The Importance of Relative Humidity

by Laurie Waggener,

March 25, 2020

Humidifier in home

With more people hunkering down at home these days, it’s critical to maintain appropriate indoor relative humidity (RH) levels to better protect yourself and your family during flu season. There is a proven link between RH and the transmission of cold and flu viruses, and hospitals go to great lengths to control RH to help prevent the spread of infection. RH of 30-60% is generally recognized as the optimal range.

It was first thought that colder air in winter combined with people spending more time indoors were the main drivers for the rapid spread of the flu. This was partially right. While more people being indoors for longer periods leads to more social interactions, cold air holds less moisture than warm air. Cold air dries out the nasal passages and mucous membranes, your body’s main line of defense for trapping airborne pathogens, which leaves you more vulnerable to infection. This is why the flu season in the US coincides with the onset of winter.

Researchers from the Centers for Disease Control (CDC) examined the ability of the flu – an infectious virus like COVID-19 – to remain active in  indoor environments. CDC researchers tested the effect of RH on the capacity of the virus released in a simulated 'cough' to re-infect cells. They found that an hour after being released in a room with RH of 23% or less, 70-77% of viral particles retained their infectious capacity, but when humidity was increased to about 43%, only 14% of the virus particles were capable of infecting cells. Most of this inactivation occurred within the first 15 minutes of the viral particles being released in the high-humidity condition. The study concludes that maintaining indoor RH at levels of 40% or greater can significantly reduce the virus' infectious capacity.

Think about your own indoor home environment as a key part in preventing infection. Consider implementing other preventative measures, including:

  • Using a humidifier to maintain optimal RH levels
  • Maintaining recommended indoor temperatures of 68.5°F to 75°F in the winter and 75°F to 80.5°F in the summer
  • Covering your nose and mouth when you sneeze or cough
  • Wiping down high touch surfaces like doorknobs and counter tops regularly with appropriate disinfectants
  • Washing your hands for at least 20 seconds with soap and water
  • Putting on hand lotion regularly to combat dry skin and prevent cracking

During this flu season, give yourself and your family members every advantage to stay healthy.

Laurie Waggener

Director, Research for Healthcare