If my mom were still around, she’d say “AHA!!” My mom, like millions of others, firmly believed cold weather caused colds. I would argue, after having chatted with doctors numerous times and hearing what I wanted to hear: “No, Mom, you have to be exposed to a cold virus to catch a cold.” Turns out we both were right, at least partially.
You can’t catch a cold without being exposed to a virus. That’s true enough. But a new study from Yale demonstrates cold viruses really dig chilling out. The Yale study, just published in the Proceedings of the National Academy of Sciences, helps reinforce what has been believed by most virologists since 1960. Viruses reproduce much more readily in nasal airways that have been chilled than when those airways are near normal body temperature. But this interdisciplinary Yale study focused more on how humans respond to the chill of winter, rather than the known viral response to the chill.
Our immune systems fend with rhinoviruses at 37 degrees Celsius in nasal cavities (approximate body temperature) rather effectively compared to chilled air of 33 degrees Celsius in those passageways.
Yale Professor of Immunobiology Akiko Iwasaki and her team found the immune response is actually impaired at the lower temperature. In mice, the study suggested once this thermal barrier is lowered, the virus can continue to replicate more rapidly even after core temperatures go back up to normal body temperatures. Iwasaki believes it is actually prudent to take measures to keep warm and keep the nasal passages warm. In other words, there may be partial credence to what had been viewed by the likes of me as a wives’ tale to do what you can to keep yourself and your nose warm.
Besides this research, there is the matter of indoor humidity. There is strong evidence low indoor humidity enables the influenza and other viruses to spread much more readily because the viruses stay airborne longer in a dry environment. When viruses are traveling on water droplets in higher humidity environments, however, they tend to fall to the floor on those sinking droplets rather than stay airborne. (The same principle has been found to be true with bioterror agents including sarin gas in laboratory tests.)
The problem during cold weather is our furnaces run more often, and dry out the indoor atmosphere drastically. Without efficient humidifiers, indoor humidity can drop to 5 percent to 7 percent. Even with good humidifiers, Dr. Philip Tierno, NYU clinical professor of microbiology and pathology, told WebMD it’s very difficult to exceed 20 percent indoors during cold weather even with most humidification. The preferred level of 43 percent to 45 percent is often beyond the reach of most heating and humidification systems in northern latitudes in the winter. A CDC study showed up to 86 percent of airborne viruses were rendered powerless to spread at that 45 percent humidity level.
Of course, there is the added risk of so many of us being trapped indoors in the winter at these low humidity levels, sometimes in small spaces. Combine these factors and it’s not difficult to understand why cold and flu season tend to spike during and after cold outbreaks. Colds and flu occur in warm parts of the country, too, but the outbreaks are worse in colder climates. They may spread southward, once they get going.
In sum my mom and some of your moms were at least partially right. Maybe it was for the wrong reason, but that’s true of some of my accurate weather forecasts too, gang.