KEEP QUIET! Infected person talking loudly could generate coronavirus droplets and lead to airborne transmission
Coughing or sneezing may not be the only way people transmit infectious diseases such as the novel coronavirus to one another. Thousands of droplets from the mouths of people who are talking loudly can stay in the air for between eight and 14 minutes before disappearing, according to a new study. The research, conducted by experts from the US National Institutes of Health (NIH) the University of Pennsylvania, could have a significant impact on our understanding of Covid-19 transmission.
The novel coronavirus is primarily spread from person to person. According to the US Centers for Disease Control and Prevention, a person can become infected with coronavirus by coming into close contact (about 6 feet) with a person who has Covid-19. One can become infected from respiratory droplets when an infected person coughs, sneezes, or talks. That’s why coughing and sneezing are important. But speech can release thousands of oral fluid droplets into the air too. Accordingly, the research team wanted to find out how many were produced and how long they could remain airborne.
In the current analysis, researchers asked people to repeat phrases and used sensitive lasers to visualize the droplets they produced, watching them decay in a closed environment. Based on previous studies of how much viral RNA can be found in oral fluids in the average Covid-19 patient, the researchers estimate that a single minute of talking loudly generates at least 1,000 virus-containing droplets. Their observations suggest these droplets stay airborne -- that is, they can linger in the air -- for longer than eight minutes, and sometimes as long as 14 minutes.
“Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2-the virus that causes Covid-19) 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 minutes,” say researchers in their findings published in Proceedings of the National Academy of Sciences (PNAS). “These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments,” they conclude.
The current study is based on an experiment that used an intense sheet of laser light to visualize and study the number of small respiratory droplets produced during human speech. The research did not involve the coronavirus or any other virus but instead looked at how people generate respiratory droplets when they speak. The experiment did not look at large droplets but instead focused on small droplets that can linger in the air much longer. The researchers write that their estimates are conservative. Some patients produce a much larger amount of the virus than average, which could increase the number of virus-containing droplets “to well over 100,000 per minute of speaking.”
The findings could help explain how people with mild or no symptoms may infect others in close quarters such as households, nursing homes, conferences, offices, and other confined spaces. “Our laser light scattering method not only provides real-time visual evidence for speech droplet emission but also assesses their airborne lifetime. This direct visualization demonstrates how normal speech generates airborne droplets that can remain suspended for tens of minutes or longer and are eminently capable of transmitting disease in confined spaces,” says the team.
Recently, an analysis found how a choir practice, where one person who did not show any symptoms, led to 87% of others developing Covid-19 in Washington, US. According to the CDC study, the transmission was likely facilitated by close proximity (within 6 feet) during practice and augmented by the act of singing. The researchers said that the singing practice provided several opportunities for droplet and fomite transmission, including members sitting close to one another, sharing snacks and stacking chairs at the end of the practice. “The act of singing itself might have contributed to the transmission through the emission of aerosols, which is affected by the loudness of vocalization,” say experts.
According to Dr Michael Cotterell, NERC Research Fellow, University of Bristol, who was not involved in the study, the laboratory research techniques used are novel and “provide compelling evidence that micron-sized speech droplets have significant lifetimes (several minutes).”
“More work is needed to understand the viability (including aerosol size dependence to rates of death) of SARS-CoV-2 in aerosol droplets, viral load, and contexts such as filtration efficiencies for various mask/face coverings, before any recommendations on the aforementioned health and safety precautions can be made in respect of mitigating airborne transmission. The lifetimes and number of concentrations of speech aerosol have been characterized in previous work by other researchers. It (current study) does not transform our understanding, but adds further evidence to building consensus on the range of lifetimes for speech-generated aerosols,” says Dr Cotterell in a statement.
Lawrence Young, professor of molecular oncology, University of Warwick, says that one of the main assumptions in this paper is that each virus particle in a droplet is equally capable of causing an infection, but it is not known that this is the case for SARS-CoV-2. “This study measured the size and spread of oral fluid droplets using a laser. It convincingly shows that normal speech generates airborne droplets that can remain suspended in the air for tens of minutes or longer. This suggests that virus from an infected individual could be transmitted this way in confined spaces. However there is no direct analysis of the presence of viruses in the droplets or their ability to pass on the infection,” says Young, who was not involved in the study. According to him, the findings add weight to the need for social distancing and raises important issues about the potential for the virus to spread in confined spaces such as offices and factories. “It also highlights the problem of virus transmission from infected individuals who do not have symptoms,” says Young.