14-07-2021

Can Mechanical Ventilation Protect us from (the next) COVID?

By Alex Xudong Sun

COVID infections in the Netherlands have been decreasing for the last few weeks. We had a 26% fall in the number of positive cases the past week. However, concerns are growing about the more contagious Delta variant that is spreading in many countries. Are those anti-COVID measures still effective towards new variants? Will some of them become new norms? After all, the Delta variant is by no means the last COVID variant we will deal with.

Since the (first) reopening in May 2020, Dutch primary schools are required to run their mechanical ventilation system at full capacity to minimize the risk of COVID transmission indoors. We monitor the performance of ventilation systems in 27 schools using a network of air quality sensors installed in 252 classrooms. Are they working effectively? Is it reasonable to invest (more) into mechanical ventilation systems?

Graph 12 01

The COVID pandemic has made people more aware of the importance of building ventilation systems. Extensive studies have found that COVID can be transmitted via aerosol in indoor spaces without enough ventilation. Also, there is a positive correlation between the spread of the virus and poor indoor air quality, especially the concentration of particular matters (PM). Just like the cholera outbreaks in the 1800s reshaped the built environment when people upgrade the street and drainage system to curb transmission, the spread of COVID and its variants has called for significant improvement of building ventilation systems. It is reasonable to anticipate that this increased awareness on ventilation will lead to more investment and, over the long run, have an impact on the valuation of real estate, especially office buildings. But are building ventilation systems really effective in lower the risk of transmission?

By establishing an air quality monitoring network, we are now able to evaluate the effectiveness of building ventilation systems in real time and link it to the occupants’ behavior patterns. In particular, we investigated the indoor particulate matter (PM) level. Studies have found that the concentration of indoor PM not only has an aerodynamic pattern similar to COVID aerosol (Miller, Shelly L., et al., 2020), but also positively correlated with the severe rate (Zhu et al., 2020). Therefore, PM level can be taken as an indicator of indoor COVID risk.

Graph 12 02

By establishing an air quality monitoring network, we are now able to evaluate the effectiveness of building ventilation systems in real time and link it to the occupants’ behavior patterns. In particular, we investigated the indoor particulate matter (PM) level. Studies have found that the concentration of indoor PM not only has an aerodynamic pattern similar to COVID aerosol (Miller, Shelly L., et al., 2020), but also positively correlated with the severe rate (Zhu et al., 2020). Therefore, PM level can be taken as an indicator of indoor COVID risk.

Graph 12 03

The first step is to understand the source of indoor PM. Outdoor PM from atmospheric pollution or traffic can “penetrate” indoors. Meanwhile, indoor PM can be generated from furnishings or occupants’ activities, for example, children moving about the classroom (Matic et al., 2017). Airflow is filtered when it flows into the circulation.

Small Diag

We use a fixed-effect model to find the correlation between indoor and outdoor PM levels, controlling daily weather, classroom, and time of the day. The indoor PM is negatively correlated with the outdoor PM before class starts (8:30). This observation is because the traffic outside classrooms increases before school starts while the classrooms are still empty. After students entering the classrooms, indoor PM starts to accumulate. From the I/O ratio plot, it is evident that indoor sources generate the majority of PM pollutants.