In a recent study published in the journal PNASResearchers at the US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) tracked the air mass of speech-generated aerosols using an alternative experimental approach that addressed problems arising from condensation with nuclei. Hybrid measurements of the coarse fraction of speech-generated aerosols, with a diameter (D) greater than five micrometers (μm), revealed that they stayed in the air for a few minutes, not hours. However, due to their high volumes and life in the air, they likely dominated the transmission of respiratory diseases, including coronavirus disease 2019 (COVID-19).
Study: Hybrid measurement of respiratory aerosol reveals a dominant coarse fraction due to speech remaining in the air for minutes. Image credits: peterschreiber media/Shutterstock
Quantitative modeling of the airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) requires an understanding of the number and size distribution of respiratory droplets in different environments. In addition, it is critical for appropriate strategies to reduce COVID-19 and to evaluate the relative transmission of SARS-CoV-2 by saliva versus airway droplets.
About the study
Breathing, coughing, sneezing and speaking, including singing, laughing, etc., are the four ways to generate respiratory droplets; however, the current study focused only on comparative analyzes of respiratory droplets generated by speaking and breathing.
The researchers used a simple, low-cost experimental setup to characterize larger and smaller respiratory droplets in the range of 0.3 to 100 m 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 slide deposition.
For larger particles, the team used video analysis of laser light scattering and optical particle sizes (OPSs) and aerodynamic particle sizes (APSs) to quantify smaller respiratory droplets. Video recording of light scattered by exhaled air measures breath droplets in very large numbers (>105 per liter). Therefore, the researchers fed all the respiratory droplets directly into a room with low humidity, in which droplets with a D 80 m did not completely dry out and sediment within seconds. They then viewed these larger droplets based on their Stokes sedimentation rate by laser light scattering and used OPS to measure their number, size and settling rate.
On the other hand, respiratory droplets with D 5 m dry out immediately after entering the atmosphere. When generated by a person infected with SARS-CoV-2, such droplets can remain viable and infectious for many hours. Importantly, coarse aerosols with a D 5 m deposit 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 underlined the amount of speech aerosol, especially those larger than 4 m. However, the hybrid measurements of the current study revealed that much of the coarse aerosol generated by speech was intermediate in size from 5 to 20 m in diameter. This aerosol remained in the air for minutes, but was too large to penetrate directly through the LRT.
Video recording of laser light scattered by breath droplets. (A) Single frame of a 120 fps video recording of exhaled breath, over a 0.7 mm thick sheet of blue laser light. Particles have undergone nucleated condensation, resulting in droplet sizes of approximately 1 to 2 m. (B) Particle count 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. Because the leaf transition time (about 3 ms) is shorter than the duration of a single frame, very few droplets are visible in successive frames. The video is available at https://doi.org/10.5281/zenodo.6131524.
The study had a number of other important findings. First, consistent with the observation that most SARS-CoV-2 infections begin in the URT, the authors noted that the air mass of the coarse speech aerosol was approximately twice that of the fine aerosol, and therefore not in the LRT was able to penetrate. †
Second, they noted that the SARS-CoV-2-containing breath aerosol from hospitalized COVID-19 patients with viral pneumonia was less than 5 m. probably during the first phase of the pandemic when they had no access to high quality respirators. More importantly, the study analysis revealed that although breathing is a continuous activity, speaking a few words per hour generates much more aerosol mass than breathing.
In addition, the authors noted that breath particles originate from the lungs of an infected person and only contain a viable virus if the infection involves the LRT. Therefore, asymptomatic transmission of SARS-CoV-2 carriers with the URT infections are involved. Speaking generates more virus-containing aerosols that can be transported over greater distances in jet-like streams before finally dispersing into the atmosphere. Therefore, studies have documented more SARS-CoV-2 superspreader events in bars, conferences and restaurants, and none in libraries and movie theaters.
Another important observation was that increased ventilation is one of the most effective ways to reduce the concentration of speech and breath aerosols, which carry the greatest risk of serious disease. However, the rapid gravity sedimentation of coarse aerosol makes it challenging to reduce SARS-CoV-2 transfer through increased ventilation.
Overall, the study showed that in the absence of COVID-19 symptoms such as coughing and sneezing, speech-induced aerosols actively transmit respiratory diseases, including COVID-19. However, new research should expand on these findings to gain a better quantitative understanding of speech aerosols generated in practice.