Engineering healthy workplaces – Considerations in response to Covid-19

How workplaces will adapt upon employees’ return to work

Covid-19 has kept commercial buildings relatively empty while workers are quarantined at home. This has left many wondering how businesses will adapt when employees return to the workplace. Office building owners will be required to re-examine the existing measures in place to reduce the spread of infection, as well as what improvements can be made. Heating, ventilation, and air conditioning (HVAC) are important systems to consider through this lens because infections such as Covid-19 can be transmitted through aerosolization.

Buro Happold’s approach to the built environment supports humans in all their complexity, focusing on people-centric systems that prioritize occupant health and wellbeing. As a result, many HVAC design measures that we routinely implement have already been taken to reduce the spread of infection within buildings. However, COVID-19 has prompted an examination of how building systems can be augmented to further mitigate the spread of airborne infections. While some new technologies show promise in helping to address this issue, existing practices can also be improved. A summary of Buro Happold’s general recommendations can be found in the table below and will be further elucidated in the following sections.  

Table 1: Air System Considerations

Air Filtration

The Minimum Efficiency Reporting Value (MERV) is a scale from 1-20 that defines the ability of a filter to trap particles. The higher the MERV rating, the more particles a filter can capture. ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality, requires that filters in HVAC systems meet a minimum rating of MERV 6.1 However, LEED, WELL, and other green building rating systems require that HVAC systems use filters that meet at least a MERV 13 rating. While MERV 13 is a good starting point, Buro Happold has specified electronic filter technology with a MERV 15 rating that can filter viruses and bacteria more effectively and at a lower pressure drop than standard MERV 13 filter systems. This product ionizes and polarizes particles between 0.3-1 micron in size.2 As a result, when particles collide, they remain joined and form a larger particle that can be captured in the physical filter media and pathogens are inactivated by the strong electric field.2 Any particles that are not captured in the physical filter are transported into the space being served. The polarity of these particles attracts other particles in the space, creating larger aerosols that do not remain floating as long, thus reducing the number of airborne pathogens that occupants may be exposed to. Filter technologies termed High Efficiency Particulate Aire (HEPA) filters, may be considered in environments where hospital grade filtration is deemed appropriate. HEPA filters would fall into a MERV rating between 17-20.

Figure 1: MERV 15 Air Filtration and Purification System – Image provided by Dynamic Air Quality Solutions

Ultraviolet Light

Ultraviolet C (UVC) lighting is used for the disinfection of surfaces and spaces in many industries and should be considered for use in commercial HVAC design. The WELL rating system awards 2 points to projects that use UV lamps to irradiate mechanical system components such as the surface of cooling coils and drain pans.10 If a design does not incorporate a forced-air cooling system, WELL allocates these points for the use of standalone UV germicidal irradiation air sanitizers in all spaces with more than 10 consistent occupants.

Alternatively, low-level far-UVC light is being studied for its potential to limit airborne pathogens.11 Preliminary research from Columbia University Center for Radiological Research shows that this form of UVC can effectively inactivate >95% of aerosolized influenza viruses. Simultaneously, this research has found that the far-UVC cannot penetrate the outer layer of mammalian skin, establishing the potential for low-level far-UVC light to be used overhead in many public spaces. However, more research is needed to study exposure risks in alternative settings and to determine whether this form of UVC can effectively inactivate Covid-19. 

In addition to implementing appropriate air filtration and purificaiton technologies, the cleaning and maintenance of air handling systems is critical to minimizing the risk of infections from airborne particles. There are multiple companies and technologies that provide these services. A Standard Operating Procedure (SOP) for cleaning air systems should be compiled using manufacturer and National Air Duct Cleaners Association (NADCA) guidelines for all facilities.3

Air Exchange Rate

Another factor that impacts the spread of contaminants is the air exchange rate of the building. The fewer air changes, the more likely that contaminants remain in a building. To ensure that occupants are provided with enough outdoor air for ventilation, the ASHRAE 62.1 Indoor Air Quality (IAQ) standard outlines recommendations for the minimum ventilation rates in various room types.1 However, Buro Happold generally applies the former LEED credit for Increased Ventilation (IEQc2) on a majority of our projects, which calls for 30% more outdoor air than required to comply with the latest version of the ASHRAE 62.1 IAQ Standard. For our projects pursuing WELL certification, 1 point can be achieved for a 30% improvement over ASHRAE 62.1 or 2 points for a 60% improvement in mechanically ventilated spaces. Lastly, Buro Happold often employs decoupled ventilation and hot/air systems with energy recovery ventilators to achieve 100% outdoor air with zero recirculation. These strategies are important to preserving occupant health because higher air change rates help to dilute contaminated air and exhaust more of this air outside.4 Additionally, studies show that buildings with higher air change rates also benefit occupant health by improving cognitive function.5

Healthcare environments have long practiced high air change rates, finding the optimal rate in critical areas such as operating rooms to be greater than 20 air changes per hour.6 Though commercial businesses do not need to provide as high a level of infection control, increasing air changes beyond the ASHRAE recommended minimum ventilation rates could be very beneficial for occupant health. In order to exhaust more air through methods beyond increasing air change rates throughout the day, building owners should consider leaving HVAC systems active for a set time after occupants have left in order to flush out contaminated air before occupants return the following day. It is important to note, however, that the benefits of increased ventilation will have to be balanced with the increase in energy demand they generate.

When considering increasing the outside air delivered to occupied work spaces, ASHRAE has the following recommendations:

  • Temporarily disable demand control ventilation and open outdoor air dampers to 100% when system capacities can maintain appropriate temperature and humidity controls
  • Keep systems running longer hours including a flushing of the system during unoccupied periods.
Figure 2: CFD Model of Airflow in an Office Space

Humidity Control

Relative humidity also has a major effect on the ability of pathogens to spread in the built environment. A recent study in a nursery school found that when relative humidity was maintained above 40%, there were fewer infectious droplets in the air and fewer children missed school.7 While ASHRAE 62.1 simply recommends maintaining a relative humidity below 65% to reduce mold growth, studies have found that maintaining a relative humidity between 40-60% may reduce the transmission of airborne pathogens in the form of a “dry aerosol” – which occurs when humidity levels drop below 40%.1,8 Additionally, dehydrated droplets remain floating longer, which increases the opportunity for the spread of pathogenic aerosols. Finally, dehydrated microbes crystallize into a solid state in which they are preserved and can remain infectious longer. Conversely, at relative humidity levels above 40%, microbial droplets retain their moisture, making them heavier and causing them to settle out of the air sooner. Moisture also allows solutes within aerosols to form dissolved salts, which creates a hostile environment that inactivates the microbes.

The EPA’s guidelines recommend providing a relative humidity between 30-50% to help control mold and maintain human comfort.9 However, it is rare for buildings other than laboratories and healthcare spaces to control humidity on the low end of the scale. This practice should be reconsidered as we look to minimize the spread of airborne infections – especially during colder weather months in areas with winter climates.

Faced with the consequences of airborne infection, designers should consider incorporating humidity control through the winter months when indoor air is dried out from heating. Indoor plants can contribute to a building’s humidification strategy through humidity provided during transpiration while also improving mental wellbeing through access to nature. Humidity can also be manufactured through many commonplace technologies. Extended humidity control will have consequences, however, such as increased energy usage and the potential to damage the building envelope with condensation if improperly controlled. Still, these challenges should be weighed against the benefits of improved wintertime humidity control in workplaces. Setting minimum indoor humidity levels for buildings can reduce the burden of Covid-19 and other upcoming seasonal viral illnesses on society, reduce absenteeism, and save lives.

Figure 3: Therm Analysis to Identify Potential Condensation Issues With Increased Humidity Levels During Winter Operation

Pressurization of Spaces from Clean to Less Clean

Healthcare facilities and research laboratories are routinely designed to pressurize spaces such that airflow pathways are directed from clean to less clean spaces in order to protect occupants and research. These pressurization techniques are often a secondary consideration in the design and operation of office buildings, where they are mostly utilized in relation to bathrooms, copy rooms, and janitor closets. To better incorporate strategic pressurization in workplaces to induce airflow that will lead to improved air quality, underfloor air displacement (UFAD) systems should be considered. These systems introduce clean supply air at the floor and accept return air at the ceiling, resulting in a once through pathway of airflow across the breathing zone of occupants before the air is returned to the central filtration system or exhausted directly out of the building. An alternative approach would be to provide supply air at a higher level and take return air at a lower level to pull clean air across the breathing zone through a method similar to how operating rooms are designed. Implementing computational fluid dynamics (CFD) analyses models to optimize these concepts should be considered a necessity in the design process of workplaces in the future.

Figure 4: Air flow in a UFAD System – one pass through the breathing zone

Air Quality Monitoring

Third party human health and sustainability certification standards (such as LEED, WELL, Living Building Challenge, and Reset) are increasingly requiring performance-based air quality results through one-time, annual, or continuous air quality testing. Advances in technology have drastically reduced the cost of sensors and they can now operate as stand-alone monitors or integrate with Building Management Systems (BMS). While there are many options, Reset Accredited Monitors are emerging as the gold standard for project specifications. Buro Happold’s WELL Gold Certified Los Angeles office utilizes sensors to measure a variety of indoor air quality metrics, including CO2, total volatile organic compounds (TVOCs), humidity, temperature, and particulate matter. Sensor data can be viewed remotely and can be integrated with wider monitoring practices to troubleshoot building health issues as well as energy demand concerns.


Covid-19 has pushed the issue of airborne sickness transmission through HVAC systems into the spotlight. In response, engineers should set new design guidelines that incorporate methods to reduce the spread of airborne sicknesses. Organizations such as ASHRAE12 and AIA13 have created Covid-19 Task Forces to address this growing concern and will soon provide official recommendations on the subject. This guidance should be followed wherever applicable in future projects as a minimum, although supplementary measures can be taken for additional precaution. Strategies for social distancing and rigorous cleaning protocols will also be key factors in getting us all back to work. Buro Happold has always met or exceeded ASHRAE standards for HVAC systems, and will continue to research and apply new technologies and industry practices to ensure the greatest safety with our work.

Article contributors: Julie Janiski, Theresa Brown, Nora McCawley, Heidi Creighton and Chris McClean.


  1. American Society of Heating, Refrigerating and Air-Conditioning Engineers. Ventilation for Acceptable Indoor Air Quality. ANSI/ASHRAE Standard 62.1 (2013).
  2. SecureAire ACS Electronic Air Purification System. SecureAire, LLC (2016).
  3. Dietz, L., Horve, P. F., Coil, D., Fretz, M., & Van Den Wymelenberg, K. 2019 Novel Coronavirus (COVID-19) Outbreak: A Review of the Current Literature and Built Environment (BE) Considerations to Reduce Transmission. Condair (2020).
  4. ACR, the NADCA Standard. National Air Duct Cleaners Association (2015).
  5. Allen, J.G., MacNaughton, P., Satish, U., Santanam, S., Vallarino, J. and Spengler, J.D. Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: a controlled exposure study of green and conventional office environments. Environmental Health Perspectives, 124.6 (2016).
  6. Swift, J., Avis, E., Millard, B., & Lawrence, T. M. Air distribution strategy impact on operating room infection control. Proceedings of Clima – WellBeing Indoors (2007).
  7. Rieman, J. “Humidity as a non-pharmaceutical intervention for influenza A”. ASHRAE (2018).
  8. Low Humidity and Its Effects on Airborne Infection. Airborn Infection and Humidity; Condair.
  9. Moisture Control Guidance for Building Design, Construction and Maintenance. EPA 402-F-13053 (2013).
  10. WELL v2; Microbe and Mold Control. International WELL Building Institute (2020).
  11. Welch, D., Buonanno, M., Grilj, V. et al. Far-UVC light: A new tool to control the spread of airborne-mediated microbial diseases. Sci Rep 8, 2752 (2018).

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