Droplets expelled from a cough or a sneeze shoot into the air and can travel metres, carried by invisible air currents, posing a silent risk to those nearby. Understanding how airborne viruses like COVID-19 spread through respiratory droplets is critical for developing public safety protocols, especially in crowded or poorly ventilated spaces.
“Generally, larger droplets tend to settle quickly, whereas smaller ones remain airborne much longer and pose long range transmission risks,” explained Chang Wei Kang, Director of the Fluid Dynamics Department at A*STAR’s Institute of High Performance Computing (IHPC), adding that airflow and evaporation further complicate these dynamics, making droplet trajectories unpredictable.
Traditional models for droplet dispersion fail to account for the solid residues, like salts and proteins, that affect droplet behaviour, prompting Kang and IHPC colleagues to develop a more accurate model of virus transmission.
They hypothesised that solid residues within respiratory droplets slow evaporation, leaving behind smaller particles that can remain airborne for longer and travel farther than pure water droplets. This increases the risk of viral transmission.
Together with former IHPC Research Scientist, Hongying Li, Kang and team worked with researchers from Xi'an Jiaotong University and Tianjin University, China; and Khalifa University of Science and Technology, UAE; to develop a mathematical model that includes variables like relative humidity, air temperature and droplet velocity to predict how these droplets behave over time—how long they remain suspended in the air and how far they travel under different conditions.
Li elaborated that their model treated the solid residue separately after recognising that these particles stay stable as droplets dry out. This allows them to persist and travel longer distances, increasing the risk of long-range pathogen transmission. “By modelling the behaviour of solid residues separately, we aimed to better capture their persistence and potential for re-aerosolisation,” said Li.
Their mathematical model confirmed that droplets with solid residues evaporate more slowly than pure water droplets. For example, at a relative humidity of 0.9, a 1.75 mm droplet can travel significantly farther than smaller droplets under lower humidity. Even after the water evaporates, solid particles left behind, which may contain viruses, can float, increasing transmission risks over distances of up to 3.3 metres.
“We found that neglecting large particles in computational fluid dynamics (CFD) simulations can reduce computational costs without sacrificing accuracy,” Li noted. “This might make CFD more efficient and accessible for broader applications, from public health to industrial processes.”
Li’s team continues to refine the model by incorporating more environmental factors and real-world data, aiming to assess disease transmission risks in settings like childcare centres to improve public health measures.
The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing (IHPC).
