Given that 1mm of oral fluid contains approximately 7m copies of the coronavirus (according to previous research), Business Insider reports that the authors of a new study at Stanford University determined that speaking loudly for just one minute can emit more than 1,000 virus-containing droplets. Their work also shows that those droplets can then remain in the air for eight minutes or longer.
While it's well understood now that the coronavirus (whose clinical name is SARS-CoV-2) can spread through droplets from coughs and sneezes, the study underscores the threat posed by asymptomatic and pre-symptomatic carriers – infected people who don't feel sick but can still pass along the virus.
"Speech droplets generated by asymptomatic carriers of SARS-CoV-2 are increasingly considered to be a likely mode of disease transmission," the researchers wrote. The study also found that louder speech produces a higher number of droplets, though the authors noted that "there is a substantial probability that normal speaking causes airborne virus transmission in confined environments."
The report says that's consistent with a recent US Centres for Disease Control and Prevention (CDC) analysis of a coronavirus outbreak at a choir practice in Washington state. Of the 61 attendees, 33 tested positive and another 22 developed suspected cases.
"The act of singing, itself, might have contributed to transmission through emission of aerosols, which is affected by loudness of vocalisation," the CDC report said.
The report says the new study found that droplets emitted via speech shrink to between 20% and 34% of their original size after they're released into the air. That slows down the speed at which they fall to ground, meaning they can remain in the air for several minutes.
The findings come as states begin lifting lockdowns, allowing businesses to reopen and permitting some social gatherings. In some places, unmasked crowds are defying social-distancing guidelines, sparking concerns about new outbreaks.
"We're not re-opening based on science," Dr Thomas Frieden, a former director of the Centres for Disease Control and Prevention, is quoted in the report as saying. "We're reopening based on politics, ideology, and public pressure. And I think it's going to end badly."
Abstract
Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission. Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second. In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration. These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.
Authors
Valentyn Stadnytskyi, Christina E Bax, Adriaan Bax, Philip Anfinrud
Abstract 2
What is already known about this topic?
Superspreading events involving SARS-CoV-2, the virus that causes COVID-19, have been reported.
What is added by this report?
Following a 2.5-hour choir practice attended by 61 persons, including a symptomatic index patient, 32 confirmed and 20 probable secondary COVID-19 cases occurred (attack rate = 53.3% to 86.7%); three patients were hospitalized, and two died. Transmission was likely facilitated by close proximity (within 6 feet) during practice and augmented by the act of singing.
What are the implications for public health practice?
The potential for superspreader events underscores the importance of physical distancing, including avoiding gathering in large groups, to control spread of COVID-19. Enhancing community awareness can encourage symptomatic persons and contacts of ill persons to isolate or self-quarantine to prevent ongoing transmission.
Authors
Lea Hamner; Polly Dubbel; Ian Capron; Andy Ross; Amber Jordan; Jaxon Lee; Joanne Lynn; Amelia Ball; Simranjit Narwal; Sam Russell; Dale Patrick; Howard Leibrand
[link url="https://www.businessinsider.co.za/loud-speech-coronavirus-droplets-study-2020-5?r=US&IR=T"]Full Business Insider report[/link]
[link url="https://www.pnas.org/content/early/2020/05/12/2006874117"]PNAS abstract[/link]
[link url="https://www.cdc.gov/mmwr/volumes/69/wr/mm6919e6.htm?s_cid=mm6919e6_e&deliveryName=USCDC_921-DM28169"]CDC analysis[/link]