Having a chinwag with your mates in a restaurant or pub is probably one of the riskiest activities for spreading Covid-19, scientists from the National Institutes of Health in the US have suggested.

In a review published in the Journal of Internal Medicine, they say unmasked speech in confined spaces poses the greatest risk of spreading the virus to others – pointing to super-spreader events that have taken place in bars and restaurants.

One of the study’s authors, Adriaan Bax, of the Laboratory of Chemical Physics, said: “We’ve all seen some spit droplets flying when people talk. But there are thousands more, too small to be seen by the naked eye.

“When the water evaporates from such speech-generated, potentially virus-rich droplets, they float in the air for minutes, like smoke, thus putting others at risk.”

Covid infection typically starts in the upper respiratory tract (URT) but can
migrate to the lower respiratory tract (LRT) and other organs.

Whereas lower respiratory tract infection can lead to shedding the virus via breath and cough droplets, infection in the upper respiratory tract enables the virus to be shed via speech droplets, researchers said. 

The viral load can be high in carriers with mild or no symptoms, which may explain why the virus spreads even when people are asymptomatic. It’s thought one in three people are carrying the virus without knowing it. 

Swabs have shown viral load is particularly high when people are pre-symptomatic, which means before they’ve even developed symptoms. 

“A large and perhaps even dominant fraction of virus transmission takes place prior to the onset of Covid-19 symptoms,” researchers said. 

“Considering that breath droplets are generated in the lower respiratory
tract and are unlikely to be rich in virus in the absence of clinical symptoms such as cough, speech droplets must represent a far more prevalent mode of presymptomatic and asymptomatic transmission of SARS-CoV-2.”

Expelled droplets rapidly lose water through evaporation, with the smaller ones
transforming into long-lived aerosols that float about in the air.

Although the largest speech droplets can carry more of the virus, they are few in number, fall to the ground rapidly and therefore play a relatively minor role in transmission, say scientists.

Of more concern is the small speech aerosol, which can linger in the air and then descend deep into the lower respiratory tract of others and cause severe disease. Since their total volume is small, the amount of virus they carry is low, but researchers suggest in closed environments with inadequate ventilation, they can accumulate, elevating the risk of direct lower respiratory tract infection.

Scientists also cautioned about speech aerosols that are intermediate-sized because they remain suspended in the air for minutes and can be transported over “considerable distances” by air currents. And obviously these bigger droplets can carry more virus. 

“The abundance of this speech-generated aerosol, combined with its high viral load in pre- and asymptomatic individuals, strongly implicates airborne transmission of SARS-CoV-2 through speech as the primary contributor to its rapid spread,” they concluded. 

Since eating and drinking often take place indoors and typically involve loud
speaking, “it should come as no surprise that bars and restaurants have become the epicentre of multiple recent super-spreading events,” the researchers wrote.

They said in addition to vaccination, mitigation strategies should emphasise the use of face masks indoors and ensuring adequate ventilation to flush out any aerosols that have accumulated in the air over time.