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.


Beneficial Humidity

There are three main benefits of maintaining a relative humidity between 40-60%:

  1. Human immune system efficiency is increased in this humidity range. Mucous membranes are moist, and overall, bodies are better able to fight viral infection.
  2. Virus float time is lowered because virus particles in moisture droplets are less likely to dry out and become aerosolized.  When contaminated droplets settle on surfaces, they are better able to be cleaned, or die without  a host to exploit.  Aerosolized virus particles are able to remain aloft in the air for long periods and travel long distances, increasing their ability to infect building occupants.
  3. Virus survival time is also lowered. Studies have shown that coronaviruses survive longer at RH levels below 40%.

Why size matters

Droplets and aerosols, two particulate sizes, are important to understand and consider. In Figure 1, particle size dictates the amount of time that particulates are likely to remain aloft in the air.  Droplets are larger particles that settle out of the air within seconds. This decreases the risk of a healthy person encountering the virus and inhaling it. Keeping particles to droplet size also allows for easier cleaning of surfaces around patients.. Aerosols are the dehydrated version of droplets.  Because aerosolized particles are much smaller, they remain along for far longer AND can travel much further away from their source, both of which increase the likelihood they will find a host to infect. Because of their size, eliminating aerosols from the air is much more difficult. As there is evidence that virus particles are aerosolized when the air is drier, keeping the air at higher humidity ranges could help prevent their formation.

Relative Humidity and Human Immune System Response

A study on hospital-acquired infections examines the effectiveness of increased humidity on 6,000 patients’ health. The hospital in the study was new, to avoid any trace contaminants. In rooms with a lower humidity, patients showed a higher rate of hospital-acquired infections. However, the patients in a room with higher humidity showed a much lower rate of infection. The graph in Figure 2 illustrates this concept and shows an inverse relationship between the two factors, relative humidity and hospital acquired infection. This seems to suggest  a link between RH levels and immunity.

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 re-balanced 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.

Nora McCawley

Nora McCawley

Buro Happold



Patrick Murphy

Patrick Murphy

Vanderweil Engineers



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Emerging research indicates that increasing the relative humidity in a space to 40% RH to 60% RH can help reduce spread of coronaviruses. While this could be done at a whole-building level in critical spaces like hospitals and laboratories, humidification is energy intensive, and in general building types–offices, for example–might be more economically implemented in small, dedicated areas to provide some additional measure of protection for particularly at-risk building occupants.


Relative humidity is a measure of how much water vapor is in the air at a given temperature.

Droplet size indicates larger particles that settle in the air within seconds. 
Aerosol size indicates small particles, dehydrated droplets, that last in the air for minutes to hours before settling.

Figure 1:  Illustration of droplet vs aerosol sizes including air settling time. (Source: Dr. Stephanie Taylor,, accessed 06/22/2020 and ASHRAE COVID-19 website, 06/22/2020)

Figure 2: Figure 1: Illustration of droplet vs aerosol sizes including air settling time. (Source: Dr. Stephanie Taylor, “Is Low Indoor Humidity a Driver for Hospital Acquired in Infection?”)

Figure 3: 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 4: This image is a key, specifying the location of each solution on the compiled strategies chart.


40 60 Relative Humidity Resource by Dr Stephanie Taylor

Website launched by Dr. Stephanie Taylor, ASHRAE task force member and Harvard Medical School consultant to urge the World Health Organization to consider and produce guidelines involving indoor relative humidity levels.

ASHRAE Position Document on Infectious Aerosols

In ASHRAE’s April 2020 Position Document on Infectious Aerosols, humidity control is listed as an effective strategy for reducing the risk of infectious aerosols in the air. The document acknowledges the evidence in favor of humidity controlling viral spread, but urges careful consideration as there are potential negative effects. In the document, immunobiologists were cited when correlating “mid-range humidity levels with improved mammalian immunity against respiratory infections.” Currently, ASHRAE uses Evidence Level A, indicating a strong recommendation with good evidence, in suggesting hospitals increase the relative humidity to 40%-60%. Evidence Level B, indicating a secondary recommendation with fair evidence, in suggesting all other buildings are carefully considered 40%-60% relative humidity.

CDC Guidance for Reopening (Coronavirus)

Guidance for Reopening Buildings After Prolonged Shutdown or Reduced Operation

COVID-19 Employer Information for Office Buildings

The CDC briefly mentions the consideration for humidity control in buildings.