Humidifiers

Fighting the Flu with Humidity

Researchers discover immune system benefits of humidity

When the temperature drops, the heat comes on. This reduces the amount of humidity in the air. It turns out this isn’t just uncomfortable causing hands, nasal passages, and throats to become dry, it also impairs the barrier function and the innate immune system in the respiratory tract.

Beyond the nose and mouth, the respiratory system includes the throat, bronchus and the lungs. These are all lined with epithelial cells, some of which produce mucus. This mucus provides the first layer of defense against viral upper respiratory infections, such as influenza and the new coronavirus disease COVID-19.

The epithelial cells have small cellular protrusions called cilia that move the mucus to promote clearance of pathogens and particles that enter the respiratory system. This process results in the phlegm and mucus that are eliminated by coughing, sneezing, and nose blowing.

Epidemiological study found a correlation between increased death from influenza (the “flu”) and a drop in humidity. Researchers studying mice found a reason for this correlation. They determined that infection at lower humidity compromised mucus clearance and the response of the innate immune system. Consequently, mice infected with influenza virus in 10% or 20% humidity became sick faster than mice infected in 50% humidity air. The virus was also more lethal to mice infected in the lower humidity conditions.

Analysis of the mucus in the trachea showed that infection in low humidity resulted in the inability to of the cilia on the epithelial cells to move it. The low humidity made the mucus too thick.

Inspection of the lungs of mice infected in either 10% humidity or 50% humidity showed that in 10% humidity the virus was present in both epithelial cells and macrophages. The mice infected in 50% humidity only had virus in the macrophages. Macrophages are innate immune cells that remove pathogens and infected cells. This indicated that lower humidity compromises the innate immune system’s ability to fight viral infection.

In contrast to the lungs of the mice infected in 50% humidity, the lungs of the mice infected at 10% humidity had reduced signs of tissue repair 6 days after infection. This is consistent with the greater severity of symptoms and faster and greater fatality rate in mice infected in lower humidity.

Analysis of the genes expressed by the lungs of mice infected in 10% or 50% humidity showed that the mice infected in 10% humidity had less activation of genes involved in mediating antiviral responses.

Collectively, these results provide an explanation for the seasonality of influenza infections. They also support a simple way to reduce risk of infection and severity of symptoms. Use humidifiers when the heat is on.

Highlighted Article

E. Kudo, E. Song, L. J. Yockey, T. Rakib, P. W. Wong, R. J. Homer, A. Iwasaki. Low ambient humidity impairs barrier function and innate resistance against influenza infection. Proc. Natl. Acad. Sci. U. S. A. 116, 10905–10910 (2019). DOI:10.1073/pnas.1902840116

Related Articles

A. C. Lowen, S. Mubareka, J. Steel, P. Palese. Influenza virus transmission is dependent on relative humidity and temperature. PLoS Pathog. 3, 1470–1476 (2007). DOI: 10.1371/journal.ppat.0030151

J. Shaman, V. E. Pitzer, C. Viboud, G. T. Grenfell, M. Lipsitch, Absolute humidity and the seasonal onset of influenza in the United States. PLoS Biol. 23, e1000316 (2010). DOI: 10.1371/journal.pbio.1000316