A unified theory of dilution ability for the control of airborne infection

Professor Li, Yuguo   (Principal Investigator (PI))

Professor Yen Hui-Ling   (Co-principal investigator)

36

2023-09-01

1132781

A unified theory of dilution ability for the control of airborne infection

SARS-CoV-2, Dilution, Airborne transmission, Ventilation, Crowding

Building and ConstructionFluid

Engineering (E)

17206723

General Research Fund (GRF) 2023/24

2023

On-going

1  To develop methods of estimating the new and unified concept of effective dilution-air flow rate that integrates the effects of the air volume/exposure time, ventilation, clean-air flow rate due to filtration, deposition and virus deactivation and their respective non-uniform spatial distributions. Current concepts of the ventilation rate or the clean-air flow rate do not directly include the effect of crowding or the non-uniform distribution. This has resulted in the inconsistent estimation of the infectious quantum generation rate in outbreak investigations and has probably also hindered the essential determination of the minimum dilution requirement for avoiding/minimising respiratory infections. The conventional ventilation rate or clean-air rate does not fully represent the dilution ability. The existing incomplete approach of investigating dilution would lead to the need for different minimum ventilation rates to be specified in different indoor settings. The new effective dilution-air flow rate integrates the effects of the air volume, clean-air rate, air distribution and exposure time into one parameter, and it equals the clean-air rate in a steady state and uniform condition. Our preliminary work demonstrated an equivalence between the effects of air volume and clean-air flow rate on inhalation exposure. The effective dilution-air flow rate incorporates the room size as a factor in evaluating infection risks. The newly defined intake fraction time is equivalent to the time of exposure for the direct mouth/nose to mouth/nose inhalation of the expired infectious quanta at the source. As an example, for an infectious quanta generation rate of 100 per hour, an intake fraction time of 36 seconds leads to the inhalation of one quantum.  2  To explore the effectiveness of effective dilution-air flow rate for respiratory infection control in multiple connected rooms. The horizontal spread of COVID-19 infection between rooms has been reported in quarantine hotels and high-rise housing estates, but the spread between buildings has rarely been reported. If two buildings are separated by outdoor open spaces, a sufficient dilution is expected. If two spaces are connected by a doorway, then the direct transport of infectious aerosols between the spaces is possible. Two or more spaces may be connected by a hub space, such as common rooms in aged care facilities and corridors in hotels. Such hub spaces may not be well ventilated, particularly at night when the spaces are unoccupied. Any transport of air into such a hub space from an infected room would not be sufficiently diluted, and subsequent transport to other rooms may lead to infections. An air-handling unit (AHU) and its associated air ducts also form a hub space. An AHU may serve many rooms, and the return air from all other rooms and the outdoor air entering all rooms dilute the infectious aerosols returning from one room. An AHU effectively sums the dilution ability of all rooms. The AHU dilution ability can be 10 or 100 times for that room. We hypothesise that because of its collective dilution ability, central air conditioning helps to reduce infection if the AHU is sufficiently large. We will investigate several relevant outbreaks in high-rise housing, in quarantine hotels, and on the Diamond Princess cruise ship to explore the roles of hub spaces. An office floor will be monitored for verification and estimating methods for the dilution ability in individual rooms and hub spaces in multi-zone settings will be developed.  3  To explore the association between the new effective dilution-air flow rate and infection rate in observed outbreaks and to determine a preliminary minimum required dilution for general indoor settings. Our team has investigated 5 COVID-19 outbreaks with access to critical data, such as data on fan coils and the associated central air conditioning system. We will conduct a systematic search and review of outbreaks of COVID-19 and other respiratory infection with available data. For each infection venue, we will calculate the effective dilution-air flow rate and intake fraction time using available parameters (e,g. Miller et al., 2021; Ou et al., 2022; Li et al., 2021; Vernez et al., 2021).We will also estimate the effective dilution-air flow rate and intake fraction time in outdoor spaces (i.e. a crowded street and a crowded neighbourhood) to explain why outdoor transmission has been rare. The ventilation rates of outdoor environments will be estimated using data reported by Hang et al. (2011). We hypothesise that the newly defined effective dilution-air flow rate better correlates with the infection rate than the ventilation rate or clean-air flow rate. This hypothesis is supported by the fact that the newly defined effective dilution-air flow rate considers all contributions to dilution of infectious aerosols. If our hypothesis is confirmed, our investigation of the association will lead to a preliminary determination of a required minimum dilution that is applicable to different indoor settings. Such an outcome will assist worldwide efforts to develop dilution guidelines for avoiding respiratory infection.