40% to 60% Relative Humidity Is Proven to Reduce Respiratory Virus Transmission

40% to 60% Calculated RH vs Weekly Influenza Cases NY

US Centers for Disease Control (CDC) collected data over nine influenza seasons in New York.

Most businesses, schools, and public facilities have taken steps to create a safer environment during the global pandemic. A study was recently conducted by John W. Birks and Kathy L. Rowlen to review respiratory illness transmission in various humidity levels. It found consistent room humidity (RH) levels between 40% and 60% drastically reduced the transmission of respiratory illnesses, such as influenza and human coronavirus. The authors of the study urge readers to humidify school classrooms urgently.

Previous studies have identified the benefits of RH, including the viability of viruses, the size distribution, 15 lifetimes of aerosolized particles containing the virus, and the susceptibility of room inhabitants to infection at 50% humidity. This study argues the effectiveness of RH levels between 40% and 60% to enhance students’ and staff’s safety. As a result, the authors implore schools and other public buildings to maintain indoor humidity levels at 50% to help fight against the transmission of all types of respiratory viruses.

40% to 60% Calculated RH vs Weekly Influenza Cases TX

US Centers for Disease Control (CDC) collected data over nine influenza seasons in Texas.

The concern of the influenza season exacerbating the current COVID-19 situation has prompted schools to reduce in-person contact. One simple way that has not been widely recommended until now is maintaining 40% to 60% RH. There is clear scientific evidence in the study suggesting readers take urgent action to install humidifiers in homes, workplaces, and schools.  There is the potential to save tens of thousands of lives per year by implementing this measure, in addition to masks, social distancing, and good hygiene.

The Correlation Between Seasonal Influenza and RH

Seasonal respiratory epidemics generally start in the late fall and end quickly in early spring. Influenza is one that commonly kills millions worldwide every year. As a result of its impact, the annual influenza epidemic has drawn more attention in the last decade. Influenza, RSV, and human coronavirus have been of particular note in scientific studies.

While outdoor RH  doesn’t show a correlation with the seasonal influenza epidemic, absolute humidity (AH)  has proven a strong anti-correlation.  In most buildings, RH drops to under 40% without the intervention of humidifiers during the winter.  Respiratory viruses and influenza are often transmitted by large droplets created when coughing, sneezing, or talking, aerosol transmission, and physical contact with surfaces that have been contaminated.  Social distancing, masks, and frequent disinfection help to reduce transmission.

Pearson correlation coefficients, R, for correlations of weekly numbers of positive influenza tests with outdoor RH and calculated indoor RH for ten states in the US

The table to the right shows some states have positive correlations of influenza cases and outdoor RH, while others have negative correlations. The average for outdoor RH is zero, which indicates no correlation. The indoor RH column has consistently negative correlation values and an average of -0.445, indicating a definitive correlation.

How Indoor RH Effects Viral Transmission

Droplets sent into the air from an occupant’s nose or mouth range in size from a single micron to several millimeters.  In rooms with higher RH, these droplets fall quickly, but in rooms with lower RH lack moisture in the air, the droplets become suspended small particles within a second due to evaporation. This process aerosolizes the virus, and it becomes as small as 3 µm and behaves like a gas. Particles can take days to settle and become mixed by air currents. Aerosolized particles can also get further into the occupant’s throat, nose, lungs, and alveoli. Rooms with little to no ventilation result in higher particle concentration and increase the number of particles inhaled by teachers and students.

Stability for aerosolized viruses most common in the winter (influenza, coronavirus, parainfluenza, RSV) at low RH (20-50%). Summer and year-round aerosolized viruses (poliovirus, adenovirus, rhinovirus) were most stable at high RH (80%).  Cough simulation studies show aerosolized particles collected over an hour had a 71-77%  infection rate at 23% RH. However, 43% RH only had 15-22% infection rates. In an RH range of 40-73%, the minimal influenza infection rate was achieved. Human infection risk is impacted by the respiratory tract’s mucus membrane’s continual exposure to air at varying RH levels.  Low RH causes epithelial cilia loss, and detachment of cells, further causing inflammation.  This injury impairs the natural antiviral defenses of the body.

SVL’s Answer to Low RH

SVL takes the health of building occupants on every job seriously. Currently, most schools do not regulate RH through their HVAC systems, and even more do not have adequate ventilation. SVL Sales Engineers have advanced knowledge to identify the best humidifiers for every job. One excellent option that SVL offers is Condair.  Condair has a line of superior humidifiers that can fit into nearly any commercial facility. They also have a 40to60RH campaign that is lead by Dr. Stephanie Taylor.  This campaign aims to gain WHO (World Health Organization) backing for 40 to 60% RH as the standard requirement.

Click on the video below to hear from Dr. Stephanie Taylor as she explains how important this effort is.

To learn about incorporating better humidity levels and getting ahead of possible outbreaks with a Condair humidifier, contact your SVL Sales Representative, or click here.