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Indoor Humidity and the Spread of Respiratory Illness

Frequent hand washing, and covering coughs and sneezes, provide a good start in reducing the spread of viruses that cause respiratory illnesses like influenza in our buildings, but more can be done. Carefully controlled indoor humidity levels are an easy and efficient way to protect occupant health.


Influenza, commonly known as the flu, is one of the most dangerous respiratory illnesses—particularly for those who are likely to develop complications like children, adults over age 65, pregnant women, nursing home residents, and patients in long-term care facilities.


The flu season occurs each year during the cold, dry winter months, with the number of cases typically peaking in February. This can be partially attributed to the fact that we spend more time indoors with others in the winter, but it can also be attributed to the role of humidity indoors.


Researchers have shown that we can transmit the flu virus by merely breathing. People with the flu generate tiny droplets that stay suspended in the air for a long time, even when they are not coughing, or sneezing.1


Furthermore, exhaled viruses float in the air for a longer time when the air is dry, increasing the chance of infection. The dry air also weakens our body’s natural defenses by causing our nasal mucous membranes to dry out and become more susceptible to infection.


Humidity and Flu Transmission

Humidity can also influence when the flu outbreak begins each year. There is a direct correlation between low humidity levels outdoors and flu transmission: In the continental United States, research has established that flu outbreaks can be reliably predicted to begin 14 to 16 days after the outdoor humidity bottoms out. So, when outdoor humidity first falls to its lowest level in the winter, experts know to expect that flu outbreaks will start showing up in about two weeks.2



A key study done in 1985 showed that the optimal conditions to minimize risks to human health occur between 40-60% relative humidity (RH) at normal room temperatures. This study is still referenced by HVAC professionals today and forms the basis of standards for healthy built environments set by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE). Maintaining RH within this 40-60% range decreases the number of viruses, bacteria, and allergens found in the environment while also preventing skin dryness and eye irritation.


Controlling Outbreaks through Controlled Humidity

This knowledge can be applied to make our indoor environments healthier. For example, in large buildings, adding a commercial humidifier to the HVAC system can ensure that relative humidity levels stay within the recommended range, reducing illness transmission and discomfort. This approach works well in schools, offices, and health care facilities.


The Mayo Clinic in Rochester, Minnesota, tested this concept in a preschool classroom. Researchers wanted to determine whether increasing the relative humidity of classrooms to 40-60% would reduce the capacity of influenza to survive on classroom surfaces or
in the air as aerosols. Commercial- sized humidifiers were installed in two classrooms, to compare against two control rooms that had no humidification. This study showed that the humidified rooms had:

  • A significant decrease in the percent of total air samples containing influenza A
  • A trend toward a decreased percent of surface samples containing influenza A
  • Samples with influenza A containing fewer “live” viruses and therefore were less infectious
  • Fewer flu-like illnesses reported


Besides posing a danger to health and wellness, flu outbreaks are detrimental in other ways. Student absences increase during the dry winter months, often due to respiratory illnesses. Chronic absenteeism or missing ≥ 10% of school days within a year, for any reason, predicts low student achievement.


When viruses spread among students, parents and teachers also tend to get ill and must take sick days. Having to rely on substitute teachers too often can negatively affect lesson plans and is also expensive.


In this way, the flu affects both employers and their businesses—accounting for $16.3 billion in lost earnings annually and costing the US approximately $10.4 billion in direct costs for hospitalizations and outpatient visits for adults.


While it’s well-known that too much water vapor in the air of a building can cause problems such as mold and fungus growth, it is important to understand the benefits that precisely controlled humidification can add to all our indoor environments.


Just as a physician will recommend that parents put a humidifier in the room of a sick child to help them breathe easier and stop the spread of germs, humidification on a large scale is key to ensuring healthy indoor air quality for institutional, industrial, and commercial applications.


By Valerie Bradt, Marketing Communications Manager, DriSteem

1 Yan, J., Grantham, M., Pantelic, J., Mesquita, P. J. B. de, Albert, B., Liu, F., … Milton, D. K. (2018, January 30). Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community. Retrieved from

2 Shaman, J., & Kohn, M. (2009, March 3). Absolute humidity modulates influenza survival, transmission, and seasonality. Retrieved from

3 Arundel, A. V., Sterling, E. M., Biggin, J. H., & Sterling, T. D. (1985). Indirect Health Effects of Relative Humidity in Indoor Environments. Environmental Health Perspectives, 65, 351. doi: 10.2307/3430203

4 Reiman, J. M., Das, B., Sindberg, G. M., Urban, M. D., Hammerlund, M. E., Lee, H. B., … Pierret, C. (2018). Humidity as a non-pharmaceutical intervention for influenza A. doi: 10.1101/273870

5 Molinari, N.-A. M., Ortega-Sanchez, I. R., Messonnier, M. L., Thompson, W. W., Wortley, P. M., Weintraub, E., & Bridges, C. B. (2007). The annual impact of seasonal influenza in the US: Measuring disease burden and costs. Vaccine, 25(27), 5086–5096. doi: 10.1016/j.vaccine.2007.03.046