The airborne spread of pathogens has assumed great importance in the public eye following the onset of the coronavirus disease 2019 (COVID-19 pandemic). In an interesting new research paper published recently on the bioRxiv* preprint server, scientists describe the dispersal of exhaled air, potentially infected, from singers and those playing wind instruments, using Schlieren techniques, a visual process that is used to photograph the flow of fluids of varying density. This could help assess measures to assess the actual spread of infectious droplets or aerosols in such situations.
It is now known that both droplets and aerosols, with a size of > 5 µm and < 5 µm, respectively, carry the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), can spread outwards depending on their size. Heavier droplets, about 100 µm in size, travel only a few seconds before they fall to the ground, reaching about 1.5 m from the source. However, the smaller particles in aerosols can remain suspended far longer in the air.
Earlier, several studies have concluded that the spread of such particles is almost nil at 0.5 m from the mouth of a professional singer, as indicated by the presence of only minute disturbances observed at a candle flame placed at this distance from the source of exhaled air. Later, it was observed that exhalation of air is much more forcible during professional singing rather than during speaking or breathing.
With wind instruments, the pattern of air escape is similar to that of singing, with the distance of spread being determined by the speed at which air escapes from the mouth or instrument and the outlet diameter.
The current study applies flow visualization techniques and anemometry to investigate the dispersal of exhaled air in terms of the pattern of spread and the velocity at which the air escapes. The scientists used two methods to observe the flow, namely, schlieren imaging using a schlieren mirror and background-oriented schlieren (BOS).
Schlieren refers to a method of photography applied to the visualization of flows of varying density by exploiting the bending or refraction of light rays when they pass across an interface separating two substances of different densities.
The advantages of these techniques are the ability to observe density gradients in transparent media, due to variations in temperature or pressure, without distorting the flow field. The measurement field of schlieren imaging is restricted by the mirror size, that is, 100 cm. To correctly visualize the spread of exhaled air beyond these limits, BOS was used.
The breathing air is warmer and more humid than the surrounding air, leading to gradients that can be captured by these techniques. The researchers looked at woodwind instruments, which release air in an initial laminar pattern followed by turbulence, and finally mixing with the surrounding room air. With singers, the air spreads most as the tone production begins and is highest when singing consonants or when precise articulation is required.
The researchers observe that both the distance to which exhaled air spreads and the angle at which air escapes are both different with the instrument and player, or singer.
Setup of the single-mirror coincident schlieren system (left) and the BOS system (right) at the Department of Building Physics at the Bauhaus-University Weimar
With woodwind instruments, air escapes from the bell, the tone holes, and is blown over (flutes) or leaks near the mouthpiece (with the oboe or bassoon). Playing the oboe or bassoon requires intermittent exhalation through the mouth and nose as well, since all the air cannot escape from the tone holes. The air travels fastest when high pitches are used, but also during intermittent exhalation. With the latter, the velocity decreases steadily thereafter.
Convective flow may also occur, accounting for air movements of about 0.02 m/s at 85 cm away from the bell. This is the farthest sensor. With the most proximal sensor, the highest velocity is observed at 45 seconds, corresponding to very transient jets produced by large emissions of breathing air.
Air escapes from the bell over much shorter distances relative to the air that leaks from the instrument at various points, or during intermittent blowing, and other practices of sound production. Air leaks can travel about 60 cm into the room from the intermittent exhalation of air through the mouth and nose between two phrases. However, it moves to within 30 cm when playing various notes. At high pitches, air hardly escapes from the bassoon bell, while the greatest velocity of airflow from the bell is at low notes. Since most tone holes are uncovered at high notes, these produce maximal airflow from these holes.
The air escaping from the bell travels different distances depending on the bore width and the breathing pressure at the moment of playing.
With brass instruments, the schlieren imaging shows that with most of these instruments, the escaping air from the bell is very turbulent because of the bigger diameter of the bell. The air blown into the mouthpiece blows into the bell.
The breathing air either travels up because of natural convection or mingles with the room air. The factors that decide the shape and the distance of the air that escapes from the bell include the musician’s physique and blowing technique, and the angle of the instrument to the mouth. The distances were measured from the bell, the mouth, or the mouthpiece. Breathing air goes out from the bell to about 25 cm at low pitches and a little more at high pitches. Air can leak from the mouthpiece when the player’s lips become tired, when playing staccato, or when the musicians are untrained or older.
With damper use, the escape of air is substantially reduced, except with the F tuba and the French horn when a stopping mute is used.
Anemometry findings confirmed the results of the Schlieren visualizations, showing that flow values are always above about 0.02 m/s. The reasons might include finger or hand movements during playing, air escape from the tone holes, taking a breath between musical phrases, or other convection airflows in the same room. With some instruments, the measured velocity first decreases as the distance from the instrument increases and then begins to increase. This effect may be due to turbulent flow, producing small vortexes that result in varying velocity.
To reduce such flow from all kinds of brass instruments, the researchers said a simple filter could be used, made of cellulose, and taped to the instrument's bell. This will work because the air that is breathed through such instruments escapes entirely through the bell.
With woodwind instruments, air escapes from the tone holes and even leaks from the mouthpiece, in addition to the bell. A filter will not hinder the spread of the air, therefore.
What are the implications?
This data could help discover the range to which exhaled air, potentially containing infectious particles, could spread during infectious airborne disease outbreaks. However, the studies only show the range of larger droplets' spread since small droplets or aerosols are not visualized by Schlieren methods. These results show that airflow does not travel more than 1.2 m into the room.
Secondly, these patterns relate to air blown out by professionally trained singers and musicians. Amateurs and learners may produce very different exhalation patterns and leakage, which may result in a larger volume of air spread into the room.
The movement of the player can also change the velocity of the breathing air, which also varies with the bell diameter and the breathing pressure. Air escaping from the mouth or leaking at the mouthpiece shows a higher velocity of up to 0.15 m/s.
Using this data, the range of spread, dimensions of escaping air, and velocity at which it escapes and spreads, can be estimated for woodwind and brass instruments and professional singers. This would help quantify the risk of viral transmission during such performances so as to develop the best safety precautions for such situations.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.