The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes the coronavirus disease (COVID-19), primarily spreads through respiratory droplets and aerosols when a person coughs, sneezes, speaks and breathes.
Superspreading events have distinguished the COVID-19 pandemic from the early outbreak of the disease. New research into exhaled aerosols suggests that a critical factor in these and other transmission events is certain individuals' propensity to exhale large numbers of small respiratory droplets.
The researchers at Harvard University showed that the capacity of the airway lining mucus to resist breakup on breathing differs significantly between individuals. They found that the levels of virus-laden aerosols increase with age, body mass index (BMI), and progression of COVID-19.
The study, published in the journal Proceedings of the National Academy of Sciences, showed that older, obese people who have an acute COVID-19 infection release more aerosols, increasing viral transmission potential.
The researchers believe that understanding the respiratory droplet generation's source and variance can help to effectively reduce COVID-19 transmission.
Respiratory droplets generation
The generation of respiratory droplets in exhaled breath happens by the force of fast airflows in the upper airways that arise when a person coughs, sneezes, breathes, and speaks. During the peak inspiratory flow during normal breathing, air rises in the trachea while the brain bronchi can achieve turbulent velocities.
The rise of the air that passes through the thin layer of mucus that lines the airways can break up the mucus surface into small droplets. The size and droplet generation depends on some factors, including surface tension and surface viscoelasticity.
When it comes to the airway lining mucus, the lung surfactant type and concentration and composition, and structure mucus can also affect the levels of aerosols dispensed in exhaled breath.
Surfactant and mucin composition and structure changes, diet, adding, and COVID-19 infection can affect droplet generation and size during breathing.
Exhaled breath particles of 74 essential workers at No Evil Foods and of 120 volunteers at Grand Rapids Community College. (A) All participants; (B) “superspreader” (of aerosol particles) participants (first decile); (C) “superspreader” (of aerosol particles) participants (second decile); and (D) “low spreader” participants. Data represent particle counts per liter of exhaled air (particle diameter larger than 300 nm) for each of the 194 individuals. Error bars represent SD sample calculations based on 3 to 12 exhaled aerosol count measurements, with each measurement an average of counts over a 5-s time interval.
To determine if COVID-19 infection and other factors are tied to infection severity risk and altered airborne droplet generation, the researchers conducted two studies in human and nonhuman primates (NHPs).
The first study involved assessing the exhaled breath of 194 human participants at two sites to assess exhaled breath particle variations in humans.
The second study focused on measuring the exhaled breath from two species of NHPs following the experimental infection by inhaling the SARS-CoV-2 virus.
The team found similar patterns in two normal human populations examined in North Carolina and Michigan and two types of NHP species with two kinds of lung infection – viral and bacterial.
Diet, lung infection, and aging can cause changes in mucus composition and structure. Moreover, phenotypical changes in airway lining mucus, including subsurface mucin chemistry and variation in surfactant composition, can change the mucus-air surface's tendency to break up into droplets.
During COVID-19 infection, the researchers observed that an extraordinarily high number of exhaled aerosols was noted in infected human participants on days 8 and 9 after the symptoms appeared. By the 10th and 11th day, the number of exhaled droplets dropped.
The team concluded that during the peak of the infection, the number of droplets being exhaled by an infected person increases then drops after.
In terms of obesity, the team found the tendency to generate respiratory droplets increases with the body mass index (BMI). Hence, being obese can be a recognized risk to transmit the virus to others through exhaled respiratory droplets.
Age may also be a factor in the transmission of the virus. The promiscuity of the droplets in the airways increases the tendency for upper airway infection to transports deeper into the lungs. It also increases the probability of expelling the aerosol into the environment and transmission of the infection.
As the pandemic continues to spread, countries and health agencies focused on vaccines and therapeutics. However, it is also important to focus on mitigation strategies to combat COVID-19. Also, the researchers noted that while waiting for an effective drug against SARS-CoV-2, the scientific community can focus on COVID-19 management through the restoration of airway lining mucus barrier function.
"Exhaled aerosol numbers appear to be not only an indicator of disease progression but a marker of disease risk in noninfected individuals," the team explained.
- Exhaled aerosol increases with COVID-19 infection, age, and obesity David A. Edwards, Dennis Ausiello, Jonathan Salzman, Tom Devlin, Robert Langer, Brandon J. Beddingfield, Alyssa C. Fears, Lara A. Doyle-Meyers, Rachel K. Redmann, Stephanie Z. Killeen, Nicholas J. Maness, Chad J. Roy Proceedings of the National Academy of Sciences Feb 2021, 118 (8) e2021830118; DOI: 10.1073/pnas.2021830118. https://www.pnas.org/content/118/8/e2021830118