Photo Credit PHIL (Public Health Image Library)
# 6983
Regular readers of this blog are aware that there has been a fair amount of research in recent years as to why influenza epidemics (in temperate zones) typically occur during the winter months.
Theories include:
- During the winter people tend to gather indoors, with less outside ventilation.
- Diminished sunlight exposure may reduced Vitamin D levels (see Study: Vitamin D And Flu-Like Illnesses)
- With schools in session, millions of children co-mingle and more efficiently share viruses
While all potential factors, they don’t satisfactorily explain our yearly winter flu season. Nor do they explain why the flu transmits reasonably well in the tropics, where there is little temperature variability.
Scientists know that during the summer, ambient air often contains 4 times as much water as it does on a cool dry winter's day. That has led some researchers to wonder about the role that relative humidity (RH) and absolute humidity (AB) play in influenza transmission.
In 2007, we looked at a study (see Cold And Dry Statistics) that appeared in PLoS Pathogens entitled Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature by Anice C. Lowen, Samira Mubareka, John Steel, and Peter Palese.
Using a guinea pig as a model host, they showed that airborne spread of the influenza virus was at least partially dependent upon both ambient relative humidity and temperature.
The link was significant, but not overwhelming.
The following year researchers Jeffrey Shaman and Melvin Kohn and found an even stronger correlation between the AH (Absolute Humidity) and the survival, and transmission of the influenza virus (see It's Not So Much The Heat, It's The Humidity).
Absolute humidity modulates influenza survival, transmission, and seasonality
Jeffrey Shaman, and Melvin Kohn
In early 2010, Jeffrey Shaman returned – joined by Virginia E. Pitzer, Cécile Viboud, Bryan T. Grenfell and Marc Lipsitch – to pen a study published in PLoS Biology called:
Absolute Humidity and the Seasonal Onset of Influenza in the Continental United States
(Excerpt)
Here we extend these findings to the human population level, showing that the onset of increased wintertime influenza-related mortality in the United States is associated with anomalously low absolute humidity levels during the prior weeks. We then use an epidemiological model, in which observed absolute humidity conditions temper influenza transmission rates, to successfully simulate the seasonal cycle of observed influenza-related mortality.
And just last December we looked at a study (see Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum) that found both extremely low and extremely high humidity were conducive to flu transmission – at least when it resides in mucus and respiratory fluids like those found in your nose, throat, or lungs.
Essentially, these researchers inoculated droplets of simulated respiratory fluids (containing salts & proteins) with influenza viruses, and tested their survivability at different humidity levels.
- At low humidity (< 50%) the droplets evaporated quickly, and the virus survived well in a dry environment.
- At high humidity (near 100%), the droplets were stable, and the virus survived as well.
But at humidity levels in-between, the droplets slowly dried out, increasing the concentration of salts and proteins to which the viruses were exposed, decreasing their survival rate.
The latest study, which appeared last week in PLoS One, suggests that maintaining a higher indoor humidity level during flu season may curb influenza transmission:
High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs
John D. Noti, Francoise M. Blachere, Cynthia M. McMillen, William G. Lindsley, Michael L. Kashon, Denzil R. Slaughter, Donald H. Beezhold
Abstract
Background
The role of relative humidity in the aerosol transmission of influenza was examined in a simulated examination room containing coughing and breathing manikins.
Methods
Nebulized influenza was coughed into the examination room and Bioaerosol samplers collected size-fractionated aerosols (<1 µM, 1–4 µM, and >4 µM aerodynamic diameters) adjacent to the breathing manikin’s mouth and also at other locations within the room. At constant temperature, the RH was varied from 7–73% and infectivity was assessed by the viral plaque assay.
Results
Total virus collected for 60 minutes retained 70.6–77.3% infectivity at relative humidity ≤23% but only 14.6–22.2% at relative humidity ≥43%. Analysis of the individual aerosol fractions showed a similar loss in infectivity among the fractions. Time interval analysis showed that most of the loss in infectivity within each aerosol fraction occurred 0–15 minutes after coughing. Thereafter, losses in infectivity continued up to 5 hours after coughing, however, the rate of decline at 45% relative humidity was not statistically different than that at 20% regardless of the aerosol fraction analyzed.
Conclusion
At low relative humidity, influenza retains maximal infectivity and inactivation of the virus at higher relative humidity occurs rapidly after coughing. Although virus carried on aerosol particles <4 µM have the potential for remaining suspended in air currents longer and traveling further distances than those on larger particles, their rapid inactivation at high humidity tempers this concern. Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus.
This study was supported by National Institute for Occupational Safety and Health NIOSH and the Centers for Disease Control and Prevention CDC.
A press short press release summarized the findings:
Higher indoor humidity inactivates flu virus particles
Infectious capacity of influenza virus particles reduced at relative humidity of 40 percent or higher
Higher humidity levels indoors can significantly reduce the infectivity of influenza virus particles released by coughing, according to research published February 27 in the open access journal PLOS ONE by John Noti and colleagues from the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention.
The researchers tested the effect of relative humidity on the capacity of flu virus released in a simulated 'cough' to re-infect cells. They found that an hour after being released in a room at a relative humidity 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 fifteen minutes of the viral particles being released in the high-humidity condition.
The study concludes that maintaining indoor relative humidity at levels greater than 40% can significantly reduce the infectious capacity of aerosolized flu virus.
A supporting point that I’ve made before is that the Chinese have long boiled vinegar in their homes to `ward off’ respiratory ailments, such as influenza.
The noxious odor was supposed to `purify' the air inside the home, and newspapers still recommend this practice during their influenza season.
It may well turn out that the `active ingredient' is really the water vapor being released, raising the absolute humidity in their homes to an unfavorable level for influenza virus survival and transmission.
