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Researchers used laser light scattering and an optical particle counter to measure the size of potentially contagious aerosols expelled when speaking and found that most prior reports failed to detect the dominant portion of speech-generated aerosols with diameters between 5 and 20 microns; the findings indicate that many SARS-CoV-2 aerosols remain airborne for minutes rather than hours and, if inhaled, such intermediate-size particles may deposit in the upper respiratory tract rather than reaching the lungs, and, thus, even in the absence of symptoms such as coughing and sneezing, particles spread through speaking may play a key role in the transmission of diseases such as COVID-19, according to the authors.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr Joel Rindelaub, Aerosol Chemist, University of Auckland
Every time someone with COVID-19 breathes, talks or coughs, they are emitting thousands of virus-laden aerosol particles into the air that others can inhale and then become infected. This study concluded that the amount of aerosols produced from talking has been previously underreported, meaning that activities that promote conversation should have an increased focus when trying to reduce COVID-19 transmission. The authors found that speaking only four words over the course of an hour led to almost 10 times more particle mass than just regular breathing over the same time.
With the large amount of aerosol particles that can be exhaled during conversations, it should be no surprise that most of the reported superspreader events have occurred at social locations, such as bars, restaurants, conferences, and church gatherings.
This study also supports the theory that COVID-19 infections begin in the upper airways. The authors found that larger particle sizes (> 5 μm) were produced during speech. At these sizes, the airborne particles wouldn't be capable of reaching the lungs during inhalation and would instead settle in the nose and throat.
Overall, the results of this paper further highlight the importance of proper mask use. When engaging in conversation, you are shooting a stream of dirty particles directly at someone's face. Sometimes you'll see people lower their mask when talking to others but really the opposite should be happening. Conversations are high risk and a high quality N95 mask can keep you and social acquaintances safe. Additionally, if you're trying to protect yourself from COVID-19, wearing a mask over the mouth but not the nose is a complete fail because the nose is where COVID infection is probably happening.
It is important to note that only one healthy individual was tested in this study, meaning that the particle size distributions observed may not be representative of a wider population or that of a COVID-19 infected patient. This also means that no data on the amount of virus within each respiratory particle size was produced from this study.
