Below is one part of a series of white papers based on our Conversation Series: New Perspectives on Health and Well-Being where we discussed a range of mitigation strategies with our community of engineers, architects, specialists, and industry professionals. We break down all the latest and greatest (and not so great) methods of disease control through engineering, design, and building maintenance strategies by their effectiveness and considerations for sustainability.


Ventilation & Health

Sick building syndrome is a term first coined in the 1970’s to describe a building whose occupants experience acute illnesses or health effects due to extended periods of time spent indoors. Assessing existing conditions in a building’s ventilation system can be an easy step before considering increasing the supply of fresh air, as inadequate ventilation is known to be a factor of occupant health. Fresh air is a topic that goes beyond the immediate response of the pandemic but can have positive long term effects for people and health indoors. Ventilation requirements set by ASHRAE Standard 62.1 outlines the minimum requirements for proper ventilation. LEED and WELL have prerequisites/pre conditions associated with indoor air quality that are based on ASHRAE Standard 62.1 and provides additional credit for exceeding the standard and increasing ventilation by 30-60%. The RESET and WELL certification standards also require measurements in the space of contaminants and VOCs to demonstrate real world air quality (see Air Monitoring for more information). 

Natural Ventilation or cross ventilation is a good solution for homes, but can be a double-edged sword in public spaces due to uncontrolled air velocity and direction. Natural ventilation relies on natural wind, aerodynamics, air velocity, direction of airflow and regional weather conditions. It is not advised that natural ventilation be the end-all-be-all solution. In the future, providing flexible spaces for outdoor working or break space at an office or learning environment may be more common practice.

How fresh is Fresh?

Fresh air and outdoor air are not always synonymous. In theory, introducing more fresh air into a space implies that the indoor air is stale and being recirculated. In Figure 2, we see a comparison of air quality based on PM 2.5 in Boston and New Delhi. Factors that need to be considered when introducing more fresh air into buildings include but are not limited to; AQI, exhaust ventilation, Air Filtration, outdoor air pollutants, energy, regional weather, building program, the direction and velocity of airflow. Considering designing systems that allow for flexibility. Highly efficient systems can be dangerous if it relies on one specific method rather than having the flexibility to accommodate a change in the environment.

Indoor Air Quality

While the COVID-19 virus is around 0.12 microns in size, the virus spreads through the air by aerosolized infectious nuclei particles. Larger droplets from coughs or sneezes with high viral loads will drop to the floor within approximately 6 feet of an infected person, hence social distancing guidelines.  However, smaller, aerosolized particles from coughs, sneezes, and even speaking or breathing that contain lower viral loads can linger in the air much longer.  By diluting or replacing indoor air with fresh air, building managers can significantly reduce the risk of occupants contracting and spreading disease. The issue in the Hong Kong restaurant case study (Figure 2) was the exhaust fans on the opposite wall of the recirculating air conditioning unit were turned off,  limiting the dilution of air and allowing infectious aerosol droplets to recirculate within the space.  Furthermore, the high velocity of supply air from the recirculating air conditioning unit is suspected of carrying larger infected droplets beyond the 6 foot social distancing zone. The lesson learned from this case study is looking at the HVAC system holistically. When considering increasing fresh air, consider the outdoor air quality first and then filtration media that the fresh air and recirculated has to go through. For existing buildings, first check that the ventilation system is balanced properly and functional. These preliminary steps can provide the most occupant health benefits while minimizing unnecessary energy penalties.

Environmental impacts of mitigation efforts

The graph above illustrates the effectiveness of filters versus fresh in comparison to cost. In this case air filtration is a better option in terms of cost and virus control (in the common cold) ASHRAE’s Position Document on Infectious Aerosols, published on April 14, 2020, suggests the minimum of a MERV-13 filter which has an efficiency factor of less than 75% of fine particulate matter (0.3-1 micron). By upgrading to a MERV-15 can increase the efficiency of particle filtration to 85-95% of fine particulate matter with minimal need for increased fan power and drop in air pressure. See our paper on Air Filtration for more information. If the building was designed to LEED standards, it is likely already meeting the suggested filtration upgrades and minimum ventilation performance. Existing, older buildings are particularly at a disadvantage, likely having HVAC systems and settings that are much lower than today’s air change per hour standards or ventilation systems that have fallen out of balance. 

Having existing systems recommissioned and rebalanced by an engineer is the best first step to take. In cases where a building manager may want to increase the fresh air supply with big energy impacts, consider suggesting a temporary operation setting that increases the fresh air but is implemented with the intention that it is TEMPORARY and will not endure a long term energy cost and emissions. Energy recovery ventilation (ERV) systems can be costly upfront but are commonly implemented in buildings designed to current building codes, PassiveHaus/PHIUS standards, and LEED. ERV systems incorporate fresh air, humidity, ventilation, and air temperature and are highly efficient in saving air conditioning costs and maintaining good indoor air quality. Displacement ventilation is recommended where possible for more densely occupied spaces in new construction projects to direct airflow away from the breathing zone. It has a wide variety of program uses and allows for flexibility of load distributions.

Factors that need to be considered when introducing more fresh air into buildings include but are not limited to; exhaust ventilation, air filtration, outdoor air pollutants, energy, regional weather, building program, the direction and velocity of airflow. Considering designing systems that allow for flexibility. Highly efficient systems can be dangerous if it relies on one specific method rather than having the flexibility to accommodate a change in the environment.

Patrick Murphy

Patrick Murphy

Vanderweil Engineers



Jacob Knowles

Jacob Knowles






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Epidemiologists, the ASHRAE Epidemic Task Force, The U.S. Centers for Disease Control, and numerous other authorities on the COVID-19 Pandemic and respiratory disease generally have advised that outdoor air can reduce the risk of transmission. There are relatively few documented cases of COVID-19 that have occurred outdoors, and case studies of prominent examples of transmission in indoor spaces point to inadequate ventilation as a contributor to transmission.  All of these points the building design and facility maintenance communities to the importance of adequately ventilating spaces and even moving activities outdoors in response to the COVID-19 Pandemic.  

Health and wellness design principles, some reinforced through WELL and Fitwel Certification platforms, also incentivize increased ventilation in occupied spaces to benefit individual health and cognition. LEED is offering Safety First pilot credits for managing indoor air quality. Additionally, the WELL v2 Reference Guide cites that “poorly ventilated spaces contribute to symptoms — such as headache, fatigue, dizziness, nausea, cough, sneezing, shortness of breath and eye, nose, throat, and skin irritation — collectively called sick building syndrome (SBS)”.


Indoor Air Quality (IAQ) can be measured and monitored based on levels of carbon dioxide (CO2), volatile organic compounds (VOC’s), and other pollutants. As the result of good IAQ, green certifications or points can be achieved through LEED™, WELL™, and RESET™.

Air Quality Index (AQI) refers to outdoor air quality by the Environmental Protection Agency (EPA) and ranges from Hazardous to Good, measuring ground level ozone, particulate matter (PM) ranging from PM2.5 – PM10 (COVID-19 is PM2.5), carbon monoxide, sulfur dioxide, and nitrogen dioxide.

 Energy Recovery Ventilation (ERV) or Heat Recovery Ventilation (HRV) systems transfer thermal energy and moisture from exhaust air to fresh intake air. This balanced ventilation solution retains minimum moisture and heat/cooling energy while relieving odors, and contaminants.

Figure 1: Outdoor Air Quality comparison between Boston and New Delhi (source:

Figure 2: Visual of the concept of aerosolization in the context of indoor space. The image on the right comes from a study on a Chinese restaurant and the spread of COVID-19. (Source:

Figure 3: Cost and Risk graph above compares effectiveness of a range of MERV and HEPA filters with the fresh air in four major US cities. (Source:

Figure 4: This chart identifies strategies, calls out sustainability factors and ranks the efficacy of COVID-19 / SARS-CoV2 mitigation and keys in a color and abbreviation linking to the larger, compiled strategy chart.

Figure 5: This image is a key, specifying the location of each solution on the compiled strategies chart.


ASHRAE Guidance for Building Operations During the COVID-19 Pandemic

Lists mitigation strategies including air filtration upgrades and supplements.

ASHRAE Position Document on Infectious Aerosols
Hong Kong Restaurant Case Study

Aerosol transmission of SARS-CoV-2 : Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly ventilated restaurant

LEED Pilot Credits

LEED Pilot Credit: Safety First Managing Indoor Air Quality During COVID-19 Credit

WELL Air & Water Quality Criteria

Why COVID-19 Raises the Stakes for Healthy Buildings

COVID-19 pushing the envelope of healthy buildings, a bigger picture article.