In a recent study published in the journal PNAS, researchers from the US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) monitored the airborne mass of speech-generated aerosols using an alternative experimental approach that addressed condensation-related issues. nucleated. Hybrid measurements of the coarse fraction of speech-generated aerosols with a diameter (D) greater than five micrometers (μm) revealed that they remained airborne for minutes, not hours. However, due to their high volumes and airborne lifetimes, they likely dominated the transmission of respiratory diseases, including coronavirus disease 2019 (COVID-19).
Study: Hybrid measurement of respiratory aerosols reveals a dominant coarse fraction resulting from speech that remains airborne for minutes. Image Credit: peterschreiber media / Shutterstock
Quantitative modeling of airborne severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission requires a good understanding of the number and size distribution of respiratory droplets in different environments. Additionally, it is crucial for appropriate COVID-19 mitigation strategies and for assessing the relative transmission of SARS-CoV-2 through saliva versus liquid-origin droplets that line the airways.
About the study
Breathing, coughing, sneezing, and speaking, including singing, laughing, etc., are the four modes of generation of respiratory droplets; however, the current study focused only on comparative analyzes of respiratory droplets generated by speech and breathing.
The researchers used a simple and inexpensive experimental setup to characterize larger and smaller respiratory droplets in the range of 0.3 to 100 μm side-by-side. In particular, nucleated condensation temporarily increases the mass of respiratory droplets, accelerating their gravitational sedimentation. This effect introduces discrepancies between studies using optical particle counters and those using microscope slide deposition.
For larger particles, the team used video analysis of laser light scattering and optical particle sizers (OPS) and aerodynamic particle sizers (APS) to quantify the smaller respiratory droplets. Video recording of light scattered by exhaled air measures respiratory droplets in very large numbers (>105 per liter). Therefore, the researchers entered all respiratory droplets directly into a low humidity chamber, in which droplets with D ≳ 80 μm did not completely dehydrate and sediment within seconds. Then they visualized these larger droplets sized according to their Stokes sedimentation velocity by laser light scattering and used OPS to measure their number, size, and sedimentation velocity.
On the other hand, respiratory droplets with D ≳ 5 μm dehydrate immediately after entering the atmosphere. When generated by someone infected with SARS-CoV-2, these droplets can remain viable and infectious for many hours. Importantly, coarse aerosols with D ≳ 5 μm settle in the upper respiratory tract (URT) and finer aerosols reach the lower respiratory tract (LRT), causing life-threatening pneumonia.
According to the authors, many previous studies have highlighted the amount of vocal aerosols, especially those larger than 4μm. However, hybrid measurements in the present study revealed that much of the coarse aerosol generated by speech was intermediate in size between 5 and 20 μm in diameter. This aerosol remained airborne for a few minutes, but was too large to directly enter the TLR.
Video recording of laser light scattered by breath droplets. (A) Single frame from a 120 fps video recording of exhaled breath, passing through a 0.7 mm thick sheet of blue laser light. The particles underwent nucleated condensation, resulting in droplet sizes of approx. 1 to 2 µm. (B) Number of particles as a function of frame number. The integral of the number is represented by the solid black line, with the scale marked on the right side. Since the sheet crossing time (about 3 ms) is shorter than the duration of a single frame, very few droplets are visible in consecutive frames. The video is available at https://doi.org/10.5281/zenodo.6131524.
The study had several other important findings. First, consistent with the observation that most SARS-CoV-2 infections begin in the URT, the authors noted that the airborne mass of the coarse aerosol was about twice as large. to that of the fine aerosol, therefore, it could not penetrate the LRT.
Second, they observed that respiratory aerosol containing SARS-CoV-2 from hospitalized patients with COVID-19 with viral pneumonia was less than 5 μm. Medical personnel were at risk of being infected with COVID-19 and were more likely to die during the early phase of the pandemic when they did not have access to high quality respirators. More importantly, the study’s analysis found that even though breathing is a continuous activity, speaking a few words per hour generates significantly more aerosol mass than breathing.
Additionally, the authors noted that breath particles come from the lungs of an infected individual and only contain viable virus if the infection involves LRT. Therefore, asymptomatic transmission of SARS-CoV-2 involves carriers of URT infections. Speech generates more virus-containing aerosols that are transportable over greater distances in jet-like flows before eventually dispersing into the atmosphere. Therefore, studies have documented more SARS-CoV-2 superspreading events in bars, conferences, and restaurants, and none in libraries and cinemas.
Another important observation was that increased ventilation is one of the most effective ways to reduce the concentration of speech and breath aerosols that pose the highest risk of serious illness. However, the rapid gravitational sedimentation of coarse aerosols makes it difficult to mitigate SARS-CoV-2 transmission through increased ventilation.
Overall, the study demonstrated that in the absence of Covid-19 symptoms like coughs and sneezes, voice-borne aerosols actively transmit respiratory diseases, including COVID-19. However, further research should expand these findings to gain a better quantitative understanding of vocal aerosols generated in real-world contexts.
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