Summary: Alongside physical distancing and improved personal hygiene, moderating indoor relative humidity with the change of seasons may help reduce COVID-19 transmission. However, the virus can still spread through direct contact and close proximity regardless of season, researchers caution.
Source: Yale
How much spring and summer will influence the COVID-19 pandemic depends not only on social distancing and behavioral measures but also on the indoor environment, according to a review by Yale scientists of their own research and that of colleagues on respiratory virus transmission.
Yale research shows that cold, dry winter air clearly promotes spread of SARS-CoV-2, the virus that causes COVID-19. As outdoor humidity and temperatures rise in spring and summer, airborne transmission risk falls both outdoors and in indoor settings such as offices and homes.
Although rising humidity can reduce airborne spread, SARS-CoV-2 can still be transmitted by direct contact or contact with contaminated surfaces. The authors suggest that, in addition to handwashing and social distancing, managing indoor relative humidity — the balance between outdoor conditions and indoor air — could be a practical measure to help slow viral spread indoors.
The review was published online the week of March 23 in the Annual Review of Virology.
“People in developed countries spend the vast majority of their lives indoors and often in close proximity to others,” said Yale immunobiologist and senior author Akiko Iwasaki. “The relationship between outdoor and indoor temperature and humidity and airborne transmission of the virus has not received enough attention.”
Iwasaki is the Waldemar Von Zedtwitz Professor of Immunobiology and a professor of molecular, cellular, and developmental biology at Yale, and an investigator for the Howard Hughes Medical Institute.
The seasonal pattern of respiratory illnesses has been observed since ancient times, with infections rising in winter and declining during spring and summer. Modern studies have identified cold, dry air as one of the factors promoting spread of respiratory viruses, including SARS-CoV-2. Work from Iwasaki’s lab and others has helped explain the mechanisms behind that seasonal effect.
Cold, dry winter air creates a threefold challenge for preventing infection. When cold outdoor air with low moisture is heated indoors, relative humidity commonly falls to about 20%, creating conditions that allow airborne viral particles to remain suspended and travel more easily. Low humidity also impairs the function of cilia — the hair-like structures that help clear pathogens from the airways — making it harder to expel viral particles. Finally, drier air can suppress certain immune responses in the respiratory tract, reducing the body’s ability to fight inhaled viruses.
The review emphasizes relative humidity as an important environmental variable. In winter, indoor relative humidity tends to be low because cold, dry outside air is reheated and circulated without added moisture.
Experimental studies cited in the review show that rodents infected with respiratory viruses transmit airborne particles to uninfected animals more readily in low-humidity environments. Based on these findings, Iwasaki recommends using humidifiers in buildings during dry winter months to raise indoor relative humidity.
At the other extreme, very high relative humidity — as often found in tropical environments — can cause airborne droplets to settle quickly onto surfaces. In poorly ventilated or crowded indoor spaces, those surfaces can sustain infectious virus for extended periods, reducing the protective benefits of higher humidity.
The review identifies an optimal range for indoor relative humidity. In animal studies, mice housed at 40–60% relative humidity showed substantially reduced airborne transmission compared with mice in very low or very high humidity conditions. Mice maintained near 50% relative humidity were better able to clear inhaled virus and mount robust immune responses.
Iwasaki stresses that these effects apply primarily to aerosol transmission dynamics. Close contact and contact with contaminated surfaces remain significant transmission routes year-round. That is why people in warmer climates and those who work in close proximity must continue standard precautions such as hand hygiene and physical distancing.
“No matter where you live — in the tropics, a temperate city, or a cold region — it’s still essential to wash your hands and practice social distancing,” Iwasaki said.
Yale’s Miyu Moriyama and Walter J. Hugentobler of the University of Zurich are co-authors of the paper.
Source:
Yale
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Original Research: Closed access
“Seasonality of Respiratory Viral Infections.” Miyu Moriyama, Walter J. Hugentobler, and Akiko Iwasaki. Annual Review of Virology. DOI: 10.1146/annurev-virology-012420-022445.
Abstract
Seasonality of Respiratory Viral Infections
The seasonal cycle of respiratory viral diseases is well documented: annual epidemics of common cold and influenza occur routinely in winter in temperate regions, and outbreaks caused by viruses such as SARS-CoV and the newly emerging SARS-CoV-2 also have winter peaks. Researchers have examined the environmental and behavioral drivers of that seasonality for many years. Two major contributors are changes in environmental parameters and human behavior. Studies demonstrate how temperature and humidity affect virus stability and transmission rates, and more recent work highlights how environmental factors—particularly temperature and humidity—modulate host intrinsic, innate, and adaptive immune responses in the respiratory tract. This review summarizes evidence linking outdoor and indoor climates to seasonality of respiratory viral infections and discusses how environmental determinants influence host responses.