Introduction: The Growing Need for Aerosol Simulation in Public Health

The COVID-19 pandemic underscored a critical vulnerability in urban public health systems: our limited ability to predict and mitigate the spread of airborne pathogens. While standard epidemiological models focus on human contact networks and population mobility, they often neglect the physical behavior of respiratory aerosols and droplets. Urban environments, with their dense populations, complex building geometries, and varied ventilation systems, create conditions where airborne transmission can be highly efficient and difficult to trace. Addressing this gap requires a more mechanistic approach—one that combines fluid dynamics, aerosol science, and computational simulation.

Recent advances in aerosol simulation technology, often referred to as aerosimulations, now offer a powerful lens through which public health officials, city planners, and building engineers can analyze, forecast, and reduce the spread of airborne pathogens in urban settings. This article explores what aerosimulations are, how they work, their specific applications in cities, their role in designing interventions, and the challenges that remain before they become a standard public health tool.

What Are Aerosimulations?

Aerosimulations are computer-generated models that replicate the movement of airborne particles—ranging from submicron aerosols to larger respiratory droplets—within a defined spatial environment. These models incorporate multiple physical factors including:

  • Turbulent airflow generated by ventilation systems, wind, or human movement
  • Particle size distribution, which determines how long particles remain suspended and how far they travel
  • Evaporation and hygroscopic growth of droplets in different humidity conditions
  • Deposition surfaces such as floors, walls, filters, and masks
  • Thermal plumes from human bodies and electronic equipment

The core technology behind most aerosimulations is Computational Fluid Dynamics (CFD), a branch of fluid mechanics that uses numerical algorithms to solve the equations governing fluid flow and particle transport. By dividing a space into millions of small cells (a mesh) and iteratively calculating velocity, pressure, temperature, and species concentrations, CFD can predict how airborne contaminants disperse over time. For urban-scale problems, these models are often coupled with meteorological data and building geometry obtained from LiDAR scans or city planning databases.

Beyond CFD: Lagrangian Particle Tracking and Zonal Models

While CFD provides the most detailed representation, full three-dimensional CFD simulations can be computationally expensive, especially for large urban areas or real-time applications. Therefore, two complementary approaches are often used:

  • Lagrangian particle tracking: Individual particles are released from a source (e.g., an infected person) and their trajectories are computed based on the airflow field. This is especially useful for determining exposure risk in indoor spaces like classrooms, offices, and restaurants.
  • Zonal or multi-zone models: The urban environment is divided into interconnected zones (rooms, floors, street canyons), and a simplified set of equations models airflow and particle exchange between zones. These are more computationally efficient and suitable for city-scale building stock analysis.

Modern aerosimulation platforms often combine these techniques, using CFD for detailed parts of the domain and zonal models for the rest, providing a balance of accuracy and speed.

Applications in Urban Settings

Urban areas present a unique combination of risk factors for airborne disease transmission: high population density, enclosed public transit, mixed-use buildings with shared ventilation, and outdoor microclimates shaped by tall structures. Aerosimulations can be applied across several scales and contexts.

Indoor Public Spaces: Schools, Offices, and Healthcare Facilities

Indoor environments are where respiratory aerosols can accumulate to high concentrations. Aerosimulations help identify areas with poor ventilation, recirculation zones, and dead spots where particles linger. For example, studies using CFD simulations of classrooms have shown that desk arrangements in front of windows, combined with upward ventilation, can reduce cross-infection risk by over 60% compared to traditional layouts. Similarly, simulations of hospital wards have guided the placement of negative pressure rooms and high-efficiency particulate air (HEPA) filters to minimize nosocomial transmission. A 2022 study published in Indoor Air used aerosimulations to demonstrate that increasing the air change rate from 6 to 12 ACH in a typical office reduced the risk of inhalation by 90% within 15 minutes after an infected occupant leaves.

Public Transportation Systems

Buses, trains, and subway stations are notorious hotspots for airborne disease. Aerosimulations have been used to evaluate ventilation designs in subway carriages, showing that opening windows only on the windward side can create cross-ventilation that expels contaminants. In subway stations, simulations can track how aerosols from crowded platforms rise and disperse through connecting tunnels, aiding decisions about where to place monitoring sensors and high-capacity fans. For example, a collaboration between the Hong Kong University of Science and Technology and the MTR Corporation used CFD modeling to optimize the placement of air purifiers in train compartments, reducing particle concentration peaks by 40% during rush hour.

Outdoor Urban Canyons and Street Markets

While outdoor transmission is generally lower risk, it can occur in crowded, narrow streets with limited natural ventilation, known as urban canyons. Aerosimulations that incorporate building geometry and wind patterns can predict how aerosols from a source (e.g., an open-air market stall or a vocal crowd) travel downwind. These models help event organizers and local governments set safe spacing between vendor stalls and seating areas. A study simulating the Notting Hill Carnival in London used a coupled CFD-meteorological model to show that positioning food stalls with backs to the prevailing wind reduced aerosol exposure for customers by more than 50% compared to stalls facing the wind.

High-Rise Buildings and Vertical Ventilation

In dense cities, tall buildings create vertical airflow phenomena such as stack effect and wind-driven pressure differences across floors. Aerosimulations can model how contaminated air from a lower floor rises through stairwells, elevator shafts, and ventilation ducts to upper floors. This was critically relevant during COVID-19 outbreaks in high-rise apartment complexes in Hong Kong and Singapore. Using aerosol modeling, building managers identified that sealing pipe chases and adding lobby‐level exhaust fans could break the vertical transmission path. The World Health Organization’s 2021 roadmap for improving indoor ventilation explicitly recommends conducting airflow simulations in multi-story buildings to manage airborne risk.

Reducing the Spread of Pathogens Through Simulation-Informed Interventions

The ultimate goal of aerosimulations is not just to understand how pathogens spread, but to design and evaluate interventions that reduce transmission. Several evidence-based strategies have emerged from simulation studies.

Targeted Ventilation Improvements

Aerosimulations can pinpoint the optimal placement of supply and exhaust vents to create “piston” flows that push contaminated air out of occupied zones. For example, in a typical open-plan office, simulations reveal that ceiling-mounted supply diffusers placed directly above employees can create short-circuit flow where fresh air bypasses the breathing zone. By redirecting supply grilles toward windows or using displacement ventilation (where cool, fresh air enters near the floor and rises as it warms), simulations show that inhalation exposure can be reduced by 70% compared to standard mixing ventilation. Building owners can use these insights to retrofit HVAC systems without major construction.

Optimized Mask Usage and Filtration Placement

While mask usage is a behavioral intervention, its efficacy depends on the local air mixing. Aerosimulations can model the effect of different mask types (N95, surgical, cloth) and face-seal leakage. They also help position portable HEPA filters for maximum effectiveness. A simulation study in a hospital ward showed that placing HEPA filters near patient beds, rather than in corners, cut the time to clear aerosols by 40%. Similarly, in a subway car, simulations demonstrated that placing filters at each end of the carriage, rather than the center, better captured particles generated by passengers seated throughout the car.

Dynamic Occupancy and Distancing Strategies

Rather than fixed social distancing rules, simulation-informed strategies can adapt to real-time conditions. For instance, a university lecture hall with a specific ventilation rate and seating geometry might be safe at 80% capacity, while another hall with poor airflow requires only 30% capacity. Aerosimulations enable dynamic occupancy models that update based on current ventilation performance, outdoor air quality, and occupancy patterns. Some cities have begun incorporating these models into their public building operating guidelines, creating a tiered system of risk levels that inform policies on events, school closures, and transit crowding.

Design of Urban Airsheds and Green Infrastructure

On the urban scale, aerosimulations can guide the placement of parks, tree lines, and green walls to modify wind patterns and enhance outdoor air dilution. Trees and hedges can act as both barriers (blocking polluted air) and as sinks (capturing particles). However, incorrect placement can trap polluted air between buildings. By simulating particle dispersion in a model of a major city thoroughfare, researchers in Berlin found that a row of trees planted in the median strip actually increased pedestrian exposure by 20% because the trees deflected exhaust back toward sidewalks. Simulation allows planners to test designs virtually before planting.

Challenges and Future Directions

Despite their promise, aerosimulations face several obstacles that limit their widespread adoption in public health decision-making.

Computational Cost and Real-Time Capability

High-fidelity CFD simulations of entire city neighborhoods can require supercomputing clusters and days of processing time. For emergency responses—such as an outbreak in a specific subway line or a hospital—this is too slow. Ongoing research focuses on reduced-order models, machine learning surrogate models, and cloud-based simulation as a service to bring turnaround times down to minutes. Early work by Google DeepMind in 2023 used a graph neural network trained on CFD data to predict indoor airflow patterns in less than a second, achieving 95% accuracy for particle concentration predictions.

Data Availability and Integration

Accurate simulations require detailed inputs: building floor plans, HVAC specifications, weather data, occupancy counts, and human movement patterns. Many cities lack comprehensive digital twins or open data repositories. Light detection and ranging (LiDAR) surveys, building information modeling (BIM), and smart building sensors are gradually closing this gap. Initiatives like the European Union’s Destination Earth initiative aim to create a digital twin of the planet, including urban microclimates, which could be linked to public health models.

Human Behavior and Stochastic Variability

People do not stand still or breathe uniformly. Activity level, talking vs. breathing, mask compliance, and movement all affect particle generation and exposure. Current aerosimulations often assume average behavior or worst-case scenarios. Incorporating stochastic behavioral models (e.g., agent-based models representing individual movements) is an active area. A noteworthy 2024 study from MIT combined CFD airflow modeling with an agent-based model of pedestrian movement in a busy train station, demonstrating that the timing of a person’s cough relative to the arrival of a train could change infection risk by a factor of 10.

Validation Against Real-World Measurements

Simulations must be validated with field data to be trusted by public health agencies. Controlled experiments using tracer gases (like CO₂ or SF₆) in real buildings or street canyons are common, but scaling up to entire cities is difficult. The U.S. National Institute of Standards and Technology (NIST) has been developing a standardized validation framework for indoor airflow models, but an analogous standard for urban outdoor models is still lacking. The ASHRAE Standard 241 (Control of Infectious Aerosols) is a step forward, providing a tiered risk assessment that can incorporate simulation results.

Ethical and Equity Considerations

Simulation tools could be used to justify unequal interventions—for instance, focusing ventilation upgrades on wealthier districts while neglecting lower-income neighborhoods with older buildings. Similarly, data from simulations about high-risk areas could lead to stigmatization. Public health officials must ensure that simulation-informed policies are equitable and transparent. The WHO’s guidelines for indoor ventilation emphasize equity in resource allocation for ventilation improvements, a principle that should extend to simulation studies.

Looking Ahead: Aerosimulations as a Core Urban Health Tool

The trajectory is clear: aerosimulations will become an integral part of urban public health infrastructure, much like weather forecasting is for disaster preparedness. We are moving from reactive modeling (simulating past outbreaks) toward predictive and prescriptive modeling (forecasting risk and recommending interventions in real time).

Several cities have already begun incorporating simulation into their planning. Singapore’s Urban Redevelopment Authority uses wind simulation software to assess ventilation in new developments and issue planning approvals with conditions to enhance natural ventilation. The city of Zurich has developed a digital twin of its downtown area that includes airflow models to evaluate pedestrian comfort and air quality, a system being expanded to model pathogen dispersion for future pandemics.

To accelerate adoption, standard curricula for public health professionals and urban planners should include fundamentals of aerosol science and simulation interpretation. Funding agencies should prioritize validation studies and the development of open-source simulation platforms that lower the barrier for cities with limited budgets. The 2021 paper in Scientific Reports demonstrating a low-cost CO₂-based surrogate for aerosol simulation offers a promising path for cities that cannot afford full CFD modeling.

Ultimately, aerosimulations are not a panacea—they must be integrated with epidemiology, behavioral science, and equitable policy. But as urbanization accelerates and the threat of airborne pandemics grows, the ability to see the invisible flow of pathogens and act on that knowledge will be one of the most powerful tools we have to protect public health.