By now, you’ve probably got your coronavirus risk-minimizing routine down pat. Mask? Check. Social distance? Acquired. Spending time indoors with people outside your pod? Hard pass. You wash your hands. (But maybe not your groceries.) And that’s great. Keep it up. Because with the virus now surging to record levels in nearly every state, and hospitals starting to buckle under the strain, doing all these things is more important than ever. But as public health experts warn of a long and deadly winter ahead, there’s one more thing some scientists say we should be talking about: humidity.

With winter comes plummeting temperatures, and the colder the air gets, the less water vapor it can hold. The way most buildings are heated only exacerbates the problem. Heating, ventilation, and air-conditioning (HVAC) systems suck in outside air and then heat it up, which zaps even more moisture out of it. These changes not only make it easier for respiratory viruses to hop from host to host, but dry air cripples the first few lines of defense your body has for preventing such viruses from establishing an infection. All of this could be a recipe for the coronavirus to wreak even more havoc in the coming months.

“A lot of indoor environments are bone-dry in the wintertime,” says Jeffrey Shaman, an infectious disease forecaster at Columbia’s Mailman School of Public Health. “That makes the virus more transmissible. And people are spending more time indoors. So a lot of factors are going to be working against us.”

A decade ago, a team of researchers led by Shaman looked at 31 years’ worth of data on influenza-like illness and weather patterns in the US. Over and over, they found that the biggest outbreaks happened in the winter when the weather was unusually dry. Lab studies with ferrets and guinea pigs showed similar patterns. The influenza virus spread most readily when the relative humidity inside the animals’ cages fell below 40 percent. (A typical indoor humidity range in warmer weather is between 40 and 60 percent.)

Aerosol scientists who study the flu, like Linsey Marr at Virginia Tech, have helped explain why that might be. In a 2012 study, her group showed that as relative humidity levels dip, the particles that people emit through talking or coughing get smaller and smaller. These particles are made up of mucus, salts, protein, and cell parts—but mostly water. The drier the air around them, the faster that water evaporates. And the smaller the particles get, the longer they can stay in the air, the farther they can travel, and the deeper into the lungs they can be inhaled. Any viruses lurking inside these particles go along for the ride.

If they land inside a susceptible person’s respiratory tract, that can spell trouble. Of course, the body is equipped with multiple layers of security to protect against would-be invaders. The first line of defense is a physical barrier maintained by the cells that line the nasal passages. Some of these cells secrete mucus—two layers of the slippery, stringy, substance, with two different viscosities. Other cells inside the nose and throat have tiny, anemone-like projections called cilia, which beat in synchrony in the more watery layer. That motion moves the thicker, top layer of mucus like a conveyor belt away from the lungs. This mucosal current catches any viruses or bacteria (or other irritants like pollen and ash) that land on it and sweeps them away to be swallowed or coughed out. But if the air is too dry, these mucus layers dessicate, squishing the cilia and immobilizing them.