Review
Why airborne transmission hasn't been conclusive in case of COVID-19? An atmospheric science perspective

https://doi.org/10.1016/j.scitotenv.2021.145525Get rights and content

Highlights

  • An atmospheric science perspective on airborne transmission is reported

  • Co-morbidity of SARS-CoV-2 vs air pollution is presented

  • Measurement of their infectivity and viability is highly uncertain due to lack of robust sampling system to separately

  • This may help us better understand the airborne transmission of COVID-19

Abstract

Airborne transmission is one of the routes for the spread of COVID-19 which is caused by inhalation of smaller droplets1 containing SARS-CoV-2 (i.e., either virus-laden particulate matter: PM and/or droplet nuclei) in an indoor environment. Notably, a significant fraction of the small droplets, along with respiratory droplets, is produced by both symptomatic and asymptomatic individuals during expiratory events such as breathing, sneezing, coughing and speaking. When these small droplets are exposed to the ambient environment, they may interact with PM and may remain suspended in the atmosphere even for several hours. Therefore, it is important to know the fate of these droplets and processes (e.g., physical and chemical) in the atmosphere to better understand airborne transmission. Therefore, we reviewed existing literature focussed on the transmission of SARS-CoV-2 in the spread of COVID-19 and present an environmental perspective on why airborne transmission hasn't been very conclusive so far. In addition, we discuss various environmental factors (e.g., temperature, humidity, etc.) and sampling difficulties, which affect the conclusions of the studies focussed on airborne transmission. One of the reasons for reduced emphasis on airborne transmission could be that the smaller droplets have less number of viruses as compared to larger droplets. Further, smaller droplets can evaporate faster, exposing SARS-CoV-2 within the small droplets to the environment, whose viability may further reduce. For example, these small droplets containing SARS-CoV-2 might also physically combine with or attach to pre-existing PM so that their behaviour and fate may be governed by PM composition. Thus, the measurement of their infectivity and viability is highly uncertain due to a lack of robust sampling system to separately collect virions in the atmosphere. We believe that the present review will help to minimize the gap in our understanding of the current pandemic and develop a robust epidemiological method for mortality assessment.

Graphical abstract

Schematic diagram showing interaction of droplet nuclei with particulate matter, radiation and oxidants in the atmosphere. POA and WSOA are primary organic aerosol and water-soluble organic aerosols components of the particulate matter.

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Abbreviations

CoV
Coronavirus
SARS-CoV-2
Severe acute respiratory syndrome coronavirus
MERS
Middle East respiratory syndrome
COVID-19
Coronavirus disease-2019
Droplet nuclei
Smaller droplets (≤5 μm) laden with CoV-2
Droplets
CoV-2 laden droplets (>5 μm) produced during talking, coughing or sneezing
PM
Particulate matter
t1/2
Half-life of COVID-19 virus
T
Ambient temperature
RH
Relative humidity (%)

Keywords

COVID-19
Air pollution
Mortality
Droplets
SARS CoV-2

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1

Droplets have diameter >5 μm (also called as respiratory droplet) whereas those with diameter ≤5 μm are termed as smaller droplets (also known as aerosols, nano-droplet or droplet nuclei).

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