Implementing Ventilation and Architectural Interventions Toward Raising Transit Pandemic Resiliency

By Sanja Zlatanic, Tom Grassi, Bernd Hagenah and Jesse Harder


Figure 1

Figure 1: Person exhaling, coughing and sneezing without a mask

With the emergence of COVID-19, public transportation agencies have strived to provide safe and healthy transit mobility, as well as a high level of confidence among riders and employees. Appropriate safety measures, procedures and protocols, if implemented, can promote safe public transit and increase the resiliency of systems during a pandemic, as well as readiness to face possible similar events in the future.

While good personal hygiene, physical distancing, the use of personal protective equipment (PPE) and implementation of other conventional measures (including surface cleaning and disinfection) are important, there are also opportunities to influence the physical components of transit stations and vehicles and their ventilation systems via planning and design tools. This study promotes the use of proper architectural and ventilation design elements (‘interventions’) to help make transit cars and stations more resilient to spread of airborne pathogens such as COVID-19 and suggests examining and refreshing planning and design criteria to achieve this objective.


The coronavirus disease 2019 (COVID-19) was first identified in December 2019. Since then, no country has been immune to the effect of this highly infectious airborne disease. The virus is primarily transmitted from person to person via nose and mouth secretions, including respiratory droplets and airborne particles (aerosols) that are produced when someone with COVID-19 sneezes, coughs or speaks (see Figure 1). Larger airborne droplets are infectious only at short distances, while smaller droplets and aerosol particles can stay in the air for hours while being transported and distributed by air flow. Larger droplets have a higher pathogen load than small aerosols. Droplets can also land on objects or surfaces and potentially remain there for hours.

Even now that a vaccine for COVID-19 has been developed there remain more than 1.65 million unknown infectious diseases in animals with no vaccine that may cause the next devastating pandemic. As vaccines are developed in response to outbreaks, responsible professionals including architects and engineers must actively evaluate feasible and practical measures to prepare for such events, including possible resurgence of new strains of COVID, in support of safety of transit systems, especially underground, and their ability to quickly adapt to new conditions.

The design measures identified in this article represent a compendium of potential actions available to transit agencies as outlined in a recent American Public Transportation Association (APTA) white paper produced by HNTB architectural and ventilation experts in collaboration with the Technical Solutions sub-group of APTA’s Vision for Transit Post COVID-19 Research and Thought Leadership Workgroup[1].

Understanding the pathogen spread and proper protection

Importance of proper ventilation. The probability of contagion spread by one or more infectious people in an enclosed space can be determined for different air exchange rates using the Wells-Riley equation. This calculation method has been used for many years and offers a mathematically simple description for the spread of airborne diseases. Figure 2 shows the general probability of infection following different travel times in a passenger car for different fresh air exchange rates (assuming only one person in the car is infected and that no masks are in use). Even though this method is generic and based on experiences with different viruses (for specific application further detailed information would be required), it can be assumed that, in enclosed spaces, the impact of fresh air exchange rates on the probability of infection is significant.
Use of face masks. It should be noted that mask wearing, particularly in an enclosed or indoor environment, significantly increases the protection of all individuals against the spread of COVID-19. Infected individuals can substantially reduce spread by wearing a mask, while uninfected individuals can reduce the amount of airborne virus they inhale when wearing a well-fitted filtration-grade mask. Masks with classifications (e.g. N95, FFP2, KN95) are recommended for self-protection, as they work effectively, but national guidelines should be sought for specific guidance.

Figure 2

Figure 2: Impact of fresh air exchange rates on the probability of getting infected (Wells-Riley equation)

Ventilation interventions

CO2 concentration as key indicator of air quality. The carbon dioxide (CO2) concentration in the air in enclosed environments is considered one of the key indicators of air quality. An increased risk of infection from airborne pathogens within enclosed rooms can be directly linked to increased CO2 levels from human respiration. Therefore, maintaining low CO2 concentrations in enclosed rooms is a key indicator that the portion of the inhaled air that has been exhaled by another person in the room is low as well (see [Ref. 1: Chapter 7.1] for further information). Monitoring the CO2 content of indoor air (using typical CO2 air measurement devices) may also be used to determine if the room ventilation is sufficient and when and where various air filtration or sanitization measures should be considered (e.g. UV-C light sanitization, use of sanitization agents) including room evacuation.

Number of fresh air exchanges and pathogen load. All effective measures against the spread of airborne pathogens aim to achieve a sustained reduction of the viral load in the air we breathe. While low air velocities in the open environment are sufficient to reduce virus concentrations, mechanical, natural ventilation and air sanitization measures are valuable considerations for enclosed environments; in the transit industry this is especially applicable to stations, underground platforms, railcars, buses, etc. The number of air changes (rate of replacing the entire volume of air within a space with filtered and/or fresh air) per hour and “age of the air” (reciprocal value of air changes) are critical factors in the control of air quality in enclosed transit facilities, stations and vehicles.

Irrespective of whether infection occurs via droplets or aerosols, a key factor in becoming infected and the severity of infection is related to the critical number of virus pathogens inhaled. The COVID-19 concentration required for infection to take place is not yet clearly defined; however, one can reasonably assume that various virus concentrations have varying effects on people. For example, to reach the critical viral threshold that leads to infection, it may take one deep breath with very high viral load, or it may take 15 breaths or so with smaller viral loads. Therefore, reduction of the pathogen load in the air via good ventilation is one of the most important elements in the mitigation of airborne viruses in enclosed spaces.

The design of ventilation (HVAC) systems for future transit facilities and vehicles will need to be evaluated with new air exchange goals in mind[1]. In general, an increase in regular air exchange comes at an increased cost compared to the cost of air conditioning (that provides adequate passenger comfort), as fresh intake air requires a greater level of conditioning than air that is recirculated within the interior environment. The level of fresh air intake can be managed through the HVAC control systems, but these systems must be designed to include provisions for adequate intake and distribution of fresh air. Primarily due to energy consumption considerations, traditional HVAC system design has focused on decreasing fresh air intake and treating recirculated air as a means of achieving interior air comfort. The need to increase the fresh air intake during a pandemic is leading engineers to reconsider this approach.

However, even when dealing with enclosed environments with a high number of hourly air changes, there might be zones where the air is stagnant or circulating locally and is not replaced sufficiently by the ventilation system. These stagnant areas may have increased levels of pathogens in the air, so additional measures would be required to improve passenger safety in the entire space. Also, it is evident that the fresh air exchange rate should consider the volume of the enclosed space, its location, and the number of occupants.

An analysis of pathogen spread within a transit car in use on some transit systems, with ventilation system activated, is illustrated below. The concentration and spread of the pathogen throughout the car are demonstrated and the impact of wearing the protective face mask illustrated.

Air Filtration. A reasonable goal is for an HVAC system to replace the total volume of air within an enclosed space at least 7.5 times per hour. There are many types of filtration now designed into HVAC systems, with more new technologies on the horizon due to increased visibility as a result of the COVID-19 pandemic. Most transit systems currently use traditional air filters, rated MERV 7–10, to physically filter the air, which is an adequate level of filtration to remove large particles from the airflow (Minimum Efficiency Reporting Values, or MERVs, report a filter’s ability to capture larger particles between 0.3 and 10 microns (µm), helpful in comparing the performance of different filters). In order to be effective against airborne droplets and other pathogen-carrying particles, however, the level of air filtration should be increased to at least MERV 13. This can be achieved in most systems through the exchange of air filters to this more restrictive type, but this does come at a cost to air circulation. Higher MERV-rated filtration systems, however, tend to reduce the amount of air that passes through them, decreasing the level of air circulation within the conditioned space.

Figure 3a

Figure 3a: CFD analysis of airborne pathogen spread in train/metro car with and without a face mask, 5 seconds after a cough

Figure 3b

Figure 3b: CFD analysis of airborne pathogen spread in train/metro car with and without a face mask, 20 seconds after a cough

Air sanitization. Irrespective of an overarching need for proper ventilation and cleanliness protocols within enclosed transit spaces, special sanitization measures could be of great value during and after the pandemic for areas with higher passenger density. A possible effective measure is hydrogen peroxide (H2O2).

Hydrogen peroxide (H2O2) is a substance found in nature and occurs in very low concentrations in the outside air. For more than 100 years hydrogen peroxide has been used in the medical field for disinfection, in daily life for cleaning, and in cosmetic products in liquid form or as an additive. Technologically, the production of hydrogen peroxide for air purification is carried out via a Photo-Hydro-Ionization (PHI) cell. A UV-light source is used in conjunction with a special surface to produce hydrogen peroxide from the moisture in the air using prevailing water and oxygen molecules. Over the past 20 years, the use of PHI technology has spread worldwide and has achieved a solid track record but requires further assessment including testing to assure consistency and quality control for specific in-situ applications. In practice, airborne hydrogen peroxide, in the concentration range of 0.02 to 0.05 ppm, is used as a sanitizer that is released into the air flow. The National Institute for Occupational Safety and Health (NIOSH) identifies the current OSHA permissible exposure limit (PEL) of 1 ppm for hydrogen peroxide, whereas the typical outdoor level is approximately 0.02 ppm.

Computational Fluid Dynamics (CFD) analysis demonstrates that hydrogen peroxide (H2O2) can effectively inactivate viral pathogens and bacteria in the air and on surfaces (see Figure 5). Results are considered representative of various agent-based air sanitizing techniques, in which fast-acting particles of the air sanitizing agent are capable of neutralizing the viral pathogen when rapidly mixed with air on a molecular level. This measure would be particularly effective for use in the following transit areas: entry/exit turnstiles; ticketing points; commercial zones; waiting areas, and escalators/stairways. Air sanitation portals situated at the upper landings of escalators leading from the mezzanine to the track or platform level (see Figure 4) could be very effective as well.

Figure 4

Figure 4: Vertical air curtain at upper landing of an escalator (Zurich Airport)

Other approaches to air treatment are currently being deployed or investigated. Systems using ultraviolet (UV-C) light to sanitize the HVAC system are being tested by some operators, while others have installed electrostatic filtration systems, which aim to remove particles from the air by electrically charging them and then removing them using screens that carry an opposite electrical charge. The impact of these measures on air purification is highly dependent on the HVAC system design and other parameters including air temperature and humidity. Therefore, there is no panacea.

Figure 5

Figure 5: CFD analysis of air sanitizer distribution via air curtain at mezzanine level of a typical transit station (using STAR CCM+, 3D Multiphysics solver)

UV-C light is capable of killing or inactivating viruses and bacteria by damaging their cells on a molecular level (DNA). The intensity and duration of UV-C light exposure are the key parameters of the efficiency of this measure: devices cannot be used close to patrons and employees due to the potential impact on human cells. When used for disinfecting rooms (without occupants), an extended period of exposure is required, depending on the intensity (from several minutes to hours). Also, UV-C light has a short range (decreasing intensity from the source) and requires UV lamps/sources to be moved around for the complete sanitization of rooms; yet, shadowed areas are not sanitized. Using UV-C technology for ventilation in trains or stations, where high-volume flow rates are required, is typically not practical, due to short impact time of UV-C light sources on high velocity air passing through the air ducts.

air purification

Air ionizers are another measure and typically use high voltage currents to charge air molecules in order to generate positive and negative ions. Studies have shown that ionized air distributed by hospital air conditioning systems can deactivate viruses. Even though the development of air ionizers accelerated during previous pandemics (including SARS) standardization of this technology has not yet been established. Therefore, the integration of such measures into HVAC systems needs careful evaluation and testing. The units can be integrated into ventilation ducts or installed when refurbishing existing air conditioning systems. Passive ionization devices without fans impact only the field near the ionizer but active devices with fans can help to swirl and propagate the ions throughout the air. These units could be suitable for offices or stations (e.g. in waiting areas, along the pedestrian path) when other systems cannot be used.

Architectural interventions

The two primary ways in which such a virus can spread are through close physical contact with an infected person and improper ventilation within a space. Simple, cost effective, and aesthetically appealing architectural interventions can readily be applied to mitigate those factors.

Figure 6

Figure 6: Peak-hour conditions before the pandemic in Penn Station, NYC

For instance, bold signage with unique messaging that encourages physical distancing is a simple and easy way to remind riders to keep a safe distance away from one another. Many transit agencies have already implemented floor decals in their transit facilities in a quick and temporary manner, given the situation. Moving forward, care and thought must be given to establishing consistent and effective standards regarding boarding, waiting and distancing requirements, and accompanying graphics for ease of rider movement. Maintaining a safe distance seems feasible in many situations, but as shown in Figure 6, during peak hours before the pandemic, passengers frequently had very little space between them on many transit lines, especially those in dense urban areas. Mitigation strategies then shift to other potential solutions.

Figure 7a: Smartphone app showing real-time passenger density

In order to keep transit services running while protecting the health and safety of riders, providing real-time data of how many riders are in transit facilities or on approaching trains/railcars can help riders make decisions on which stations to use and trains/railcars to board, minimizing crowding and facilitating physical distancing. Integrating real-time information can enable seamless and just-in-time connections, eliminating unnecessary wait times and associated over-crowding situations. This data can be collected via sensors mounted on turnstiles and above vehicle doorways that would provide accurate passenger counts using a combination of infrared and 3D image pattern technologies. Providing this real-time data through mobile phone applications and dynamic signage would inform riders of the onboard densities of incoming vehicles and help them choose the car, train or bus they would board.

Figure 7b: Station screens showing real-time passenger density

Similar smartphone applications have already been launched and are being tested by numerous agencies, including MTA New York City Transit (NYCT) and the Taipei Rapid Transit Corporation (TRTC) in Taiwan (see Figure 7). The TRTC’s mobile phone app assigns density information for each car according to a color code; for example, green symbolizes a “comfortable” number of riders in a vehicle, yellow an “average” density, orange “crowded,” and red “very crowded.”

A recent study was conducted to explore potential connections between infection by airborne pathogens, riders’ physical distancing and co-travel time. This study found the risks of riders being infected while traveling together in the same vehicle is much higher among individuals seated in the same row than among those seated in different rows. Therefore, retrofitting existing seating through the implementation of protective barriers made of transparent materials, introduction of reversible seating that can change direction, or seating with taller backs that can serve as barriers, can all be effective measures to assist physical distancing and minimize the spread of pathogens. Full-height platform screen doors (PSDs) can help to separate station air on the platform from the air coming from adjacent tunnels (see Figure 8). Furthermore, PSDs help to reduce the amount of fresh air on the platform to be pulled into the tunnels by leaving or passing trains. PSDs can therefore help to maintain the quality of the air on the platform level, avoid or reduce rail dust and dirt entering the station and reduce the energy needed to prepare and provide the required air quality.

Figure 8

Figure 8: Full-height platform screen doors in use on Hong Kong’s MTR system

Figure 9

Figure 9a: Self-Cleaning UVC Escalator Handrail

Instead of bar turnstile-type fare gates, wing gate-type fare gates can be installed to promote touch-free fare collection. Gates are in the closed position and automatically open when the fare is collected, allowing riders to pass through without contact. The gates close automatically after passage. Supplemented with the proper use of effective signage, this automatic fare gate can promote one-way passenger flows, avoiding bidirectional conflicts.

As noted above, UV-C light is known to kill viruses and bacteria by damaging cells on a molecular level. Several transit agencies have been testing the use of UV-C light for self-cleaning escalator handrail sterilizers, as seen in Figure 9.

Figure 9b

Figure 9b: Self-Cleaning UVC Escalator Handrail

Air sanitization interventions, outlined above, should be located strategically within stations, so that their protective capabilities are accessible to the maximum number of riders. Such appurtenances, in form of portals at faregates, stairs and/or escalators, and ventilation ducts and associated electrical and mechanical equipment require proper architectural treatment that enhances the station’s aesthetics, maximizes their efficiency, and provides clear messaging and reassurance of active air treatment measures being implemented.

Figure 10

Figure 10: Illustrations of a station platform and faregate entrance with a range of possible architectural interventions


Below is a sample Evaluation Matrix for an underground station. Similar matrices were developed for aboveground stations, rail cars and buses as well, to assist transit facility owners and operators in evaluating their facility and vehicles in terms of their pandemic or post-pandemic resiliency, for both passengers and employees. The various types of architectural and ventilation intervention measures were evaluated for the following criteria: Effectiveness: Rated as high, moderate or low; Ease of application: Rated as high, moderate or low; Capital investment (initial cost): Rated as low, moderate or high; Lowering of operations and maintenance cost: Rated as to whether or not the measure will result in lowering long-term operation and maintenance costs (initial capital costs might be offset by lower operation and maintenance cost later; this would likely benefit the owner’s operating efficiency); Achieving equity: Rated as to whether the measure will result in all riders and employees being treated in a fair and unbiased way while providing universal access.

After evaluation of all criteria, an overall rating was developed for each measure. No weightings were applied to the various criteria, although some criteria might be considered more important, or more practical, than others to various owners and operators with respect to their specific conditions, goals, and resources.

HNTB has produced a video to demonstrate some of these potential interventions in an underground transit facility, which can be found at:

Once a matrix is completed for a given facility, feeding that information into an integrated Decision Tree will provide the transit agency and/or operator with a direct guide as to the interventions that are most appropriate for that unique situation.

Sample decision trees presented in the APTA white paper[1] represent suggested methodology and are intended as a starting point/framework for transit agencies to take stock of their facilities. By answering a series of customized, prioritized yes/no questions, agencies can determine the available measures that would be most suitable for promoting pandemic-safe mobility for their patrons and employees. These decision trees can be used to assist in initiating the process to find the most appropriate ways to deliver critical and healthy transit service to patrons and employees.

interventions to promote safe mobility


1. For a full list of references, supporting documentation and studies, see: American Public Transportation Association (APTA), Sept. 2020. Transit Leadership in the Post-COVID-19 Mobility Landscape: Measures to Promote Safe Mobility:

About the Authors

Sanja Zlatanic: As chair of HNTB’s National Tunnel Practice, Sanja Zlatanic, PE, SVP, is responsible for growing the firm’s professional services for tunnels and complex underground structures within the U.S. and select international infrastructure, transportation, transit and rail markets. She leads an exemplary group of tunnel and ventilation experts who practice technical excellence, innovation, and serve as thought leaders. In a course of her carrier, Sanja worked on some of the most complex national and international tunnel projects.

Tom Grassi: Tom has over 35 years of professional experience in managing, planning, designing, and constructing some of the most transformative transportation rail programs of that time. Since joining HNTB, he has successfully held leadership roles on several critical national transportation projects. He was elected as College of Fellows in the American Institute of Architects in recognition of his unique contributions to the profession and to society. Recently, he has published and lectured at numerous forums on architectural interventions in a post-COVID environment.

Bernd Hagenah: Bernd has more than 23 years of experience in tunnel ventilation, tunnel climate and aerodynamics and tunnel safety, contributing to many significant infrastructure projects throughout Switzerland, Austria, France, Germany, Sweden, Israel, Australia and the United States. He has worked on road and rail tunnel projects such as; the Gotthard Base Tunnel, Brenner Base Tunnel, Metro Vienna, Metro Melbourne, Metro Brisbane, in addition to several highway tunnels in Europe and the French Nuclear Waste Depository (CIGEO). He is an internationally recognized leader in the field of designing safety equipment and electromechanical equipment including ventilation systems for underground transit systems, both during construction (dust control, fresh air supply, fire protection) and revenue service.

Jesse Harder: Jesse has 20 years of experience in the planning, design and construction of transportation facilities including pedestrian, road and rail tunnels, bridges, airport people movers, subway metros, bus transit and commuter rail systems. His expertise in the areas of mechanical ventilation and fire life safety systems has led to innovative solutions to critical hazards related to fire, flood and earthquake safety. He has served in key roles on major infrastructure projects both nationally and internationally.

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