Note this is an updated version of an essay published in May, 2020, under the headline “One Key Factor in Whether COVID-19 Will Wane This Summer”

The first reference to the seasonality of infectious respiratory disease was recorded around 400 B.C., when the renowned ancient Greek physician Hippocrates wrote the earliest account of a winter epidemic of such an illness. Ever since, we have pondered the impact of seasonal change on respiratory disease prevalence. And rightly so, because even before COVID-19, respiratory diseases were having a profound impact on global health. In the United States alone, the Centers for Disease Control (CDC) reports that influenza has caused up to 61,000 deaths annually since 2010—and the World Health Organization (WHO) suggests that, globally, 650,000 deaths are associated with seasonal flu each year.

So far, scientists have identified at least nine distinct viruses that can cause respiratory tract infection and that demonstrate seasonality in their outbreak pattern in temperate regions. Of these, three viruses—influenza viruses, human coronaviruses and human respiratory syncytial virus (RSV)—clearly peak during winter months.

One obvious possibility is that seasonal changes in climate directly cause a spike in respiratory illness. However, the reality may be much more complex. In fact, the answer to seasonal occurrence of disease is more likely to be linked to our indoor environments rather than those outside.

Today, most of us are likely to spend up to 90 percent of our time indoors. This is a significant issue because our buildings have become more sophisticated over the last century or so with the introduction of central heating systems and the development of increasingly airtight, insulated building shells. The result is that we are more and more disconnected from daily and seasonal outdoor climatic fluctuations, especially in winter.

Research, including our own, is beginning to illustrate that there is a relationship between the aerial transmission of viruses and temperature and humidity, which is impacted by both indoor and outdoor environments.

It is obvious that in winter, indoor heating causes a difference between indoor and outdoor temperature. But what we are increasingly coming to understand is that by heating our buildings we are causing a reduction in the level of indoor relative humidity (RH), which has a significant impact on disease spread. For example, measurements of humidities in 40 residential apartments in New York and in six high-quality commercial buildings in the Midwest showed that indoor RH dropped to below 24 percent in the winter. The evidence suggests, in other words, that when cold outdoor air with little moisture to start with is brought indoors and warmed to a temperature range of 20 to 24 degrees Celsius (68 to 75 degrees Fahrenheit) indoor relative humidity plummets.

This comparatively moisture-free air provides a clear path for dispersal of airborne particles of viruses such as SARS-CoV2, the pathogen that causes COVID-19. The SARS-CoV-2 virus survives better at low temperatures and low humidity. Estimated virus half-life was more than 24 hours at 10 degrees C (50 degrees F) and 40 percent relative humidity, but only 90 minutes at 27 degrees C (80 degrees F) and 65 percent relative humidity. Our own research indicates that dry air also reduces the ability of our body’s cilia—hairlike projections on cells lining airways—to remove viral particles and prevent them from reaching the lungs. Finally, the immune system’s ability to respond to pathogens is suppressed in drier environments. Indeed, a study conducted in New South Wales, Australia, demonstrates an inverse relationship between relative humidity and transmission of SARS-CoV-2.

As the COVID-19 pandemic continues, this research could play a vital role in how we manage and counter the disease. Until we have enough vaccines to cover a large portion of human populations, we must keep practicing social distancing, mask wearing and avoiding crowding indoors. In addition to these measures, we can increase indoor humidity to combat the spread and prevent more severe disease from COVID-19.

This is why I and others specializing in immunobiology and infection control are urging the scientific community and others to support our petition, which calls on the WHO to urgently put the link between indoor air humidity and the transmission of viruses, including SARS-CoV-2, at the front of the global health debate. We are requesting that the WHO produce clear guidelines on the minimum lower limit of air humidity in buildings. We recommend maintaining relative humidity between 40 to 60 percent to maximize the benefits of humidity but not the drawbacks of too much humidity that promote mold growth.

We hope that through this move we will reduce the spread of SARS-CoV-2 and other airborne viruses and safeguard residents, students, patients and employees—which is crucial for protecting public buildings, such as nursing homes, hospitals, schools and offices. This is not just about getting America, and the world, back to work. It is also to offer protection for our health care workers. While of course there is a complex web of influences at play, we now know enough about indoor relative humidity’s impact on disease for it to be viewed as a significant factor. Indoor air control is the next frontier to improve human health and reduce transmission of various types of viruses, including SARS-COV-2.