Virus-carrying droplets disperse as infected people breathe, scientists decode how

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As people breathe, they emit droplets of various sizes that don't necessarily follow the airflow faithfully, scientists say. Representative image: Shutterstock
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London: Scientists have developed a new framework to understand the dispersion of droplets of different sizes which are ejected as people breathe, findings that shed more light on the transmission pattern of diseases such as COVID-19.

The study, published in the journal Physics of Fluids, used mathematical formulae to determine the maximum range of small, intermediate and large-sized droplets.

According to the researchers, including those from the University of Edinburgh in the UK, the findings have important implications for understanding the spread of airborne diseases like COVID-19 since their dispersion tests revealed the absence of intermediate-sized droplets.

"We wanted to develop a mathematical model of someone breathing that could be explored analytically to examine the dominant physics at play," said Cathal Cummins, a co-author of the study from the Heriot-Watt University in the UK.

As people breathe, they emit droplets of various sizes that don't necessarily follow the airflow faithfully, the scientists said.

The current study, according to the researchers, provides a general framework to understand the droplet dispersion.

They said the model provides formulas to predict when such droplets will have short ranges.

"Our study shows there isn't a linear relation between droplet size and displacement -- with both small and large droplets travelling further than medium-sized ones," said Felicity Mehendale, a co-author of the study from the University of Edinburgh.

"We can't afford to be complacent about small droplets," Mehendale said, adding that personal protective equipments (PPEs) used by healthcare workers and clinicians are effective barriers to large droplets, but may be less effective for small ones

The scientists noted that they are currently working on plans to manufacture an aerosol extractor device to keep clinicians safe during a wide range of aerosol-generating procedures routinely performed in medicine and dentistry.

They said one such extraction unit placed near the droplet sources can effectively trap droplets if their diameters fall below that of a human hair.

"This has important implications for the COVID-19 pandemic," Cummins said.

"Larger droplets would be easily captured by PPE, such as masks and face shields. But smaller droplets may penetrate some forms of PPE, so an extractor could help reduce the weakness in our current defense against COVID-19 and future pandemics," he added.

According to Mehendale, a better understanding of the droplet behaviour will help inform the safety guidelines for aerosol-generating procedures, and is also relevant during the current and future pandemics, as well as for other infectious diseases.

"This mathematical model may also serve as the basis of modelling the impact on droplet dispersion of ventilation systems existing within a range of clinical spaces," she added.

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