Joseph G. Allen, Akiko Iwasaki and Linsey C. Marr write:

We have a great set of tools that can help slow the spread of this virus: masks, social distancing and hand-washing, as well as healthy building strategies such as ventilation and filtration.

And there is one more healthy building tool that we can use this winter: maintaining relative humidity in the 40-to-60-percent range.

Relative humidity is the term for how much water vapor is actually in the air compared to how much it can hold. Think of it like a sponge: At 100 percent, the sponge is totally soaked; at 50 percent, it holds half as much water. Warmer air can hold more water vapor; it’s like a bigger sponge. As fall turns to winter and we start heating the air, our indoor environments become more dry, often hitting 20 percent relative humidity, well below the ideal 40 to 60 percent.

Humidity can affect transmission in three ways. First, it influences our body’s ability to fight off infection. The first line of defense in the lungs relies on the combined effect of mucous in the respiratory tract and the upward beating of the hairlike cilia on lung cells. The sticky mucous acts to catch particles, including viruses, as they descend into the lungs. Once trapped, the cilia push the mucous and particles up to the throat, where they are swallowed harmlessly. It’s a “mucociliary escalator,” and it works great — most of the time.

One problem: In dry air, there is less mucous, and the cilia don’t beat as fast or in the right direction, as new research shows. This means fewer virus particles are captured or cleared out of the respiratory tract, thus allowing more of them to reach the deepest part of our lungs, where they do the most damage. [Continue reading…]