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School of thought

Duncan Curd references a Mayo Clinic study proving the reduced capacity of influenza A to survive in classrooms benefitting from humidification

| | Jul 11, 2021 | 4:54 am
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Around the world, people are awaiting relief from mask mandates and social distancing guidelines, and the return to “normal”, whatever that may look like. However, in the wake of what we know now, we should not be willing to forget indoor air quality issues that have plagued us even before the COVID-19 pandemic.

Duncan Curd

While hand washing and wearing masks, along with frequent cleaning and disinfecting, have been helpful in reducing the spread of COVID-19 and even the seasonal flu, experts have known for a long time that there is more that can be done.

A key study done in 1986 showed that the optimal conditions to minimise risks to human health occur between 40% and 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).

When the relative humidity level in a building is not controlled to fall within this 40-60% RH range, there is an increased risk of the transmission of illness spread by viruses or bacteria, skin dryness and eye irritation. These issues can be experienced by people in any building – including schools, offices, nursing homes, senior living communities and health care facilities.

The Mayo Clinic, in Rochester, Minnesota, in the United States, 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 A, or the seasonal flu, to survive on classroom surfaces or in the air as aerosols. Commercial-sized humidifiers were installed in two classrooms, to control against two classrooms that had no humidification.

The study showed that the humidified rooms:

  • Had a significant decrease in the percentage of total air samples containing influenza A
  • Displayed a trend towards a decreased percentage of surface samples containing influenza A
  • Yielded samples with influenza A that contained fewer “live” viruses and were, therefore, less infectious
  • Resulted in fewer flu-like illnesses

While the COVID-19 pandemic seems to be slowly coming to an end, facility managers and building owners should consider a plan for future safeguards, including the addition of a commercial humidification system that will provide safe, hands-free, broad protection for building occupants against airborne viruses, such as COVID-19 and the seasonal flu, while also reducing the spread of bacteria, the proliferation of allergens, and other occupant discomforts caused by dry air.

We are all eager to get our lives back on track, and one of the first steps to reopening is to ensure that the relative humidity is at a healthy level in each building. It is simple to take RH readings with a hygrometer to quickly determine if a facility is optimising its defence against COVID-19 and other viruses. In general, a target of 45% RH is recognised as providing positive benefits while being easily achieved.

While there is a short-term, day-to-day “set it and forget it” benefit to a commercial humidification system, there is also a longterm “get it and keep it” benefit, as the system can last for up to 20 years. And with health benefits beyond COVID-19, that longterm benefit delivers a valuable return on investment year after year after year. Eventually we’ll see an end to the current pandemic. When we do, the humidification system will still hold its value, protecting against other infections and common discomforts caused by dry air.


1 Arundel, A V, et al. “Indirect Health Effects of Relative Humidity in Indoor Environments.” Environmental Health Perspectives, U.S. National Library of Medicine, Mar. 1986, www.ncbi.nlm.nih.gov/pmc/articles/PMC1474709/.

2 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: 10.1101/27387

Duncan Curd is Global Business Development Leader, Sales Department, DriSteem. He may be contacted at duncan.curd@dristeem.com

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