Deep dive on indoor air quality

Research oriented discussion on air movement, indoor air quality (IAQ) and indoor transmission of aerosol infectious agents

Although many studies have shown that elevated air movement can improve the occupants’ perceived air quality (Melikov & Kaczmarczyk, 2012; Schiavon et al., 2017; Zhai et al., 2015), studies on the relationship between air movement and indoor air quality are comparatively limited. Increased air movement does not remove indoor pollutants from a space like ventilation does. However, air movement changes the airflow pattern in the space, which can reduce an occupant’s exposure to indoor pollutants and infectious agents in several ways: as discussed below.

Firstly, air movement changes the room airflow pattern by redistributing the supply air, thus diluting local sources of pollutants in the room. In spaces with stagnant air where the ventilation effectiveness is lower than 1, the sources of air pollutants can accumulate in the room locally. Using air-moving devices, such as ceiling fans, the air within the space can be fully mixed, resulting in the ventilation effectiveness of the room to approach 1 (which is a measure of complete air mixing) (ASHRAE Standard 62.1, 2016, Table 6-B) and lowering of the air pollutants levels. However, for other rooms with ventilation effectiveness above 1, such as displacement ventilation (Yang et al., 2019), using large fans will decrease the effectiveness of indoor pollutants removal by design using such systems.

Increasing air movement has been reported to impact the dynamics of indoor air. Using a ceiling fan or a larger stand fan, air pollutants in the room were fully mixed resulting in a more uniformed but lower pollutant concentration (Benabed et al., 2020). The authors reported that the air pollutant released from a source was dispersed much faster and reached a more uniform concentration within the space using a fan blowing directly to the source than without a fan. In a recent study on indoor airborne transmission of the SARS-CoV-2 virus, fan use was found to help disperse viruses from the breathing zone to the unoccupied upper part of the room, thus, potentially lowering infection risk. Another study shows that using ceiling fans reduced the pollutant concentration in the exposed person’s breathing zone by more than 20% (Li et al., 2021). For upper-room ultraviolet germicidal irradiation system (UVGI) systems typically relies on the natural convection of air within the space to disinfect microorganisms brought from the breathing zone up to the irradiated zones near the ceiling. Using ceiling fans greatly improved the UVGI effectiveness system by mixing the microorganisms-laden air up to the irradiation zone, thus disinfecting them at a higher rate compared to those without using ceiling fans (Pichurov et al., 2015; Rahman et al., 2014; Rudnick et al., 2015; Zhu et al., 2014).

Secondly, elevated air movement increases the deposition rate of airborne particles onto indoor surfaces such as fan blades and room furniture, floor, ceiling and walls (Lai, 2002; Thatcher et al., 2002). This process can be understood by the reduction in boundary layer thickness because of increased air movement, thus facilitating the mass transfer of particles onto the surfaces. Although resuspension of particles may also increase with increased air movement (Salimifard et al., 2017), the resuspension rate is several orders lower than that of deposition. The air movement needed to resuspend deposited from surfaces into the air is typically on an order of 10 m/s (Mukai et al. 2009) which is higher than most fan speeds.

Lastly, the World Health Organization (WHO) recommends using stand fans (or pedestal fans) by placing them close to an open window to enhance ventilation for naturally ventilated spaces (World Health Organization, 2021). Evaporation of airborne droplets and water-containing particles can be enhanced via air movement. A simulation study on the ejection process of saliva droplets in the air to mimic real events of human cough showed that increased air velocity will result in the saliva droplets going further with a decreased concentration and liquid droplet size in the wind direction (Dbouk & Drikakis, 2020). For infectious bioaerosols, this evaporation process and increased mechanical stress may decrease microorganisms' viability within the airborne droplets. In indoor spaces that adopt increased air movement and elevated temperature, the higher air temperature and relative humidity (RH) may affect viability of airborne microorganisms (Chin et al., 2020; Dabisch et al., 2021). Lin et al. (2020) have noted that RH controls droplet evaporation. The evaporation affects droplet chemistry (solutes become more concentrated as water is lost) which in turn affects the virion’s microenvironment and viability. For SARS-CoV-2, changing the room temperature and relative humidity from 23 °C [73 °F] and 40% to 28 °C [82 °F] and 60 % will reduce the virus half-life from 44.8 minutes to 22.6 minutes (SARS-Airborne calculator).