Why do people get sick in the winter? It's not the cold, it's the humidity

Getting it Down Cold

It’s a given: Spend time out in the harsh winter weather, and you’re likely to pick up a cold or flu, right?

But is that at all logical? How can weather prompt a virus?

Of course, cold weather isn’t the source of sickness. But the winter elements can provide a link in the dastardly chain of events that results in sneezes, coughs, fevers, headaches, and a couple of really lousy days in bed.

Mexico, 1919

Peter Palese, M.D., had one of those moments when details connect and a light bulb flashes on in the mind.

He was reading a scientific paper, published in 1919, about the horrific Spanish influenza pandemic that caused at least 50 million pneumonia deaths worldwide. The paper detailed the effects of the pandemic in a laboratory in New Mexico where guinea pigs died of pneumonia.

That was Dr. Palese’s light bulb moment.

Ethical problems prohibit the study of flu transmission from person to person (you can’t deliberately infect human subjects), and mice don’t transmit the flu from one to another. Why researchers never used guinea pigs for such a study is unclear, but Dr. Palese immediately realized the potential.

After running a simple trial to confirm that guinea pigs actually transmit flu virus, Dr. Palese and his colleagues at the Mount Sinai School of Medicine in New York mounted a more ambitious study that included twenty different experiments to determine flu transmission at various temperature and humidity levels.

Results showed that the optimal temperature for transmission was five degrees Celsius (41 degrees Fahrenheit), and the optimal relative humidity was 20 percent.

Dr. Palese’s results also show that at 41 degrees F., the infected guinea pigs duration of peak transmission was about 40 hours longer than that of guinea pigs kept at 70 degrees F. There was no transmission of the flu at 86 degrees F. and above.

Humid & healthy

Dr. Palese’s results seem counterintuitive. After all, organisms thrive in warmth – so why would a nasty winter virus flourish in Montreal, but not Miami?

Because it’s not the cold, it’s the humidity.

In a recent issue of the journal PLoS Pathogens, Dr. Palese explains that when humidity is high, respiratory droplets exhaled from an infected person tend to take on water, increase in size, and quickly settle out of the air. But when the air is cold and relatively free of moisture, those exhaled droplets remain light and airborne. In addition, indoor heating usually makes the air very dry, so exhaled droplets tend to fly instead of fall.

A New York Times report on this study notes that Dr. Palese doesn’t suggest moving into a greenhouse for the winter: “The best strategy, he says, is a flu shot.”

With all due respect to Dr. Palese, I’d much rather take my chances in a greenhouse. Like Cancun, for instance. Unfortunately, most of us aren’t able to winter on Mexico’s Caribbean coast, and many of us certainly don’t want to have anything to do with a flu shot, so we’ll have to fight off flu the old fashioned way: by strengthening our immune systems.

In the e-Alert “Fantastic Four” (10/3/05), HSI Panelist Jon Barron offers details about four excellent pathogen destroyers that have been shown to be an immune system’s best friends when an influenza bug tries to invade.

“Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature” PloS Pathogens, Vol. 3, No. 10, 10/19/07, pathogens.plosjournals.org
“Study Shows Why the Flu Likes Winter” Gina Kolata, The New York Times, 12/5/07, nytimes.com
“Effect of Refractive Error Correction on Health-Related Quality of Life and Depression in Older Nursing Home Residents” Archives of Ophthalmology, Vol. 125, No. 11, November 2007, archopht.ama-assn.org