April 25, 2009
Higher Absolute Humidity Would Slow Flu Spread?

Looking for ways to cut your odds of getting influenza? A February 2009 PNAS paper by Jeffrey Shamana Melvin Kohn points toward a way: a decline in absolute humidity might explain much of the increase in incidence of flu during the winter.

Influenza A incidence peaks during winter in temperate regions. The basis for this pronounced seasonality is not understood, nor is it well documented how influenza A transmission principally occurs. Previous studies indicate that relative humidity (RH) affects both influenza virus transmission (IVT) and influenza virus survival (IVS). Here, we reanalyze these data to explore the effects of absolute humidity on IVT and IVS. We find that absolute humidity (AH) constrains both transmission efficiency and IVS much more significantly than RH. In the studies presented, 50% of IVT variability and 90% of IVS variability are explained by AH, whereas, respectively, only 12% and 36% are explained by RH. In temperate regions, both outdoor and indoor AH possess a strong seasonal cycle that minimizes in winter. This seasonal cycle is consistent with a wintertime increase in IVS and IVT and may explain the seasonality of influenza. Thus, differences in AH provide a single, coherent, more physically sound explanation for the observed variability of IVS, IVT and influenza seasonality in temperate regions. This hypothesis can be further tested through future, additional laboratory, epidemiological and modeling studies.

Humidifiers in offices and other closed spaces might cut the incidence of the flu. If you live by yourself them a humidifier at home probably isn't going to lower your risk much unless you have visitors. However, stores, businesses, and government offices could probably reduce the spread of flu by keeping the air much more humid.

What other changes do we experience in winter that might account for winter flu outbreaks? Our blood vitamin D levels drop and vitamin D is an immune system modulator. Also see here. So make sure you get enough vitamin D if the swine flu that has crossed over into humans in Mexico turns into a big pandemic.

Update: Check out a Google Maps views of H1N1 swine flu spreading in humans. Note that the first map isn't actually a case map. Check out links on left.

Update II: Here is a Google map of real and suspected human H1N1 swine flu cases. This tracking by individual cases will become unwieldy in a few days. But for now it gives a good sense of how this flu is spreading.

Share |      Randall Parker, 2009 April 25 10:43 PM  Pandemic Prepare Business

beb said at April 26, 2009 3:07 PM:

I would warn of one danger of a humidifier, mold. I bought one and within a month or two found myself in a doctors office with a sinus infection. I was given all the warnings of allergins in my home and washed everything down with no luck until it dawned on me that I had this tub of water being constantly sprayed into my room.

Easy to fix. They make filters and sterilizing solutions for them. So it isn't something that you have to worry about, you just have to know about it. I didn't know and nobody thought to tell me when I bought the machine.


Fred said at April 26, 2009 3:17 PM:

Anyone else notice, it's on each coast, on the North Border and South Border of the United States and right smack in the heartland. Not started on one coast and spread say started on the east coast and spread westwardly. It popped up on each coast, and North and South and in the heartland all at once......Yeah, I know on t.v they say it's not bio....I'm not believing much from the media these days.

Thaden said at April 26, 2009 3:48 PM:

It would be interesting to learn the annual rates of influenza infection in areas like Seattle compared to the rest of the U.S., since humidity levels in Seattle remain very high during winter, with very little sunshine for vitamin D production. Maybe compare this with another region that keeps high humidity and gets more winter sunshine- maybe Florida??

TAF said at April 26, 2009 7:15 PM:

In as much as Mexico is both sunny and humid, I have to take this theory with a large grain of salt.

And where is bird flu most prevalent? Southeast Asia, where is it...very humid, and frequently quite sunny.

I think I would look elsewhere for a reason as to why the regular flu becomes more prevalent in winter.

Placebo said at April 28, 2009 1:40 PM:

If this high humidity hypothesis is true, the mechanism may be related to stabilizing the liquid lining the airway surface. In 2004, Howard Stone et al., wrote a paper about this effect in the same journal. It is available for free online. Below is the abstract.

Inhaling to mitigate exhaled bioaerosols.
Proc Natl Acad Sci U S A. 2004 Dec 14;101(50):17383-8.

"Humans commonly exhale aerosols comprised of small droplets of airway-lining fluid during normal breathing. These "exhaled bioaerosols" may carry airborne pathogens and thereby magnify the spread of certain infectious diseases, such as influenza, tuberculosis, and severe acute respiratory syndrome. We hypothesize that, by altering lung airway surface properties through an inhaled nontoxic aerosol, we might substantially diminish the number of exhaled bioaerosol droplets and thereby provide a simple means to potentially mitigate the spread of airborne infectious disease independently of the identity of the airborne pathogen or the nature of any specific therapy. We find that some normal human subjects expire many more bioaerosol particles than other individuals during quiet breathing and therefore bear the burden of production of exhaled bioaerosols. Administering nebulized isotonic saline to these "high-producer" individuals diminishes the number of exhaled bioaerosol particles expired by 72.10 +/- 8.19% for up to 6 h. In vitro and in vivo experiments with saline and surfactants suggest that the mechanism of action of the nebulized saline relates to modification of the physical properties of the airway-lining fluid, notably surface tension."

diana said at April 28, 2009 4:25 PM:

"In as much as Mexico is both sunny and humid, I have to take this theory with a large grain of salt."

Make it an iodized grain.

An esp. virulent strain of flu virus can break out at any time and will infect susceptible people. How do you know what their blood Vitamin D levels are? Even in a sunny place people can stay indoors or cover up when they go outside. There is such a thing as flu season, and and blood Vitamin D levels are inversely proportional to outbreak. Whether that's a causal relationship, no one knows. But the fact that Vitamin D is intimately implicated in human immune regulation isn't conjecture; it's proven.

Remember: the 1918 epidemic hit the US in May and no one much noticed because the strain appeared to be weak and mostly people didn't die. Then it broke out w/a vengeance in November, killing thousands. Vitamin D levels? I'd take my supplements just in case.

Randall Parker said at April 28, 2009 8:12 PM:


A very cool report. Thanks for that.

diana said at April 29, 2009 11:19 AM:

Added word about Mexicans: they have brown skin, and thus do not synthesize Vitamin D very efficiently. Also, they wear clothing - another good way to cut down on Vitamin D synthesis.

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