The reality is this: As the chilly autumn air shifts toward colder temperatures this winter, the possibility of outdoor dining and socializing under heat lamps and outdoor dining igloos will likely become a memory at best. And with this comes the increased risk for not only contracting, but transmitting Covid-19.
But it’s the quality of the air—specifically, colder and drier air with lower humidity that makes the virus much more virulent, by virtue of increasing its spread. Moreover, any physical distancing that was effective outdoors won’t be as useful inside.
And this is just one of the main concerns that will magnify the pandemic as we begin to move indoors, as recent data indicates a continued rise in cases, hospitalizations, and deaths throughout the U.S. in the past month, based on data from the Covid Tracking Project.
Dr. Fauci had recommended in early August that as we approached the fall and winter, the U.S would need to keep daily cases below 10,000 nationwide order to gain control of the pandemic, and prevent another catastrophic surge in the number of deaths nationwide. But with the U.S. hitting an all-time high in daily new Covid-19 cases on Friday last week—at more than 80,000 cases—this grim milestone clearly portends a more deadlier winter ahead.
In a nutshell, SARS-CoV-2 will be more able to survive and spread this winter due to lower temperatures, combined with reduced humidity and drier conditions inside your breathing passages (respiratory tract). As the weather gets colder, the air gets drier and retains less humidity. When we turn on the heat in our homes, it further dries the air and the cells and tissues inside our nose, airway passages, reducing the ability of one of our natural defenses—mucus—from removing viruses such as SARS-CoV-2 as well as other debris and viruses, and bacteria.
Research on influenza and MERS indicates that there is a higher chance of developing infections when the relative humidity declines from about 40-60% (typical in warmer weather) to about 20%. Case reports from China and Seattle during the early days of the pandemic also noted similar findings. A cohort climate modeling study published in JAMA looking at 50 international cities also confirmed that SARS-CoV-2 is more stable and remains on certain receptors in our airways as the relative humidity drops and stays at 20%. This also explains why we are more apt to contract cold viruses and influenza in colder temperatures.
So what can we do indoors to reduce the risk of infection indoors? Simply put, wearing a mask, limiting the number of people (and exposure time) along with optimizing ventilation and filtration indoors are key aspects to this approach. When we are outside, any viral particles released are likely to be carried away by wind currents. As a result, the caution for people to stay 6 feet apart may not be enough in schools, offices, restaurants and other indoor settings.
But in reality, 6 feet of separation is not adequate. While large droplets may fall to the floor within 6 feet, smaller droplets and aerosols, in particular, are carried much further, up to 20 feet in some cases. Aerosol transmission becomes much more relevant as we move indoors this winter. In fact, superspreading events, which ultimately occur between persons with more than 6 feet of separation are the result of airborne transmission, based on recent research by Martin Bazant and John Bush at MIT.
Airborne transmission via aerosols depends on many factors including the ventilatory and filtration parameters within indoor spaces, dimensions of the space itself, number of persons, cumulative exposure time, respiratory rate, face mask use, infectiousness of the respiratory aerosols.
What’s most concerning is that the CDC remains murky about the role of airborne transmission, unconvinced of the importance of its contribution to transmission of SARS-CoV-2. The CDC continues to be focused on what they view as a more significant role for large droplet transmission related to coughs and sneezes where such droplets typically fall to the floor within 6 feet.
Bazant and Bush have also developed a guideline to assess the risk of airborne transmission that factors in the size of the indoor space, exposure time, human activities, air filtration, and ventilation. They created a spreadsheet and online app that can be used by public health agencies as well as the public at large.
Dr. Stephanie Taylor, a graduate of Harvard Medical School has organized an online petition calling on the World Health Organization (WHO) to examine evidence regarding humidity and respiratory health, urging that the minimum lower limit of indoor relative humidity (RH) in public buildings should be 40-60%, an important way to reduce the transmission of respiratory infections. At 0-40% relative humidity, airborne droplets containing viruses shrink by evaporation, become lighter and stay afloat for longer periods of time, increasing the risk of infection and transmission. But at 40-60% relative humidity, these airborne droplets containing viruses are much heavier and fall out of the air to the ground, reducing the potential for transmission.
Prominent researchers including Dr. Akiko Iwasaki, an immunologist at the Yale School School of Medicine have signed the petition. “Studies have shown that there is a sweet spot in relative humidity. Air of between 40% and 60% shows substantially less ability to transmit viruses and allows our nose and throat to maintain robust immune responses against them,” said Iwasaki on the petition’s website. It is also for this reason that she recommends the use of humidifiers in people’s homes during the winter.
But even with such a change in humidity in public buildings, it’s essential to realize that face masks are still more effective than ventilation or air filtration systems due to their ability to stop aerosols and large droplets as they emerge from a persons’s mouth. This allows more people to safely spend time within indoor settings, when compared to a room with no masks worn with adequate ventilation and air filtration systems.
In fact, universal masking in public could significantly reduce the number of people who die of Covid-19 in the U.S by February, 2021—by nearly 130,000—based on a modeling study by the Institute for Health Metrics And Evaluation (IHME) at the University of Washington, published late last week in the journal, Nature Medicine projects.
This is a big yield for such a small ask. We should heed the findings of this study. It provides evidence for the argument that a federal mandate on universal masking could saves tens of thousands of lives.