Air transmission of Covid-19 is being underestimated – which could impact guidelines on social distancing, ventilation systems and shared spaces, warns new research.
Researchers at Heriot-Wyatt University and University of Edinburgh found evidence that both small and large droplets can travel relatively long distances through the air – and not always in predictable directions with airflow.
The World Health Organisation (WHO) warned aerosol transmission of Covid-19 is being underestimated as a study reveals droplet spread from humans does not always follow airflow.
Researchers say the new findings on droplet migration may have important implications for understanding the spread of airborne diseases such as COVID-19.
It comes after top US infectious disease specialist Dr Anthony Fauci admitted during a Monday JAMA interview that there is much unknown about how coronavirus spreads through the air and that he himself needs to ‘study’ papers that suggest big droplets can travel further than six feet.
Scientists of the study at Heriot-Watt University and the University of Edinburgh in Scotland echoed his sentiments that a better understanding of different droplet behaviors and their spread based on droplet size is also needed.
Scientists in the UK found that both small and large droplets from people coughing or just breathing can travel far distances and unpredictable directions, suggesting the airborne danger of coronavirus – but they invented a device to ‘extract’ tiny infectious particles not stopped by masks from the air (file)
Government guidelines will need to be considered if air transmission is proven to be significant.
Dr Cathal Cummins, an assistant professor at Heriot-Watt University in Edinburgh, said: ‘The flow physics of someone coughing is complex, involving turbulent jets and droplet evaporation.
‘And the rise of Covid-19 has revealed the gaps in our knowledge of the physics of transmission and mitigation strategies.’
One such gap in the physics is a clear, simple description of where individual droplets go when ejected.
Dr Cummins added: ‘We wanted to develop a mathematical model of someone breathing that could be explored analytically to examine the dominant physics at play.’
The team created a mathematical model that clearly shows small, intermediate and large-sized droplets.
They found simple formulas can be used to determine a droplet’s maximum range.
This has important implications for understanding the spread of airborne diseases such as Covid-19 because their tests revealed the absence of intermediate-sized droplets, as expected.
As a person breathes, they release droplets of different sizes that do not necessarily follow the airflow.
Dr Cummins said: ‘We represent breathing as a point source of both air and droplets and include a point sink to model the effect of extraction of air and droplets.
‘To take their size and density differences into account, we use the Maxey-Riley equation, which describes the motion of a small but finite-sized rigid sphere through a fluid.’
This work gives researchers a general framework to understand the droplet dispersion.
The model shows that bimodality, or having two modes, could actually be a property of the droplets themselves.
Researchers provided formulas to predict when such droplets will have short ranges and say both large and small droplets can travel further than medium-sized ones.
A diagram from the study shows the varied and far trajectories that simple heavy breathing can send both small and large particles
Co-author Dr Felicity Mehendale, an academic surgeon at the University of Edinburgh, said: ‘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.
‘We can’t afford to be complacent about small droplets. PPE is an effective barrier to large droplets but may be less effective for small ones.’
As a solution, Dr Mehendale came up with the idea of creating an aerosol extractor device.
The team is working on plans to manufacture the product to keep clinicians safe during a wide range of aerosol-generating procedures routinely performed in medicine and dentistry.
Extraction units placed near the droplet sources can effectively trap droplets, if their diameters fall below that of a human hair.
Dr Cummins said: ‘This has important implications for the COVID-19 pandemic.
‘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 defence against Covid-19 and future pandemics.’
Dr Mehendale said a better understanding of the droplet behavior will help ‘inform the safety guidelines for aerosol-generating procedures.
She added: ‘It will be 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.’
Findings were published in the Physics of Fluids journal from AIP Publishing.