Reducing the Transmission of Viruses with Humidification Part 1: Optimum RH Range

By Chad Edmondson

Who would have imagined less than a year ago that our collective daily lives would be so greatly impacted by a single, air-borne virus? Yet here we are, recalibrating “normal” in the wake of COVID-19 (SARS- CoV-2). And while social distancing and masks dominate much of the public conversation, building humidification is the hot topic among HVAC professionals.

As HVAC professionals, most of us are aware that 40-60% RH is the optimum range for health and wellness. Many of us are also familiar with the following graph from the 1986 Sterling and Arundel study that is the basis of standards for healthy built environments set by the ASHRAE:

A number of other more recent studies have confirmed the role that humidification can play when it comes to reducing the transmission of viruses, bacteria, and allergens. (Links to these studies can be found at the end of this blog.)

Precisely how does humidity impact the spread viruses? It has to do with aerosols, defined by the CDC as tiny particles or droplets suspended in the air. We expel aerosols every time we cough, sneeze, speak, sing, etc., and if we are sick, these aerosols contain the virus we are infected with.

The drier the air, the smaller and lighter these particles/droplets become due to increased evaporation. Smaller, lighter weight droplets stay suspended in the air for longer periods of time and travel greater distances, increasing the chance for virus transmission. They can also be inhaled more deeply into the lungs, increasing the severity of the infection.

Figure 1 shows data collected during a cough simulation under various RH conditions, indicating that high humidity reduces the infectivity of influenza.

Figure 1: “Influenza virus was coughed into the examination room and NIOSH samplers collected aerosol samples for 60 minutes from the manikin’s mouth, 10 cm to the right and left of the mouth, and at positions P1 and P2 within the room. At constant temperature (20°C), the RH was varied over 7–73%.The percentage of virus that retained infectivity relative to that prior to coughing is shown. A, The percentage of infectious virus from all fractions (>4 µm, 1–4 µm, and <1 µm) was determined by the viral plaque assay (VPA) and is shown. B–D, The percentage of infectious virus within each aerosol fraction is shown. Data are means ± standard error (n = 5).”

CITATION: McMillen CM, Lindsley WG, Kashon ML, Slaughter DR, et al. (2013) High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs. PLOS ONE 8(2): E57485. https://doi.org/10.1371/journal.pone.0057485. Editor: Lijun Rong, University of Illinois at Chicago, United States of America

Indoor RH that is lower than 40% not only increases the airborne time of viral particles by as much as 36 to 72 hours, it allows for the re-suspension of these particles after they have already settled.

Conversely, moisture in the air weighs viral particles down so that they drop to surfaces more quickly where they can be cleaned with routine sanitization practices. In the event of a respiratory infection occurrence, maintaining a 40 to 60% RH helps to support the body’s natural immune response. If the air is too dry, it can cause mucous to become thickened or reduced. Cellular recovery and tissue repair is also better at higher humidity, which is why burn units always maintain a humidity of at least 40%.

The Latest from ASHRAE

Despite these compelling reasons to maintain at least 40% RH in indoor spaces, buildings in the US are frequently kept anywhere between 20 and 60% RH, while standards in the UK and Japan allow for more elevated RH, 35 to 50% and 45- 65% respectively.

Current ASHRAE guidelines are expected to change. ASHRAE has established the ASHRAE Epidemic Task Force to help address the challenges of the current pandemic and future epidemics as it relates to the effects of heating, ventilation, and air-conditioning systems on disease transmission in healthcare facilities, the workplace, home, public and recreational environments. Per the task force’s recommendation, ASHRAE recently released the following statements:

Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.

Ventilation and filtration provided by heating, ventilating, and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air. Unconditioned spaces can cause thermal stress to people that may be directly life threatening and that may also lower resistance to infection.

In addition, the recently published ASHRAE Position Document on Infectious Aerosols recognizes the role of the built environment in reducing the transmission and impact of the virus, and more importantly, has extended this to commercial buildings that currently lack minimum RH guidelines.

Over the next several blogs we’ll discuss how humidity impacts virus activity, the human body’s response to airborne pathogens, and how to optimize indoor relative humidity (RH) in the fight against infectious diseases.

Other resources:

1986 – Arundel et al., Indirect health effects of relative humidity in indoor environments

2007 – Lowen et al., Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature

2012 – Noti et al., Detection of Infectious Influenza Virus in Cough Aerosols Generated in a Simulated Patient Examination Room

2012 – Yang, Marr, Mechanisms by Which Ambient Humidity May Affect Viruses in Aerosols

2013 – Welty, Airborne Influenza in Dry Wintertime Indoor Air: Is 50% RH Indoor Humidity One Cure for “Flu Season”?

2018 – Reiman et al., Humidity as a non-pharmaceutical intervention for influenza A

2019 – Iwasaki et al., Low ambient humidity impairs barrier function and innate resistance against influenza infection

2020 – Van Dormelen, How Long Will Coronavirus Live on Surfaces or in the Air Around You?

2020 – Gough, Humidity helps in the fight against COVID-19, virologists report

2020 – Wei Luo, The role of absolute humidity on transmission rates of the COVID-19 outbreak