August 6, 2025
Waiting in Line: Why Six Feet of Social Distancing May Not Be Enough
In the wake of the COVID-19 pandemic, public health guidelines emphasized maintaining six feet of distance between individuals as a primary method to inhibit the spread of airborne viruses. This seemingly straightforward rule quickly became ubiquitous signage in queues at grocery stores, vaccination centers, and cafes worldwide. However, a novel study led by undergraduate physics majors at the University of Massachusetts Amherst in collaboration with researchers at the University of Cadiz now challenges this convention by revealing the complex fluid dynamics of aerosol transmission in line settings. The findings indicate that the conventional six-foot guideline may not adequately address the true behavior of airborne particles when people are in motion.
The intricacies of how viral aerosols travel are rooted in the physics of fluid flow — an area where intuition often falls short. Aerosol particles, microscopic droplets expelled during breathing, talking, coughing, or sneezing, interact with surrounding air currents in ways that are affected by numerous environmental factors. When individuals stand still or move slightly, the air around them is disturbed in three dimensions, creating turbulent plumes that can carry pathogens farther or closer than expected. The team from UMass Amherst embarked on this research to decode these mechanisms under realistic conditions, particularly focusing on the environment of people waiting in lines, where steady movement and pauses are inherent.
To tackle this challenge, the researchers employed a unique combination of experimental modeling and computational fluid dynamics simulations. Traditional studies often assess aerosol dispersion in static or uniformly ventilated spaces, but rarely address dynamic scenarios like queuing, where people periodically advance forward and stop. Two physics undergraduates, Ruixi Lou and Milo Van Mooy, spearheaded the project by designing life-like human models using 3D printing technology. These models, equipped to “exhale” colored dyes mimicking aerosolized particles, were placed on a conveyor system to simulate the advancing movement typical of waiting lines. This innovative setup allowed precise measurements of how exhaled aerosols propagate relative to individuals ahead and behind.
Simultaneously, the team collaborated with experts led by Rodolfo Ostilla at the University of Cadiz to perform sophisticated numerical simulations, integrating parameters such as temperature gradients, airflow patterns, and human motion dynamics. Their approach encapsulated the interplay between body heat, ambient air temperature, and subtle air currents generated by walking that together dictate aerosol trajectories. This multifaceted methodology enabled validation of experimental results and deeper exploration of varying environmental conditions too difficult to replicate physically.
Unexpectedly, the experiments uncovered a counterintuitive “downwash” effect caused by the movement of individuals in lines. Contrary to the common assumption that warm aerosol plumes rise due to buoyancy, the researchers observed that walking and pausing in sequence generate air currents that push the aerosol clouds downward. This phenomenon results in exhaled particles sinking towards the floor area rather than dispersing upward into the ambient air, raising concerns because lower-level aerosol concentrations could increase exposure risk for the next person in line.
Moreover, the ambient temperature relative to human body temperature emerged as a critical determinant of aerosol behavior. In unconditioned spaces where room temperature approaches the warmth of exhaled air, downward movement of aerosols is amplified, causing particles to hover at heights where subsequent line members might easily inhale them. Conversely, in climate-controlled environments with cooler ambient temperatures, buoyant forces lift particles higher, decreasing immediate inhalation risk. This nuanced dependence on environmental conditions indicates that fixed social distancing rules do not universally apply and must be adjusted based on venue settings.
The implications of these insights are profound for public health guidelines and architectural planning. Fluid dynamics reveal that safe spacing in static air does not account for the temporal and kinetic variables introduced by human motion. Varghese Mathai, assistant professor of physics at UMass Amherst and senior author, emphasizes that “there are no hard-and-fast rules about social distancing that will keep us safe or unsafe. The fluid dynamics of air are marvelously complex and general intuition often misleads, even for something as simple as standing in a line.” The study advocates for a dynamic understanding that incorporates both spatial separation and temporal factors, including airflow ventilation and crowd movement patterns.
This research also shines a light on the importance of multidisciplinarity in pandemic response strategies. By integrating physics, engineering, and epidemiology, the study brings clarity to scenarios where previous assumptions fell short. It highlights that transmission risk is not merely a function of physical distance but of how aerosols physically move within a space altered by human behaviors and environmental conditions. Such precision is crucial as societies emerge into the post-pandemic era, aiming to reopen safely without relying solely on blunt instruments like fixed distancing rules.
Public spaces designed for queues may benefit from rethinking layouts to mitigate aerosol accumulation. Possible interventions could include improved ventilation systems that direct airflow away from breathing zones, staggered waiting protocols that minimize line density, or even architectural features that disrupt downward aerosol movement. The findings suggest that a one-size-fits-all distance guideline lacks the sophistication necessary for real-world disease control and emphasizes ongoing research to tailor solutions uniquely suited for different environments.
It is worth noting the novel experimental approach undertaken by Lou and Van Mooy, whose ingenuity in combining physical and computational models elevates the study’s robustness. The use of 3D-printed human forms on conveyor belts is an inventive adaptation to bypass the impracticality of studying aerosol dispersion in real crowds, where safety concerns and uncontrollable variables prevail. This method allowed precise, repeatable measurements of aerosol spread synchronized with simulated human motion, offering insights impossible to glean from conventional static setups.
In conclusion, the UMass Amherst-led study reframes our understanding of airborne disease transmission in seemingly mundane contexts such as waiting in line. It underscores that six feet of separation, while a useful heuristic, is insufficient without accounting for the complex physics of aerosol movement influenced by human motion and environmental factors. The research beckons policymakers, architects, and public health officials to incorporate fluid dynamical principles into evolving guidelines to better protect public health as communities navigate current and future airborne pathogen threats.
By peeling back the layers of aerosol transmission dynamics, this work illuminates an often-overlooked dimension of pandemic mitigation—how the invisible flow of air around us subtly shapes exposure risks. As our knowledge deepens, so too should our strategies, evolving from simplistic mandates to scientifically grounded, context-sensitive measures that truly safeguard health in shared spaces.
Subject of Research: Fluid dynamics of airborne transmission and aerosol plume behavior in moving human queues
Article Title: Fluid Dynamical Pathways of Airborne Transmission while Waiting in a Line
News Publication Date: 6-Aug-2025
Web References: https://doi.org/10.1126/sciadv.adw0985
References: Lou et al., 10.1126/sciadv.adw0985
Image Credits: Lou et al., 10.1126/sciadv.adw0985
Keywords: aerosol transmission, social distancing, fluid dynamics, airborne pathogens, COVID-19, human motion, aerosol plume, public health guidelines, airflow, temperature effects, 3D modeling, computational simulation
