Decay rate estimation of respiratory viruses in aerosols and on surfaces under different environmental conditions

Emerging infectious diseases have a significant impact on public health and economies (Jones et al., 2008). Approximately 15 of 57 million (>25%) annual global deaths are related to infectious diseases (Morens et al. 2010). Out of these, the majority (3.96 million) are due to respiratory infections (Morens et al., 2010). Acute respiratory diseases are the most widely reported infections among individuals of all age groups (Monto 2002). Respiratory viruses replicate in the respiratory tract and are subsequently transmitted by respiratory secretions, causing infections ranging from asymptomatic to symptomatic (Kutter et al., 2018; To et al., 2020; Zhao et al., 2020). Respiratory tract infections are caused by various respiratory viruses, including severe acute respiratory virus coronavirus 2 (SARS-CoV-2), severe acute respiratory syndrome coronavirus (SARS-CoV) (Casanova et al., 2010; Darnell and Taylor 2006; Lai et al. 2005), Middle East respiratory syndrome (MERS-CoV) (Chu et al., 2020; van Doremalen et al., 2021; Gabutti et al., 2020; Su et al., 2016), influenza viruses (Harper, 1961; Hirose et al., 2022; Izumikawa, 2019; Sutton et al., 2013), and respiratory syncytial virus (RSV) (Moreira et al., 2018; Paynter 2015).

Most cases of outbreaks are associated with indoor environments (Chau et al., 2021; Qian et al., 2021). Superspreading events have been reported in indoor environments such as restaurants (Y. Li et al., 2020; Majra et al., 2021; Qian et al., 2021), households (W. Li et al., 2020; Qian et al., 2021), buses (Majra et al., 2021; Mangili and Gendreau 2005; Shen et al., 2020; Tsuchihashi et al., 2021), airplanes (Flight et al., 2020; Mangili and Gendreau 2005), trains (Pestre et al., 2012), classrooms (Charlotte 2020; Rothamer et al., 2020), and healthcare facilities (Bin et al., 2015). Most transmissions led to clusters within an exposure period of less than 12 h. For instance, during a 10-h nonstop commercial flight, a cluster of 16 infected individuals was reported from one probable index case (Flight et al., 2020). The above-mentioned studies concluded that clusters of infections were established within 2–10 h, highlighting that a critical time interval for successful transmission may be governed by the decay rate of the viruses in indoor environments.

Infected individuals expel large droplets and aerosols into the air via expiratory activities such as coughing, sneezing, breathing, and speaking (Klompas et al., 2021; Yin et al., 2022). There are three dominant modes of respiratory virus transmission (1) droplet transmission: the direct inhalation of relatively larger droplets (>5 μm); (2) airborne transmission: the inhalation of tiny droplets, aerosols, and droplet nuclei floating in the air (<5 μm); and (3) fomite transmission: viruses can remain viable on inanimate surfaces for hours to even days and cause indirect contact through fomites (Castaño et al., 2021; Chaudhuri et al. 2020; Delikhoon et al., 2021; Kutter et al., 2018; Patel et al., 2020; Pease et al., 2021). A previous study reported that the sizes of 87% of exhaled particles are less than 1 μm, emphasizing the importance of considering aerosol transmission in long-range transmissions (Zhang et al., 2020). Recently, the World Health Organization and U.S. Centers for Disease Control and Prevention have issued scientific declarations on the importance of aerosols in SARS-CoV-2 transmission (Klompas et al., 2021). Meanwhile, droplet transmission causes short-range transmission (<1 m), and these droplets remains in the air for a short period (<17 min) (Kutter et al., 2018). Droplets travel directly from the mouth or nose of the infected individual to the nostrils or mouth of susceptible individuals and cause deposition on the upper respiratory tract and mucous membranes (Arslan et al., 2020; Biryukov et al., 2020a; Kutter et al., 2018; Miller et al., 2021). Fomite transmission occurs by the rapid deposition of larger droplets on inanimate surfaces (Castaño et al., 2021; Karia et al., 2020). Influenza virus is predominantly transmitted via fomites (Nicas and Jones 2009) and causes infection by gaining entry via hands and subsequently through facial membranes such as the nose, mouth, or eyes.

Environmental factors such as temperature, humidity, and solar radiation can affect the stability of viruses in aerosols and surfaces (Casanova et al., 2010; Gamble et al., 2021; Paynter 2015; Schuit et al., 2020; Wood et al., 2010). The relative humidity (RH) around the surface can affect the evaporation rate and concentration of compounds such as salts and proteins in the droplets, which influence the decay rate of viruses (Guo et al., 2021). At a low humidity, owing to the high evaporation rate, respiratory droplets reduce in size and form tiny droplets and droplet nuclei (Paynter 2015). Conversely, respiratory droplets are larger at a high humidity and settle faster on surfaces because of their lower evaporation rate (Paynter 2015). Higher temperature and higher humidity levels have a synergistic effect on virus decay compared to lower temperature and humidity (Chan et al., 2011). However, few studies have indicated an increased daily incidence at lower temperatures and humidity (Chan et al., 2011). Additionally, solar radiation affects the viability of viruses in the environment; ultra violet light is a natural environmental virucide which disrupts viral replication by causing the formation of photodimers (Sutton et al., 2013). Therefore, determining viral decay rates under different environments is vital for adopting interventional strategies and decisions initiated by policymakers and government health authorities (Dublineau et al., 2011).

Although we found a few prediction modeling approaches used in wastewater epidemiology (Kadoya et al., 2021; Zhu et al., 2022) and virus transmission dynamics (Vuorinen et al., 2020), there have been limited modeling studies that predict viral decay rates in indoor environments (Guillier et al., 2020). To the best of our knowledge, no study has focused on viral decay rates at shorter time intervals or on virus-specific or surface-specific decay rate estimation based on environmental conditions. Therefore, the objectives of our study were to predict the decay rates of respiratory viruses (SARS-CoV-2, SARS-CoV, MERS-CoV, influenza virus, and RSV) and develop estimation models for decay rates on different surfaces under diverse environmental conditions (temperature, relative humidity, and solar radiation), which would help determine effective control measures.

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