Share on PinterestA new review investigates the role of aerosols in the transmission of respiratory viruses. Julia Forsman/Getty ImagesInterdisciplinary researchers recently conducted a review to investigate the airborne transmission of respiratory viruses.The authors conclude that most of the respiratory viruses, including SARS-CoV-2, spread via aerosols at both short and long ranges.The researchers write that airborne transmission may be the most dominant transmission route for all respiratory diseases.
In 2016, in the United States, lower respiratory tract infections were the seventh most common cause of death, contributing to around 95,992 deaths.
Traditionally, experts believed contact with contaminated surfaces and inhaling droplets from coughs and sneezes were the main transmission modes of respiratory diseases.
However, a comprehensive review of 206 studies in the journal Science finds that aerosols may be the most dominant transmission route for several respiratory diseases.
Aerosols are tiny particles of “microscopic liquid, solid or semi-solid particles that are so small that they remain suspended in air,” say the authors.
Talking, breathing, coughing, and sneezing can all produce aerosols, but because talking and breathing are regular, these activities transfer more of the virus than more occasional coughing and sneezing.
According to the review authors, aerosols “can remain suspended for many seconds to hours, travel long distances, and accumulate in air in poorly ventilated spaces.”
In contrast, droplets are larger particles that originate from coughing and sneezing that can carry infection over short distances. A distance of up to 0.2 meters (m) provides the optimum distance for the transfer of infection by droplets.
Aerosols carry more of the virus and can penetrate deeper into the lung tissue than droplets, which are too large to reach the lower respiratory tract.
The authors hope that a better understanding of this transmission route will help design better ways to prevent infection.
“This review outlines a new paradigm that would be helpful in assuring better air quality and preventing airborne infectious diseases in the future.“
– Prof. Chia Wang, corresponding author
A new review
Prof. Chia Wang told Medical News Today: “I decided to conduct the literature review on this topic at the beginning of the outbreak of COVID-19, with a goal to understand better the transmission pathways of SARS-CoV-2 and other respiratory viruses.”
Dr. Shahyar Thomas Yadegar, acritical care medicine specialist, pulmonologist, and medical director of the ICU at Providence Cedars-Sinai Tarzana Medical Center in Tarzana, CA, also spoke with MNT. Discussing the importance of the review’s findings, he said:
“Small viral particles remaining in the air, long after an individual [with the infection] has left an area, can still [cause infection in] an otherwise healthy [person]. It should sound an alarm on rethinking how healthcare providers and public health officials approach respiratory diseases, especially COVID-19, that continues to evade full scientific and clinical understanding.”
The review provides three key reasons why aerosol transmission is the best explanation for the spread of SARS-CoV-2:
Aerosols are the only explanation for super spreader events since not everyone could have touched a single contaminated surface. Similarly, not everyone with an infection could have stood within less than 1 m from the index case, or indeed at the optimum transmission distance of less than 0.2 m.Aerosols explain the marked differences in transmission rates between inside and outdoor environments because ventilation affects only aerosols. Droplets behave the same indoors as outdoors, while aerosols, because they are lighter, clear more easily by air currents in the outside environment. Aerosols are also the best explanation for the variability in virus transmission between people. Just 10–20% of individuals account for 80–90% of infections. Some people produce higher quantities of aerosol than others during normal speech and breathing, while the amount of droplet production during coughing and sneezing is less variable.
A range of experimental methods corroborates these findings. These approaches include animal experiments, epidemiology, laboratory and clinical studies, airflow simulation studies, and air sampling of contaminated sites at a distance from the source of infection.
These lines of research show there are similar levels of corroboration for transmission of other respiratory viruses by aerosol, particularly for the influenza virus.
The authors breakdown in depth how airborne transmission occurs to inform policymakers about how to counteract transmission by aerosol.
Through each of the processes, from the generation of virus-laden particles, to transport through the air, inhalation by a susceptible host, and deposition in their respiratory tract, the virus needs to stay viable. Knowledge of what can disrupt each of these stages can lead to interventions to disrupt transmission.
Humidity as an example
Relative humidity impacts both the aerosol size and how well the human respiratory tract can clear inhaled aerosols.
Below a relative humidity of about 80%, respiratory aerosols can reach a size up to 40% smaller than usual. This means the virus can travel further and penetrate deeper into the respiratory tract.
The mucus from the lungs that helps clear inhaled particles is less efficient in low relative humidity.
Some respiratory viruses are more susceptible to changes in relative humidity than others, which explains why influenza and some common cold viruses are seasonal.
Some research shows there is an impact of humidity and possibly temperature on the transmission of SARS-CoV-2.
However, it will be difficult to disentangle the effects of access to tests, behavior in cold weather, and measures to prevent transmission, such as mask wearing. This might not be possible until the majority of people have some protection from vaccination or infection.
Implications of the review
While discussing the implications of the study, Prof. Wang told MNT: “One of the important points in this review is that airborne transmission is a major transmission pathway not only for SARS-CoV-2 but also for many other respiratory viruses, which have been previously considered as droplet-driven.”
“With the improved understanding of airborne transmission,” she continued, “it is time to take this transmission pathway into serious consideration and add aerosol precautionary measures to prevent outbreaks of other respiratory viruses.”
Dr. Shahyar added: “While treatment of these diseases continues to require further research, these findings take a big step towards decreasing disease prevalence. With lower disease rates, hospitalizations and mortality will also decrease, leading to better patient outcomes.”
Preventive measures
Experts suggest proper ventilation, open space, disinfection of toilet areas and their proper use, and sanitizing personal protective equipment are highly effective in limiting the concentration of SARS-CoV-2 RNA in aerosols.
Airflow and ventilation can influence aerosols greatly. Ensuring proper natural ventilation and air filtration — along with maintaining physical distance, wearing masks, and avoiding crowded spaces — can help reduce airborne transmission of respiratory diseases.
Since the pandemic, the use of plexiglass among workers has become widespread. However, there is little evidence to support its effectiveness against blocking infectious aerosols.
According to the authors of the new review, these barriers “can impede the airflow and even trap higher concentrations of aerosols in the breathing zone and has been shown to increase transmission of SARS-CoV-2.”
The way forward
To prevent respiratory diseases, which are responsible for a substantial economic burden, governments, along with healthcare professionals, should implement aerosol protective measures.
These, the authors suggest, could include proper ventilation in indoor areas, avoiding recirculation of contaminated air, ensuring universal masking, using UV sterilization lamps, and air filtration to remove airborne particles effectively.
“An improved understanding of aerosol transmission will allow for better-informed controls in numerous ways. For example, by understanding that aerosols are largely influenced by airflow and ventilation, it allows people to consider the importance and effectiveness of ventilation systems, such as ensuring sufficient ventilation rates and avoiding recirculation to reduce the risk of exposure to virus-laden aerosols,” Prof. Wang told MNT.
“An improved understanding of the aerosol filtration efficiencies of various masks would allow one to make better choices in what masks to wear and how to wear them properly to protect against aerosol transmission,” she continued.
“Also, the improved understanding of the physical plexiglass barriers, which can impede the airflow, trap higher concentrations of aerosols in the breathing zone, and therefore increase the infection risk, would allow for a proper adjustment on this ineffective control.”
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