When researchers at Universitat Rovira i Virgili in Catalonia heated exhaled air to 37 degrees Celsius—the temperature of a person with a slight fever—and released it into a climate chamber kept at 7 degrees Celsius, something unexpected happened: the sneeze cloud stayed together longer and traveled much farther than it would have in warmer air. This simple observation, buried in a new study published in Physics of Fluids, could reshape how we design ventilation systems and safety protocols in schools, hospitals, and public transport.
For decades, scientists have understood that coughs and sneezes expel clouds of microscopic particles capable of carrying viruses and bacteria. The speed of the exhalation, the shape of the person's respiratory system, the size of the room—all of these factors matter. But the role of temperature has been largely overlooked, even though it turns out to be surprisingly powerful.
Nicolás Catalán and his team at the URV's ECoMMFiT research group conducted 180 experiments in controlled conditions, testing sneeze behavior across three ambient temperatures: 27 degrees Celsius, 17 degrees Celsius, and 7 degrees Celsius. They used a sophisticated simulator that reproduces realistic coughs and sneezes, varying both the intensity of exhalation and whether air passed through the nose or mouth. High-speed cameras and laser lighting systems captured every detail of particle dispersal.
The results were consistent and striking. When the temperature difference between exhaled air and ambient air widened—say, a 30-degree gap rather than a 10-degree one—the particle cloud remained more concentrated and traveled significantly greater distances before dispersing. The physics behind this is elegant: the density differences between warm exhaled air and cold surrounding air create buoyancy forces that alter the trajectory and structure of the respiratory particle cloud, keeping concentrations higher for longer.
The study also confirmed something earlier URV research had suggested: the nose itself is a powerful filter. When airflow passes partially through the nasal cavity, the horizontal range of the particle cloud shrinks while vertical dispersion increases. When exhalation happens exclusively through the mouth—as with a cough—the cloud advances much more horizontally. These differences become even more pronounced when temperature is factored in, meaning that the same person coughing in December versus July could have very different transmission patterns.
What makes this research particularly valuable is its precision. Until now, studies on respiratory aerosols have relied on either numerical simulations or experiments with human volunteers, both of which make it difficult to isolate variables. The URV simulator allows researchers to control flow rate, temperature, and respiratory geometry with laboratory exactitude, generating experimental data that can feed into computational models capable of predicting aerosol dynamics with far greater accuracy.
The implications ripple outward. In schools where winter heating creates large temperature gaps between indoor air and exhaled breath, sneeze clouds could travel several feet farther than building designers assumed. Hospitals might need to recalibrate their isolation protocols depending on the season. Public transport systems could optimize their ventilation strategies based on expected temperature gradients. The findings suggest that even small design choices—thermostat settings, duct placement, air exchange rates—could materially reduce disease transmission risk in spaces where people gather.
As Nicolás Catalán explained, the team wanted to understand how much ambient temperature could alter particle cloud dynamics. What they found was that temperature matters far more than anyone had measured before, opening a new frontier in respiratory disease control.