Why Flu Spreads, Thrives in Winter
Flu Mystery Solved? Why It Flourishes in Winter
Matt Kaplan for National Geographic News | February 9, 2009 | news.nationalgeographic.com
Why the flu strikes hard during the winter but nearly vanishes in the summer has baffled epidemiologists for decades.
Now a new study may have the answer: Influenza germs last longer and pass from person to person more effectively in lower absolute humidity—i.e., when it’s cold outside and the air is dryer.
Absolute humidity is a measurement of the total amount of water vapor in the air at a given temperature. Relative humidity, a percentage, is the ratio between the water vapor present and the air’s saturation point, a figure that changes with the temperature.
Earlier studies of humidity and the flu had focused on relative humidity and found no link, said lead author Jeffrey Shaman at Oregon State University, whose findings appear tomorrow in the journal Proceedings of the National Academy of Sciences.
After reanalyzing these past studies, which also contained data on absolute humidity, Shaman’s team found a much more powerful connection.
“Absolute humidity conditions explain most of these changes,” Shaman said.
The researchers do not know exactly what it is about low absolute humidity that the flu virus likes. But they suggest that absolute humidity levels be raised in buildings such as hospitals and medical clinics where the disease most often spreads.
“This gets us a big step closer to one type of mechanism” for how the flu spreads, said epidemiologist Marc Lipsitch of Harvard University.
“One really key question is how much influenza is transmitted in tropical locations”—places with high absolute humidity year-round—”and how this compares to temperate parts of the world,” he said.
“A satisfactory explanation would not only explain the seasonal cycles in temperate areas, but also the much less seasonal, but still substantial, burden of influenza that seems to be present in the tropics.”
Converting relative humidity to absolute humidity is easy:
The absolute humidity of the ambient air is the saturation humidity multiplied by the percentage relative humidity and divided by 100: Habs = (Hsat )(RH / 100) where: Habs = absolute humidity at a given temp Hsat = saturation humidity at the same given temp RH = percentage relative humidity at the same given temp First, you must calculate the saturation humidity at each of your temperature data points: Hsat = (7000)(18.02 / 28.85)(PS)/(Pamb – PS) where: Hsat = grains of water per pound of air 7000 = grains per pound 18.02 = molecular weight of water 28.85 = molecular weight of dry air (taken as 79 vol % nitrogen, 21 vol % oxygen) PS = vapor pressure of water at temperature T Pamb = ambient pressure (You will need a good set of steam tables to look up the vapor pressure of water at each of your temperature data points) Example calculation of Hsat at 68 oF and an ambient atmospheric pressure of 14.696 psia: From the steam tables, the vapor pressure of water is 0.3389 psia at 68 oF. Hsat = (7000)(18.02/28.85)(0.3389)/(14.696 – 0.3389) = 103.2 grains of water/lb of air Now if your relative humidity data point at 68 oF was say 60%, then: Habs = (103.2)(60/100) = 61.9 grains of water/lb of air.
This gives new meaning to ‘hot and bothered.’ I decline to comment on the uh…humidity.
sounds like you’ve got extra free time today.
Free time? Me? Here’s few more in case anyone has plans to do some tech diving:
PSIG = Pounds per Square Inch (Gauge) PSIA = Pounds per Square Inch (Absolute) ATM = Atmospheres ATA = Atmospheres (Absolute)
FSW = Feet of Sea Water FSWA = Feet of Sea Water (Absolute) FFW = Feet of Fresh Water FFWA = Feet of Fresh Water (Absolute)
“T” formula Partial Pressure
Total Pressure
% Gas
Partial Pressure = Total Pressure X % Gas
% Gas = Partial Pressure / Total Pressure
Total Pressure = Partial Pressure / % Gas
Calculating PO2: PO2 = Depth in atmospheres absolute (ATA) X percentage O2
PO2 = ((d+33)/33)XFO2 or PO2 = ((d/33)+1) X FO2
Calculating MOD: MOD = ((Maximum PO2 (normally 1.6 or 1.4) / % Fraction of Gas (%O2)) -1 atmosphere) X 33
MOD=((PO2/FO2) – 1) x 33 or MOD = (33 X PO2/FO2)-33
Calculating EAD: Salt Water ((1- FO2/.79) X (D+33)) – 33
Fresh Water ((1- FO2/.79) X (D+34)) – 34
Calculating Surface Consumption Rate (SCR) in PSI/MIN:
Salt Water: ((PSI used/Bottom Time) X 33) / Depth + 33
Fresh Water: ((PSI used/Bottom Time) X 34) / Depth + 34
Converting SCR from PSI/MIN to Cubic Feet/MIN:
Cubic Feet/min = SCR X Tank Volume (full)/Fill Pressure
Anyother equations you need? Here’s another for calculating my free time:
t=time (hrs)
t=24=24 hrs. a day…
wow. now imagine if you used your free time for the power of good.
If I used my free time for the power of good I might actually become Spider Man. That would be cool…ME, a Superhero! “Look out! Here comes the Spider Man.” Getting caught in my web means real trouble for the bad gals! Perhaps the Riddler is more fitting…very hard to say.