but only in the medium-cost range. (See the summary graph below)
Original data is from the article “Integrating IAQ control strategies to reduce the risk of asymptomatic SARS CoV-2 infections in classrooms and open plan offices”, table 1. Graphic by author. (The author uses the condition of 20% outdoor air, mixing ventilation, and MERV 8 filter per ASHRAE standard 62.1 for the baseline that relative to the risk reduction factor (RRF) in the estimation.)
What‘s the takeaway
Source Control: N95 has the highest impact on reducing risk, but not cost-effective
One effective strategy for source control is wearing a face mask. And each type of face mask has various impacts by factors. For example, if by a factor of 20, 4, and 2 with facemasks (N95, Surgical mask, Cloth mask, respectively), it means the application could reduce the emission rate by 95% (1-1/20), 75% (1-1/4), and 50% (1-1/2).
Ventilations: Displacement Ventilation is better than Mixing Ventilation but costs more.
There are several strategies for ventilation, such as using personal ventilation, double/ triple the ventilation rate per person, installing semi-open partitions, or using mixing or displacement ventilation. However, many existing buildings are not designed to meet the displacement ventilation systems. Therefore it costs more to implement this system or convert the current system to displacement ventilation.
As you can see, both these two highlighted scenarios can reach a maximum of 196 RRF. But with a displacement ventilation system, it is definitely at a high cost.
Air Cleaning: HEPA helps, but not by much
The primary air cleaning strategy here involves applying an air filter or purification of outdoor air supply. In these two scenarios, replacing the MERV filter with the HEPA filter only increase the RRF from 1 to 30 factors. If you have cost concerns, MERV 14 would be a good enough option.
By the way, a few vocabularies of Air Quality you might want to know:
MERV (Minimum Efficiency Reporting Value) is an effectiveness scale for air filters that ASHRAE established. Typically, the residential buildings use MERV 1-4, and commercial building uses 5-8. However, during the coronavirus infection, ASHRAE recommends upgrading the air filter to at least MERV 13.
HEPA (High-Efficiency Particulate Air) can capture most airborne germs. However, many ventilation systems are not designed to use HEPA filters. But the good news is many portable air cleaners are designed to install with HEPA filters.
Mixing Ventilation controls the density of indoor pollutants by bringing in enough outdoor air to dilute indoor contaminants. Typically, the air supply in Mixing Ventilation is located close to the ceiling and discharges the air at a high velocity. This greater volume of air is sufficient to mix the existing room air before reaching the breathing zone.
Displacement Ventilation supplies air at a low speed from the bottom of the room. Airflow moves up slowly due to the help of buoyancy (aka "Stack Ventilation"- warmer air rises and moves out). When bottom supplied-air reaches a certain height, it stratified removing contaminants from the breathing zone then exhaust from the ceiling.
Breathing Zone: When we talk about air quality, it is assumed that the contaminants are homogeneous within one space. Therefore, the breathing zone is defined as the area within 10 inches radius of an occupant's nose and mouth. And its concentration is equivalent to the concentration inhaled by the occupants.