A study from UC Davis, published in Nature and called "Aerosol emission and superemission during human speech increase with voice loudness," finds that:
"The rate of particle emission during normal human speech is positively correlated with the loudness (amplitude) of vocalization."
To explain: As we all know, coughing or sneezing sends potentially virus-carrying droplets into the air. These coughed or sneezed particles are often large enough for the eye to see, about 50 nanometers or larger.
But the mere acts of breathing or talking also sends droplets into the air. These droplets are much smaller and invisible, at approximately 1 nanometer, but they're still "sufficiently large to carry a variety of respiratory pathogens" includes the measles virus and influenza virus. The study reckons these smaller particles "are potentially more infectious than larger sneeze- or cough-generated droplets" (emphasis ours) for three reasons:
1. Light weight. These smaller particles float around in the air for a longer period of time, unlike the heavier sneeze- and cough- generated particles that gravity takes care of.
2. Smaller size. The smaller particles "have a larger probability of penetrating further into the respiratory tract of a susceptible individual to initiate a lower respiratory tract infection."
3. Higher count. The study states that "speech can release dramatically larger numbers of particles compared to coughing," about 2 to 10 times more, according to their cited research.
Now that you've understood that, let's get to the volume part. Here's what they found by using an "aerodynamic particle sizer (APS) placed in a laminar flow hood" while volunteers spoke into it:
1. The particle emission rate during speech is linearly correlated with the amplitude (loudness) of vocalization, for four different languages tested (English, Spanish, Mandarin, and Arabic).
2. The particle size distribution is independent of vocalization loudness or language spoken.
3. Some individuals emit particles at a rate more than an order of magnitude larger than their peers, i.e., they behave as "speech superemitters."
What's interesting is that the researchers were not able to nail what makes people superemitters. They looked at age, gender and BMI but found no correlation. Also, while the study found that increasing voice volume increases particle emissions, not all of the superemitters spoke loudly.
In an effort to learn why, study leader Sima Asadi is following up with a new study, according to UC Davis' Chemical Engineering newsletter. This study will look at "how different 'phonemes,' the units of sound that make up words and phrases, relate to particle emission. The theory is that speaking certain words, phrases and languages impacts how many particles a person releases, though she (Asadi) notes that more research needs to be done to confirm."
How fascinating would it be if certain words caused increased particle emission?
Lastly, we should note that this study was conducted last year or earlier, well prior to the COVID-19 outbreak; the paper was published on February 20th of 2019. That explains why there are no direct mentions of COVID-19.
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This explains the vast outbreak in the US.