By StudyFinds Analysis, Reviewed by Steve Fink

During the COVID-19 pandemic, most public indoor spaces required people to stand six-feet apart for social distancing when standing in line. (Photo by Prostock-studio on Shutterstock)

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Researchers discover that common indoor conditions, like mild temperatures and stop-and-go walking, can trap airborne particles where people are most vulnerable.

In A Nutshell

• Waiting in indoor lines, even with six-foot spacing, can allow infectious aerosols to accumulate at face level due to air currents created by human movement.
• The study finds that “comfortable” indoor temperatures (72–86°F) may actually increase infection risk by allowing breath particles to hover where others inhale them.
• Simulations and lab experiments show that faster walking speeds and more extreme temperatures (either hot or cold) help move particles away from breathing zones.
• Current public health guidelines focused on static distancing may be insufficient for queued environments with dynamic air flows.

AMHERST, Mass. — It turns out that the “six-foot” rule wasn’t as effective as scientists thought. New research now shows that waiting in line at the grocery store, airport security, or vaccination clinic might actually pose a much higher infection risk than previously understood.

Scientists have discovered that the widely-adopted six-foot social distancing rule offers surprisingly little protection in waiting lines. Even more striking: comfortable indoor temperatures, the kind most buildings maintain year-round, may actually create the worst conditions for airborne infection spread.

Research published in Science Advances reveals that temperatures between 72°F and 86°F can heighten infection risks by allowing infectious breath particles to linger at face level with minimal dilution. Both significantly hotter and colder temperatures actually suppress transmission risks.

“Depending on the walking speed, an intermediate temperature range can potentially heighten the infection risks by allowing the breath plume to linger; however, colder and warmer ambients both suppress the spread,” the researchers wrote in their paper.

This discovery stems from complex air flow patterns that emerge when people periodically start and stop moving in lines, a scenario that creates competing air currents scientists call “fluid dynamical counter-currents.” These invisible forces battle each other in ways that can trap infectious particles right where people breathe.

How Scientists Tested the Hidden Risks of Standing in Line

Researchers from the University of Massachusetts Amherst and University of Cadiz conducted an investigation combining laboratory experiments with advanced computer simulations to understand how diseases spread in waiting lines.

“We wanted to know how the aerosols we breathe out are transported, but it turns out this is very difficult to do in a real waiting line,” says study co-author Ruixi Lou, who is now a graduate student at the University of Chicago, in a statement.

Teams built a scaled-down waiting line using a conveyor belt system with 3D-printed human-shaped figures and cylindrical models in a water tank. By using water instead of air and scaling down the physical dimensions, they could precisely control variables while maintaining the same physics that govern real-world air flow patterns.

Using fluorescent dye to mimic exhaled breath particles and high-speed cameras to track movement, researchers observed how infectious particles travel when people alternately walk forward and wait stationary—the stop-and-go pattern typical of most real-world lines.

Computer simulations complemented the physical experiments, modeling air flow patterns across different temperature conditions and walking speeds. Researchers tested temperatures equivalent to a range from 72°F to 95°F and various walking speeds typical of indoor spaces.

How Air Currents Spread Disease in Lines

Research uncovered two primary forces that determine where infectious breath particles end up: downwash currents and thermal buoyancy.

When people walk forward in a line, they create powerful downward air currents, similar to the wake behind an airplane wing. These “downwash” currents normally push infectious particles toward the ground, away from other people’s breathing zones.

However, warm exhaled breath naturally rises due to buoyancy, creating an upward current that opposes the downwash. Temperature difference between human breath (around 98.6°F) and ambient air determines how strongly breath particles rise.

At intermediate temperatures, precisely the range where most indoor spaces operate, these two forces nearly cancel each other out. Instead of infectious particles being pushed safely downward or carried upward and away, they remain suspended at head height where the next person in line can easily inhale them.

Why Six-Foot Distancing Doesn’t Work in Moving Lines

Increasing physical separation between people in line, which, of course, was the cornerstone of pandemic-era public health guidance, showed minimal effectiveness in reducing disease transmission.

Researchers tested separations equivalent to traditional six-foot distancing and found that “physical separation, unexpectedly, has only a minor effect on aerosol spreading.” Risk remained highest for people directly behind infected individuals regardless of distance, since insufficient time exists for released particles to disperse during typical waiting periods.

This challenges current public health recommendations that focus primarily on static distancing measures rather than the dynamic air flow patterns created by human movement.

“Ultimately, there are no hard-and-fast rules about social distancing that will keep us safe or unsafe,” says senior author Varghese Mathai, assistant professor of physics at UMass Amherst, in a statement. “The fluid dynamics of air are marvelously complex and general intuition often misleads, even for something as simple as standing in a line. We need to take space and time into account as we come up with our public health guidelines.”

How Walking Speed Affects Disease Transmission

Research revealed that walking speed plays a role in infection risk. Faster-moving lines create stronger downwash currents that more effectively push infectious particles away from breathing zones.

Typical indoor walking speeds during start-stop conditions range from 1.1 to 1.9 mph, significantly slower than normal unimpeded walking speeds of about 3.3 mph. At these slower speeds, the periodic movement creates circulation patterns that can trap disease particles near their release point.

When ambient temperatures allow breath buoyancy to balance against walking-induced downwash, this causes infectious particles to hover at breathing level for extended periods, creating peak infection risk conditions.

Indoor Lines at Airports, Stores, and Clinics Could Be a Breeding Ground

These findings have immediate relevance for countless daily situations where people wait in lines: grocery stores, pharmacies, airports, government offices, and entertainment venues.

Research suggests that maintaining indoor temperatures outside the human comfort zone, either significantly warmer than 86°F or cooler than 72°F, could serve as a disease prevention strategy. In warmer conditions, increased thermal buoyancy carries breath particles up and away from face level. In cooler conditions, reduced buoyancy allows downwash currents to more effectively push particles downward.

However, researchers acknowledge this presents practical challenges, as extreme temperatures affect human comfort and may not be feasible in many settings.

Study Limitations and What’s Next

Research focused on simplified waiting line scenarios with equally-spaced individuals and assumed poorly ventilated indoor spaces. Real-world conditions involve additional variables like humidity, ventilation systems, varying line configurations, and individual behavioral differences.

Researchers studied only small airborne particles (less than 10 micrometers) released during normal breathing, not larger droplets produced by coughing or sneezing. Laboratory experiments used scaled-down models rather than actual humans, though computer simulations validated the physical principles across different scales.

Findings apply specifically to periodic start-stop movement patterns typical of waiting lines, not continuously moving situations where different air flow dynamics would emerge.

Time to Update Public Health Guidelines

The research exposes fundamental gaps in current public health approaches to airborne disease prevention. Most existing guidelines assume static interactions (people standing or sitting in fixed positions) rather than the complex air flow patterns that emerge from human movement.

“Current guidelines of increasing physical separation appear to have a limited impact on reducing aerosol transmission. This work highlights the need for updated transmission mitigation guidelines in settings where physical separation, interaction duration, and periodicity of movements are factors,” the researchers concluded.

Rather than relying solely on distance-based rules developed for stationary interactions, effective transmission prevention in waiting lines may require considering temperature control, ventilation design, line configuration, and movement patterns, all of which are factors largely absent from current public health recommendations.

Paper Summary

Methodology

Researchers used laboratory experiments and computer simulations to study airborne particle transmission in waiting lines. They built a scaled laboratory model using a conveyor belt system with 3D-printed human figures in a water tank, controlling walking speeds, waiting times, and temperature conditions. Fluorescent dye represented exhaled breath particles, tracked using high-speed cameras and particle imaging velocimetry. Direct numerical simulations modeled the same scenarios with precise mathematical accuracy, testing temperature ranges equivalent to 72°F-95°F and walking speeds typical of indoor spaces.

Results

Research found that intermediate temperatures (72°F-86°F) create the highest infection risk by allowing breath particles to linger at face level, while both hotter and colder temperatures reduce risk. Physical distancing showed surprisingly little effectiveness in waiting lines. Two competing air currents determine particle movement: downwash from walking (pushes particles down) and thermal buoyancy from warm breath (pushes particles up). When these forces balance at comfortable indoor temperatures, infectious particles remain suspended at breathing height with minimal dilution.

Limitations

Research focused on simplified waiting scenarios with equally-spaced individuals in poorly ventilated spaces. Research examined only small airborne particles from normal breathing, not larger droplets from coughing or sneezing. Laboratory experiments used scaled models rather than actual humans. Findings apply specifically to periodic start-stop movement patterns, not continuously moving lines. Real-world variables like humidity, ventilation systems, and individual behavioral differences were not fully addressed.