Sustainably Protect Humans Against Harmful Aerosols

This blog discusses a new approach to determine and validate effective measures to mitigate the risks of airborne particles based on the distribution of aerosols carrying pathogens exhaled by humans in closed spaces.

by Rüdiger Magg
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Lessons learned from a global pandemic - and the remaining problem

Many countries worldwide recently succeeded in containing the spread of the COVID-19 disease by drastic confinements, regular testing, and an increasing vaccination rate. However, these measures are unsuitable for an extended period of time nor is it possible to vaccinate humans against every harmful aerosol. There is an urgent need for actionable and effective procedures to sustainably contain the risks of infection without harming the economy and society.

This pandemic taught us how easily and vehemently pathogens can be transmitted via airborne particles. This fact is accepted and documented by WHO, ASRAE and several national building and public health authorities. In closed spaces with insufficient ventilation, one single person can easily infect a large number of others. Nevertheless, investigations of infectious events repeatedly showed that humans became infected only in certain areas of the room, depending on the position of the spreading person and the ventilation situation. Interestingly, the infected person does not need to be in the room when the infection happens. The contaminated air they left behind can be enough to infect others entering the room at a later time. Improving the air quality with effective ventilation systems has the potential to mitigate the effect of diseases with airborne transmission. This includes seasonal illnesses, like influenza, common colds, and other virus infections as well. As a consequence, proper ventilation for optimized airflow matters.

Effects & consequences of the problem - a need for action

The latest guidelines and advice to reduce infection risks by regular fresh air supply are still based on limited knowledge about the individual situation. In fact, fundamental influencing factors and their dependencies for being able to effectively maintain air quality remain unaddressed:

  • How uniform is the air exchange based on the selected ventilation method?
  • How do the number of people in the same space, their activities, the ventilation configuration, temperature, etc. influence the air quality and air circulation?
  • What measures could improve the uniformity of air exchange?
  • What are the most effective protection measures to minimize infection risks while considering the associated investments?

In the case of natural ventilation by opening windows:

  • How wide and how many windows should be opened to exchange the air equally?
  • What is the best position for CO2 sensors to warn early enough that windows should be opened?
  • What is the best position for a CO2 sensor to inform about the acceptable level of fresh air that windows can be closed?

In the case of air filter devices:

  • How effective are air filter devices to improve the air quality in the selected situation?
  • What is the best number, performance, and position of air filter devices to achieve the highest effectiveness?

The absence of satisfying answers to these questions causes companies to struggle in getting acceptable risk appraisals of their facilities. Transport associations and operators have difficulties in winning back people's trust in the safety of public transportation. Planners, operators, and facility managers have difficulties ensuring safety for the people using their facilities. There is a need for solution approaches that consider the nature of individual environments, ventilation systems, and occupancies. 

What are appropriate and effective protection measures?

Computational Fluid Dynamics (CFD) is predestinated to provide insights into the airflow and resulting variations of fresh air supply. Still, this technology is often associated with time-consuming, costly, and complex work to simulate the dynamics of air turbulence or droplet movement and thus not helpful to support individual assessments. However, there is a possibility to use this technology for analysis of ventilation-related aerosol distribution or air quality at any area in the considered space, with little time required and high information value.

Physics regimes considered by ESI’s simulation methodology
Physics regimes considered by ESI’s simulation methodology

Based on the open-source CFD software OpenFOAM, ESI developed a solution that allows you to evaluate ventilation situations in a small amount of time. This solution is based on physically validated simulation models for turbulence, buoyancy, droplet size distribution, heat flux, radiation, as well as transient impulses. 

We combined these underlying flow regimes in a steady-state simulation methodology which enables us to analyze the air exchange process while considering influencing factors like ventilation configuration, temperature, humidity, gravity, as well as peoples’ activities like breathing, talking, or shouting.

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Simulation of a typical breathing cycle

Zones of dead air (recirculations) can be determined via age-of-air simulation. Areas in which the air-exchange rate is below the nominal value can be visualized. These are important data to estimate the exposure to aerosols of each individual staying in these areas. The longer it takes to replace stale air with fresh air the more particulates carrying pathogens can accumulate. Considering an infected person at any position in the room the dispersion and concentration of contaminated exhaled aerosols can be calculated accordingly.

 

Scenarios where we can apply this approach

Assessment of natural ventilation possibilities

Many offices, schools and universities, administrative facilities, community centers, religious and cultural facilities do not have ventilation systems and need to rely on natural ventilation. For operators and users, it is unclear at what time and to what extent the used air can be replaced by fresh air.

The introduced simulation methodology allows determining the effect of opening windows under various wind and temperature conditions on the air exchange. This enables operators to make informed decisions on meaningful ventilation intervals or the need for additional improvement measures.

Comparison of the effect of different opening angles of windows on the airflow and air exchange efficiency
Comparison of the effect of different opening angles of windows on the airflow and air exchange efficiency

Verification of occupancy recommendations

In open-plan offices and, even more frequently, public transportation, many people meet in confined, enclosed spaces for an extended period of time. To evaluate the risk of workers or passengers being exposed to a critical number of pathogens, further aspects must be considered in addition to the ventilation system and the number of people. These are people’s activities like breathing, talking, and shouting, as well as protection measures such as wearing a mask.

The proposed simulation methodology allows examining the air quality under these conditions and the effect of different occupancy levels and climate conditions. The comparison of manifold settings of all influencing parameters allows identifying best and worst cases scenarios as well as acceptable occupancy levels considering predefined thresholds of aerosol concentration. Also, informed decisions can be made on which additional protection measures like installing air filters or glass shields between seating rows would be necessary and effective.

Left: Low occupancy, air exchange rate ~ 9/hr; Right: Medium occupancy, air exchange rate ~ 9/hr
Left: Low occupancy, air exchange rate ~ 9/hr; Right: Medium occupancy, air exchange rate ~ 9/hr

Validation of safety (distance) rules under different ventilation conditions

Supplying large halls evenly with fresh air is a challenge. Ventilation systems should be designed to protect workers in factories or manufacturing plants from cumulated exhaust gases or airborne pollutants from the machines and increase occupancy comfort by avoiding high velocities.

Our simulation approach allows determining how efficiently the fresh air reaches the operators and how the emission of harmful aerosols from contamination sources might affect operators working in the vicinity. In case of extreme respiratory events like coughing or sneezing, trajectories of exhaled particles illustrate direct contamination of the environment. Additionally, the effectiveness of work plans and work instructions aiming to protect workers from excessive exposure to potential toxicants can be easily investigated in an interactive Virtual Reality environment and the effect of alternative ventilation configurations and designs can be evaluated comparatively.

Interactive Virtual Reality scenario of an assembly line to validate distance and probability of direct contamination.
Interactive Virtual Reality scenario of an assembly line to validate distance and probability of direct contamination.

Validation of air filters

Displacement ventilation in medical treatment rooms and operating theatres are designed to deliver uniform, non-turbulent air to the controlled parts of the room. Nevertheless, identifying zones of re-circulations is insightful to minimize the risk of pathogenic aerosol distribution, so that people, the equipment can be placed with safety and wellbeing in mind. The effectivity of air filter devices for air quality improvement at various positions of the room can be easily validated.

Left: Fresh Air Index with displacement ventilation  Right: Fresh Air Index with additional air filter
Left: Fresh Air Index with displacement ventilation  Right: Fresh Air Index with additional air filter

Investigation of the influence of fans and diffusers on air circulation

Fan types and diffusers applied to ventilation inlets can have a significant influence on the distribution of fresh air in the considered space. Especially in small rooms the informed selection of an effective fan–diffuser combination can make the critical difference between good and poor air quality in relevant areas.

Fresh Air Index in the mobile medical unit with central ventilation inlet and different fan – diffuser combinations.
Fresh Air Index in the mobile medical unit with central ventilation inlet and different fan – diffuser combinations.

 

Communication and Visualization

Aerosols are invisible to humans. Consequently, humans cannot perceive danger zones in which virus-laden aerosols can accumulate.  

For decision making, it is of high importance to use media that can visualize such zones, and which can be easily understood by everyone. Visualization in an interactive virtual environment allows intuitive communication with all affected stakeholders. Depending on available data and whether the analysis refers to a facility under construction or an existing one, either Virtual Reality can be the right technology for visualization, or Augmented Reality can be used to illustrate the results directly in the context of the real environment.

A Virtual Reality walk-through illustrates the stale air pockets with dark grey point clouds and airflow with arrows of an operating theatre.
A Virtual Reality walk-through illustrates the stale air pockets with dark grey point clouds and airflow with arrows of an operating theatre.
Augmented Reality visualization of stale air pockets and air velocity augmented to the real environment.
Augmented Reality visualization of stale air pockets and air velocity augmented to the real environment.

It's good to know that solutions are already available and ready to use that support industry in quickly analyzing the risk of infection of people in closed spaces like offices, production lines, public transportation, as well as in public facilities like trade shows, theatres, or restaurants to prepare ourselves for a more resilient future.  

Do you need to know how to improve air quality in your area of responsibility? Register for our complimentary workshop - as part of our OpenFOAM conference - on October 21, 2021: Air Quality lessons learned from COVID-19 - Smart ventilation supported by CFD and sensor data

Can't wait? Watch our On-Demand Workshop: COVID-19 Minimize the risk of infection & increase confidence in safety


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