A study by Dr. Akiko Iwasaki at Yale University, facilitated by Condair Ltd., has shown that relative humidity (RH) of 50 percent provides protection from influenza. Previous studies have clearly shown that environmental conditions such as dry air affect transmission of the flu virus, this is the first to show the mechanism through which humidification decreases illness and mortality in an animal model that applies to humans.
The work by Dr. Iwasaki’s immunobiology laboratory provides answers to questions about the health benefits of balanced humidification.
Dr. Iwasaki answers these questions:
- What effect does balanced indoor humidity have on people exposed to the flu virus?
- What are the protection mechanisms of balanced humidification?
It has been recognized for decades that influenza outbreaks occur in the winter when indoor heating is needed. The reasons that people get the flu during the wintertime has been vigorously debated, with explanations for this seasonality including exposure to cold temperatures, low indoor or outdoor humidity, indoor crowding with increased short distance transmission through contact or sneezed droplets, and decrease in healthy blood levels of vitamin D due to less exposure to sunlight.
Understanding the drivers behind seasonal influenza, and therefore knowing what steps one can take to decrease the associated morbidity and mortality, is not just theoretically interesting. The topic is important as seasonal influenza infections continue to increase and cause at least half a million deaths annually, worldwide. Providing balanced humidity is a protective intervention that supports vaccination, efforts that could save millions of lives.
Dry indoor air has been shown to worsen influenza outcomes, and controlling relative humidity between 40 – 60 percent has been demonstrated to minimize individual steps of the viral infection and transmission process. This is the first study that clearly reveals the physiological mechanisms behind improved outcomes when balanced humidification is provided.
This research used genetically modified mice with immune responses similar to humans, making these animals an ideal model.
Data from this study provides the following insights:
1. Mice infected with Influenza A were able to maintain a healthy body weight and temperature, surviving longer in ambient 50 percent RH compared to 20 percent RH with twice as many still alive at the end of the experiment.
2. Mice in 10 percent RH were less able to clear the virus from their lungs, and showed ongoing viral growth. Conversely, mice in 50 percent RH were more resistant to the virus, had less tissue damage from inflammation and had overall milder symptoms.
3. Low humidity of 10 percent RH diminished the ability of the natural respiratory tract barrier to protect the lungs. This barrier, the mucociliary clearance system, is made up of tiny hairs (cilia) washing particles upward and away from deep, vulnerable lung tissue in a layer of mucus. Dehydrated and viscous mucus restricted cilia movement so the influenza virus was settled in deep tissues.
4. Once damage from infection had occurred, low humidity conditions of 10 percent RH diminished airway repair.
5. The protective immune response to influenza A was stronger at 50 percent RH compared to 10 percent RH.
This study reinforces the need to maintain indoor RH 40–60 percent in homes, schools, offices, hospitals, airplanes, and all other occupied spaces.
These findings are a true breakthrough in infectious disease control. Dr. Iwasaki shows that balanced humidification increases our immune defenses and therefore can be applied to other diseases, not limited to influenza.
“ASHRAE must recognize this excellent study as evidence to support a minimum RH level in occupied commercial buildings,” says Dr. Stephanie Taylor, a member of the ASHRAE’s Environmental Health Committee.