Experimental study finds how coronavirus can spread while speaking

Based on experiments and simulations, Princeton University researchers have found that air emitted during plosive speech sounds — where a consonant (P, B, T, D, K and G) is produced by stopping the airflow using the lips, tongue tip or body followed by a sudden release of air — lead to significantly enhanced directed transport of the virus. When the speech contains a train of such puffs a continuous, turbulent, jet-like flow is formed and is capable of transporting air and droplets to over two metres in just 30 seconds.

Now, there is one more study showing how important it is to wear a mask when speaking and improve ventilation to cut the risk of spreading novel coronavirus.

Based on experiments and simulations, a team of researchers led by Howard A. Stone from Princeton University has been able to show how air emitted during plosive speech sounds, where a consonant is produced by stopping the airflow using the lips followed by a sudden release of air, lead to significantly enhanced directed transport of the virus.

They found that during certain sounds like consonant ‘P’, ‘B’, the vocal tract is blocked temporarily either with the lips or with the tongue tip (T, D) or body (K, G), so that the pressure builds up slightly and then is released rapidly, producing the characteristic burst of air of these sounds. They found consonants such as ‘P’ and ‘B’ forms vortical puffs that travel to a distance of approximately one meter. Peak velocities are seen at the emission of the sound P with values close to 1.2 to 1.5 metres per second. When the speech contains a train of such puffs a continuous, turbulent, jet-like flow is formed. Such jet-like flows from continuous puffs are capable of transporting exhaled air and droplets out to over two meters in just 30 seconds of conversation.

“We can expect stronger propagation when breathing after exercising, as the volumetric flow rates are increased, which could make breathing in such a case closer to blowing,” they say.

This work should inform public health guidance for risk reduction and mitigation strategies of airborne pathogen transmission,” the researchers write in a paper published in the journal Proceedings of the National Academy of Sciences.

Visualising the flow when saying a sentence like “We will beat the corona virus”, the researchers found different parts of the sentence had different velocities and angles.

“Our results show that typical airflow speeds at one-two metre distances from a speaker are typically tens of centimetres per second. This means that the ambient air current may be dominant at such distances from a speaker,” they note.

Based on the experimental and numerical results, the researchers note that exhaled materials reach 0.5 to 1 metre in just one second during normal breathing and speaking, and in fractions of a second in the case of plosive consonants.

During extended discussions and meetings in confined spaces would result in the surrounding air potentially containing exhaled air over a significantly longer distance, they say.

“It follows that in conversations longer than 30 seconds it is better, in our opinion but based on the results, to move beyond two metres of separation, and to stand to the side of a speaker — outside of a cone of 40-50 degrees (half angle), further reduces possible inhaled aerosol. Most significantly, our results illustrate that two metres do not represent a “wall,” but rather that behaviour can help minimise risk by increasing separation distances and relative position for longer conversations when masks are not used,” they caution.

Published in The Hindu on September 26, 2020