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Why flu spreads more during winter February 10, 2009

Posted by federalist in Energy, Healthcare.

Shaman & Kohn are getting a lot of press for their article documenting a correlation between absolute humidity (AH) and flu virus transmission and survival.  But it was already well established that flu incidence peaks during the winter.  Shaman & Kohn have merely “rediscovered” this fact through a correlated variable: Apparently nobody covering their article is aware that AH and outside air temperature are nearly perfectly correlated on a seasonal scale.

Humidity itself is an interesting subject: For a given temperature and pressure there is an absolute limit to how much moisture air can hold.  The hotter the air, the higher that limit.  Relative Humidity (RH) indicates the ratio of actual water vapor to theoretical water vapor in the air.  When air is cooled, holding all else constant, RH increases because the capacity of the air for holding water decreases with temperature.  When RH hits 100% and the air is cooled further the moisture begins to condense right out of the air.

This presents all sorts of problems for indoor air quality.  Ideal indoor relative humidity is 30-50%.  Any higher and fungus begins to thrive.  Any lower and humans begin to dry out, which leads to health problems, some of which we’ll come to shortly.  During the summer even relatively dry outdoor air can hold enough water vapor to cause elevated humidity when cooled to comfortable indoor levels.  Fortunately modern air conditioners include condensers that remove enough moisture from the air they cool to keep humidity in line.  But many houses still have problems in basements, which are naturally kept cool by underground heat mass: As the outside summer air makes its way inside and cools in the basement, its relative humidity shoots up.  Without dehumidifiers basements get that musty smell indicative of fungus thriving on the moisture.

In the winter we have the opposite problem: Cold winter air, even at 100% humidity, dries out as it is heated indoors.  Without artificial humidifiers the RH of heated air can fall to single digits, which is drier than many deserts.  Humans acclimated to more temperate weather do not handle dry air well: their skin and sinuses dry out and crack.  Dry mucus membranes are more vulnerable to pathogens.  Which is why the onset of winter in temperate climates causes a spike in influenza: Eyes and noses are irritated from the dry air, so people are constantly touching them, sneezing, and coughing.  If that weren’t enough to get pathogens out of their body and onto their hands, then when they go outside the cold gives them a runny nose.  Now everyone’s hands are covered in respiratory pathogens, and they’re constantly putting their hands near the dried out mucus membranes those pathogens crave to infect.

No surprises here: Humidify your air during the winter to protect your respiratory membranes.  Wash your hands and keep them away from your face when they haven’t been washed.

Unfortunately Shaman & Kohn’s full article is not freely available (thanks to the mercantilism of the AIP, its publisher).  However, they rely heavily on data from this 2007 article by Lowen, Mubareka, Steel, and Palese, which is public.  This latter research on guinea pigs confirms that influenza viruses spread well in both cold and dry conditions.  Shaman & Kohn seem to have fixated on some data from 1961 (Harper) that show airborne virus survivability most strongly linked to absolute humidity.  I believe this could obfuscate critical dynamics of flu propagation.  E.g.,

  • Host animals might become more susceptible to infection when exposed to low relative humidity because their membranes dry out.
  • Host animals might engage in more transmissive behavior in low relative humidity because dry membranes are irritated, prompting them to both express viruses (through coughing and sneezing) and contaminate their more vulnerable membranes through touching.
  • Cold temperatures may, for separate reasons, also increase transmissive behaviors: e.g., runny noses both expressing the virus, and encouraging contamination by the hands.

Again, saying something is correlated to cold and/or dry conditions is roughly equivalent to saying it is correlated to absolute humidity.  But Shaman & Kohn’s strongest statistical result, airborne survivability of the virus, may not be critical to influenza’s propagation under cold and/or dry conditions because contact transmission and host vulnerability is also enhanced under cold and low-humidity conditions (for different reasons).

In any case, everyone seems to agree that the best way to minimize flu incidence is to avoid exposure to cold and dry air.



1. Ray Hicks - February 12, 2009

CO2 Carbon dioxide and RH measurement for a healthy envioronment. While high relitive humidity RH does aid the spread of respitory infection, and keeping track of RH is certainly an indicator of potential problems. CO2 is a better indication if the space adiquite ventilation. the quality of ventilation is mesured by CO2 concentrations, normally 600 to 900 Parts Per Million, as the number increases people in the area are infact rebreathing now infected air. For more referance on this and other indoor air quality issues http://www.co2meter.com.

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