Study of expired breath shows droplets emitted during speech can linger in air for eight minutes or more: another reason to wear a mask in public.

photo by Juraj Varga courtesy of pixabay.com

A “brief report” published in the Proceedings of the National Academy of Sciences on May 13 shows that small saliva droplets emitted during speech can linger in the air for eight minutes or more.  This study did not involve people infected with SARS-COV-2, the virus that causes COVID-19, partially for safety reasons.  However, it did demonstrate that these droplets are big enough to contain infectious virus and small enough to “float” in ambient air for minutes.

It makes us beware of stagnant indoor air and provides data from which we can strongly recommend that people wear masks whenever in social situations.  We can be sure that the virus is transmitted through the air we breathe while speaking as well as by contact with contaminated objects or hand-to-hand.

The research used sheets of laser light to illuminate saliva droplets emitted by people during normal speech; they found an average of a thousand droplets per second ranging from roughly 1 to 500 microns (thousandths of a millimeter); those less than 10 microns can literally float in the air almost indefinitely.  Each droplet can contain viruses in addition to 95-99% water, dead epithelial cells, bacteria, and other debris.  On drying out, such particles maintain their infectious load but can float even more effectively.  This is the mechanism by which the measles virus can stay in the air for two hours after a measles patient leaves the room, waiting to infect the next susceptible person to come along.

We don’t yet know how many virions (individual virus particles) it takes to establish an infection in a susceptible person; it may be as few as one or as many as several thousands.  In any case, the probability of exposure is nonzero– not a reassuring prospect.

From the study’s abstract:

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.

It has long been recognized that respiratory viruses can be transmitted via droplets that are generated by coughing or sneezing. It is less widely known that normal speaking also produces thousands of oral fluid droplets with a broad size distribution (ca. 1 μm to 500 μm) (1, 2). Droplets can harbor a variety of respiratory pathogens, including measles (3) and influenza virus (4) as well as Mycobacterium tuberculosis (5). High viral loads of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been detected in oral fluids of coronavirus disease 2019 (COVID-19)−positive patients (6), including asymptomatic ones (7). However, the possible role of small speech droplet nuclei with diameters of less than 30 μm, which potentially could remain airborne for extended periods of time (1, 2, 8, 9), has not been widely appreciated.

In a recent report (10), we used an intense sheet of laser light to visualize bursts of speech droplets produced during repeated spoken phrases. This method revealed average droplet emission rates of ca. 1,000 s⁻¹ with peak emission rates as high as 10,000 s⁻¹, with a total integrated volume far higher than in previous reports (1, 2, 8, 9). The high sensitivity of the light scattering method in observing medium-sized (10 μm to 100 μm) droplets, a fraction of which remain airborne for at least 30 s, likely accounts for the large increase in the number of observed droplets.

The amount by which a droplet shrinks upon dehydration depends on the fraction of nonvolatile matter in the oral fluid, which includes electrolytes, sugars, enzymes, DNA, and remnants of dehydrated epithelial and white blood cells. Whereas pure saliva contains 99.5% water when exiting the salivary glands, the weight fraction of nonvolatile matter in oral fluid falls in the 1 to 5% range.

The independent action hypothesis (IAH) states that each virion has an equal, nonzero probability of causing an infection. Validity of IAH was demonstrated for infection of insect larvae by baculovirus (15), and of plants by Tobacco etch virus variants that carried green fluorescent protein markers (16). IAH applies to systems where the host is highly susceptible, but the extent to which IAH is valid for humans and SARS-CoV-2 has not yet been firmly established. For COVID-19, with an oral fluid average virus RNA load of 7 × 10⁶ copies per milliliter (maximum of 2.35 × 10⁹ copies per milliliter) (7), the probability that a 50-μm-diameter droplet, prior to dehydration, contains at least one virion is ∼37%. For a 10-μm droplet, this probability drops to 0.37%, and the probability that it contains more than one virion, if generated from a homogeneous distribution of oral fluid, is negligible. Therefore, airborne droplets pose a significant risk only if IAH applies to human virus transmission. Considering that frequent person-to-person transmission has been reported in community and health care settings, it appears likely that IAH applies to COVID-19 and other highly contagious airborne respiratory diseases, such as influenza and measles.