As students go back to school this fall and workers return to offices after summer vacations, we will need healthy indoor air to keep us all safe. In May, the US Centers for Disease Control (CDC) reiterated its advice to ventilate indoor air to reduce the spread of Covid-19, this time recommending a firm target of five exchanges of air per hour. This advice seems to be so clearly common sense that it might be wondered if anyone could possibly dare to question it. But constantly bringing in fresh outdoor air in winter will make it very hard to keep indoor air humid, with potentially disastrous consequences.
As everyone knows, sicknesses like flu are seasonal in countries with temperate climates such as the United States: Illness peaks in winter and nearly disappears in summer. Over the years, the explanations for why this is so have included theories about the virus-killing powers of sunlight, immune-boosting doses of vitamin D in summer and, during the school year, the many exposures to infections in the classroom. But one of the factors that is looking to be increasingly important is low indoor relative humidity.
In winter, we heat our homes. As air warms, it still contains the same amount of water as before, but, because warmer air can hold more water, it feels drier and the relative humidity is lower. If the outside air is zero degrees Celsius, for example, with a relative humidity of around 60 percent (normal for winters in the northern US), heating that air to 20 degrees C would send the relative humidity down to 20 percent. That will feel uncomfortable — most people prefer a relative humidity of around 30 percent to 60 percent. But more importantly, there’s increasing evidence that humidity at or above 40 percent is healthier, both because it helps the body to fight off infection and because it limits viral transmission.
Studies have shown that increasing the relative humidity is good for our nasal passages, for example, helping them to build up mucus to defend us against agents of disease. Low humidity, on the other hand, has been found to encourage the evaporation of large, virus-containing droplets that are spewed out when we speak. This evaporation leads to smaller and more insidious droplets — smaller droplets are lighter and can stay aloft for longer, travel further through the air, and penetrate deeper into the lungs. Viruses have also been found to decay faster at higher humidity. As the evidence for these effects has mounted, some have called for us to harness their power against illness: In 2020, for example, researchers wrote a piece in the Washington Post encouraging people to keep indoor relative humidity above 40 percent in the fight against Covid-19.
At the start of the Covid-19 pandemic, disease outbreaks didn’t seem to be strongly linked to the seasons, in part because everyone’s lack of natural immunity swamped environmental effects on disease transmission. But a seasonal signal has emerged. The tie to humidity has become clear, too. In 2022, researchers crunched data from 121 countries and found that indoor air humidity of less than 40 percent was very strongly linked to the spread and severity of Covid-19 outbreaks. It’s hard to rank the power of humidity against other mitigation measures, such as increased ventilation or masks, but it’s clearly important.
Yet the CDC makes no recommendations about indoor air humidity when it comes to viruses. Why ignore the importance of humidity? I think part of the reason is that no one really understands the chemistry of how and why relative humidity plays such an important role for virus survival. Experiments in my lab are helping to spell this out.
Work from my team and others shows that the surface of tiny water droplets (including the aerosols spewed out of our mouths) naturally contain plenty of reactive oxygen species, such as the familiar disinfectant hydrogen peroxide, and an even more potent class of chemicals called hydrated hydroxyl radicals. We have found that the concentration of these reactive oxygen species in water droplets increases as the relative humidity of the air surrounding the droplet increases. Exactly why this happens we don’t yet know. We have also found that these reactive oxygen species kill bacteria and viruses, provided the bacteria and viruses remain in contact with the surface of the water droplet for a sufficiently long time.
If the humidity is below 40 percent, as is often the case for indoor air in the wintertime, the droplets quickly evaporate to nothing, and the disinfecting power is lost. Viruses are perfectly happy in this desiccated state. With mid-range humidity, evaporation is slowed down and there is sufficient time for the disinfectants to kill the viruses inside the droplets.
The evidence seems compelling that keeping the indoor relative humidity sufficiently high, but not so high as to cause mold and mildew, should be an important part of any strategy to control and mitigate the spread of viral airborne infections. This is particularly true in the wintertime when we dry out the air by heating, but it also applies to summertime if we overuse an air conditioner. The sweet spot is thought to be 40 percent to 60 percent relative humidity.
Of course, we can increase indoor relative humidity using humidifiers. But here’s the problem: It’s going to be extremely hard using existing systems to keep air moist while also ventilating it as frequently as the CDC recommends. Constantly bringing in fresh air will mean constantly needing to add moisture, and humidifiers will simply struggle to keep up. One solution might be to filter the air and recirculate it, keeping its moisture, rather than bringing in fresh outdoor air so often.
I urge paying more attention to making the relative humidity of indoor air more like that found summertime in the outdoors. That includes on airplanes, where extremely low humidity, typically just 5 percent to 10 percent, contributes to the spread of disease. We spend roughly 80 percent to 90 percent of our time indoors: We must strive to make that air as natural and healthy as possible.