During cold and flu season, humidity is our friend.
Sweet spot is at 40-50% RH.
I live in the Northern Hemisphere and we are now entering meterological winter. Heat turned on and blasting away, zapping our indoor relative humidity, dropping it into the 20’s %. So I got my bedside steam vaporizer hissing away at night and my radiators are topped with evaporation water pans. These winter habits were initially started to reduce static discharges on my cpu’s, but inadvertently, all along I was wiping out circulating respiratory viruses.
Optimal humidity levels are needed to allow for the flow of mucin along the mucosa surfaces of the nasopharynx and lungs. The mucopolysaccarides trap the pathogens and cilia act like flippers pumping them along to the esophagus where they can get dumped into the gastric juices for destruction. Our innate immune system is well designed for this. We just need to cooperate with it for our optimal protection.
Mannequin
Where did we learn this info? Enviromental engineers who design hospitals and laboratories tested parameters to optimize conditions. They examined the role of relative humidity (RH) in the aerosol transmission of influenza in a simulated examination room containing coughing and breathing mannequins. Total virus collected for 60 minutes retained 70.6–77.3% infectivity at RH <23% but only 14.6–22.2% at RH >43%.
At low RH, influenza remains maximally infectious but inactivation of the virus at higher RH occurs rapidly after coughing. Although virus carried on aerosol particles <4 microM have the potential for remaining suspended in air currents longer and traveling further distances than those on larger particles, if they are floating in high humidity, they are inactivated.
Mice and guinea pigs
So low ambient humidity is known to enhance viral transmission; what is the mechanism of action? What is its impact on host response to influenza virus infection and disease outcome? Experiments housing Mx1 congenic mice in low relative humidity showed that it makes them more susceptible to severe disease following respiratory challenge with influenza A virus. Inhalation of the dry air impaired their mucociliary clearance, innate antiviral defense, and tissue repair. In multiple lung cell types, single-cell RNA sequencing revealed that induction of IFN-stimulated genes in response to viral infection was diminished.
… Dr. Peter Palese found a peculiar comment in an old paper published after the 1918 flu pandemic: the author of the 1919 paper stated that upon the arrival of the flu virus to Camp Cody in New Mexico, the guinea pigs in the lab began to get sick and die. Palese tried infecting a few guinea pigs with influenza, and sure enough, the guinea pigs got sick. Importantly, not only did the guinea pigs exhibit flu symptoms when they were inoculated by Palese, but the virus was transmitted from one guinea pig to another. Now that Palese had a model organism, he was able to begin experiments to get to the bottom of the flu season. He decided to first test whether or not the flu is transmitted better in a cold, dry climate than a warm, humid one. To test this, Palese infected batches of guinea pigs and placed them in cages adjacent to uninfected guinea pigs to allow the virus to spread from one cage to the other. The pairs of guinea pig cages were kept at varying temperatures (41°F, 68°F, and 86°F) and humidity (20%-80%). Palese found that the virus was transmitted better at low temperatures and low humidity than at high temperatures and high humidity.
Humans
But what about real life experience with human beings? Are we just like guinea pigs? Well those environmental industrial engineers tested out their theories by surveilling the incidence of infections arising from 10 hospital patient rooms over a period of time that varied in ambient temperature and humidity. Lowest level of cases occurred when RH was above 40%.
In the temperate regions, seasonal influenza virus outbreaks
correlate closely with decreases in humidity. Absolute humidity (AH) is defined as absolute amount of water in the air, and relative humidity (RH) is defined as the relative proportion of water in the air in comparison to the maximum water vapor. In Japan, they looked at outside temperature and absolute humidity and correlated it with the time course of flu cases incidence. They collected meteorological and influenza epidemiological data from 46 prefectures in temperate Japan for 1991-1995 and 1999-2009, and analyzed 2,392 sets of weekly compiled data for each season year. When compared to Relative Humidity, Absolute Humidity values were better correlated and predictive. Influenza epidemics predominantly occurred at low AH (<10 g/m3).
So yes, we humans are just like the guinea pigs. This flu season be sure to up that room humidity. And, of course, vitamin C, vitamin D and fresh air wouldn’t hurt.
As for me, my next experiment is to test out my Grandma’s chicken and dumpling soup, which is sure to cure all that ails ya. Mmmm, mmmmm … good.
REFERENCES
JD Noti et al. (2013) High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs. 8:(2) e57485 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0057485
E Kudo et al. Low ambient humidity impairs barrier function and innate resistance against influenza infection. Proceedings of the National Academy of Sciences, May 13, 2019; DOI: 10.1073/pnas.1902840116
The Reason for the Season: why flu strikes in winter BLOG DECEMBER 1, 2014 https://sitn.hms.harvard.edu/flash/2014/the-reason-for-the-season-why-flu-strikes-in-winter/
Shoji M, Katayama K, Sano K. Absolute humidity as a deterministic factor affecting seasonal influenza epidemics in Japan. Tohoku J Exp Med. 2011 Aug;224(4):251-6. doi: 10.1620/tjem.224.251. PMID: 21705850
Humidity, Human Health and Implications in improving Health Care
Presenter: Steve Brandt, P.E.
Developed from research conducted by Stephanie Taylor, MD http://www.cscos.com/wp-content/uploads/Mechanical-3.1-Brandt-Humidity.pdf
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