During a presentation on moisture management at the Better Buildings: Better Business conference this past week in Wisconsin Dells, WI, a builder asked this question:

“Given the Relative Humidity Optimum Zone table everyone cites [Sterling et al, 1986; Environmental Health Perspectives Vol 65, pgs. 351 – 361; see main image], should we be running the relative humidity (RH) in our homes higher, say 50% to 70%?”

Authors’ note: The image above is a slide from a presentation from AHRI. For more on the latest in humidity research in buildings, we recommend you check out the full presentation here.

Who’s qualified to answer this question?

It was a great question but not one that I am qualified to answer. But I do know someone who is at least qualified to review and comment on what information I could gather: Dr. Nathan Yost, retired pulmonary physician and building scientist (and also one of my six brothers).

Nathan worked with me at Building Science Corporation in the early 2000s, becoming one of perhaps a dozen professionals in the US to speak with authority on the relationships between indoor air quality, human health, and building science. I feel pretty lucky that I am one of a very few who can call him out of retirement on occasion for his unique expertise.

Below are the major points of discussion that Nathan and I worked up on this topic: viruses and relative humidity, with some focus on the novel coronavirus responsible for the current pandemic.

Nathan Yost’s overview

Transmission of the novel coronavirus (COVID-19) can occur in 2 ways: (1) in respiratory droplets and (2) transfer from a surface to a person’s mucous membrane (nose, eyes, or mouth), after it has been deposited on that surface from an infected person (so called face-to-hand to hand-to-face).

Transfer via respiratory droplet that are produced by normal talking and breathing requires that the two individuals come into close contact, hence the so-called “six-foot rule.” Coughing or sneezing can send viral particles beyond six feet. The good news is that the corona virus does not remain suspended in air for very long, probably a few minutes. The science of airborne pathogens is very complex. What applies to COVID-19 does not necessarily apply to other virus or even other coronaviruses.

It depends on the type of virus

Not all viruses “prefer” higher RH (that 50% – 70% shown in Figure 1). From Sterling et al: “High relative humidity tends to favor the survival of viruses composed entirely of nucleic acids and proteins, whereas lipid containing viruses prefer low relative humidities.”

The COVID 19 virus is encased in a lipid membrane so this particular virus actually prefers lower RH. And interestingly, the lipid-encased viruses are (fortunately) much more susceptible to soaps, because soaps readily break down the lipid membrane.

Manipulating RH to manage viruses

It seems there is not much research on the impact of manipulating RH to manage viruses. I could find only two studies on this topic. Here is the main takeaway from the first study:

“Preventing Airborne Disease Transmission: Review of Methods for Ventilation Design in Health Care Facilities” (November 2011; Aliabadi, Bartlett, Rogak, Green)

4.5. Viability and Infectivity

4.5.1. Pathogen Response to Environmental Conditions. The effect of environmental factors, such as temperature and relative humidity, on the survival of some aerosolized pathogens has been studied. However, the literature is limited to a few diseases, and a large class of aerosolized pathogens are yet to be analyzed. The effect of OAF [outside air filtration] (relying on natural ventilation) and electromagnetic radiation (relying on daylighting and UV disinfection) on the survival of pathogens need to be studied in greater detail. As far as the building code is concerned, the recommendations for temperature and relative humidity in functional spaces of health care facilities are very conservative. Future research should reveal more detailed mechanisms for pathogen survival behavior as a function of temperature and relative humidity so that the building code will recommend more specific environmental conditions to reduce airborne infection risk in a pathogen-specific fashion.

The second study I found also had a few takeaways:

“Humidity as a non-pharmaceutical intervention for influenza-A” (September 25 2018; Reiman, Das, Pierret)

“Strong cyclical reduction of absolute humidity has been associated with influenza outbreaks in temperate climates.”

“This suggests the future potential of artificial humidification as a possible strategy to control influenza outbreaks in temperate climates.”

“One approach is to maintain relative humidity (RH) between 40–60%, the proposed optimal range for reducing growth opportunities for viruses, bacteria, and fungi. Our previous study demonstrated that classroom humidification to RH of 40–60% may be a feasible approach to increase indoor RH to levels with the potential to reduce influenza virus survival and transmission as predicted by modeling analyses.”

Nathan Yost’s perspective

Does either temperature or relative humidity affect how long the virus remains suspended in the air or viable? The answer is yes, but probably not very significant. The statement has been frequently made that the virus will “go away” once it gets warmer and more humid in April. Yes, influenza infections decrease in the spring and summer in part because that virus doesn’t live as long in humid conditions. The higher the RH the more quickly the virus falls to the floor. But human behavior also changes as weather warms—we spend more time outdoors, less time very close to other people.

So, there probably is no reason to attempt to change the interior temperature or RH with respect to COVID-19. Maintaining RH between 35-55% minimizes the growth of many pathogens (other than COVID-19) and prevents drying of mucous membranes. Dry eyes and cracked lips affect behavior (hands touching face) and allow some viruses to more easily penetrate natural defenses. Building science is important in controlling RH throughout a building. Too much infiltration of cold, dry air can reduce RH below 35%. Poor building design or construction can create cold surfaces on which condensation occurs providing water for potential growth of fungi or bacteria.

Human-to-human vs surface-to-human transmission

Manipulating the relative humidity of indoor environments has more influence on surface-to-human transmission whereas social distancing, limiting contact, and hand washing have more influence on human-to-human transmission. The latter directly targets transmission while the former is quite indirect. And we have lots of research on the latter and very little on the former.

Nathan Yost on air purification

Why doesn’t ventilation, air filtration, or air purification make much, if any difference? The virus does not remain airborne very long and the virus does not grow on building surfaces. Unlike many bacteria and fungi, viruses grow in living cells. Yes, many viruses can remain “alive” or infectious on surfaces for hours to days, but they do not multiply on these surfaces. There is much we do not know about COVID-19 because it is a new, or novel virus. Some reports suggest that this virus can remain infectious on surfaces for up to five days. But, building materials do not provide a reservoir where the virus grows and remains for long periods of time.

Final perspective by Nathan Yost

Building science and design can play an important role in reducing infections caused by COVID-19, influenza and many viruses: reducing the number of surfaces that people touch and installing easy to clean surfaces that people will touch frequently. Think hand rails, door knobs, light switches. I love when bathroom doors at interstate rest stops have a “hook” that allows one to use ones forearm to open the door.

Keep abreast of COVID-19 updates through the Center for Disease Control.

-Peter Yost is GBA’s technical director. He is also the founder of a consulting company in Brattleboro, Vt., called Building-Wright. He routinely consults on the design and construction of both new homes and retrofit projects. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years, and he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? Contact Pete here.

Nathan Yost is a retired pulmonary physician and building scientist and Peter’s brother.

Illustration courtesy of the author.