As we continue to work on curbing the COVID-19 illness, it is clarifying to understand the three steps in disease transmission where we can exert control. 

The first step, attempting to limit the amount of infectious virus introduced into our surroundings by a sick person, is done by wearing masks and social distancing. The second step, decreasing transmission in our buildings, is generally where HVAC engineers focus their efforts. The third step of increasing our own physical immune defenses and overall health is left up to our health care providers and to us individually. 

This column will focus on the third step, increasing the ability of our immune system to protect us from the COVID-19 disease if we are exposed to the SARS-CoV-2 virus. If you are an engineer or building manager who deals with HVAC systems, please do not stop reading because, surprisingly, this step involves you. Perhaps even more surprising is that bats are teaching us about the relationship between indoor air, immune responses to SARS-CoV-2, and the COVID-19 disease. 

Why can bats, small mammals with many similarities to humans, live with coronavirus infections without appearing sick? In fact, bats easily live with many different viruses that make humans extremely sick. Because of the similarities in physiology, yet differences in viral illness outcomes, bats are key to understanding the functioning of a protective, yet not overly active and damaging, immune system. 

An obvious first place to investigate is the different genetic makeup of bats and humans. Metagenomic comparisons of the chromosomes of both species, however, did not reveal any answers. Scientists finally found several powerful environmental, non-genetic factors that are key in modulating bat and human immune responses. The biggest difference discovered is in the “innate immune system,” the first line of defense against invading pathogens. In particular, bats exposed to SARS-CoV-2 immediately produce larger amounts of interferons, proteins that play a key role in the timing and extent of the overall immune response that prevents the viral disease.

Conversely, humans, especially those who develop severe COVID-19 disease, have both delayed and diminished interferon production. A delay in interferon production is often followed by an often-fatal hyper-inflammatory response known as a “cytokine storm.”

The next question is: What modulates interferon production, and could this be the difference between bats and humans? One known driver of human interferon production is seasonality with a decrease during winter months and an associated increase in viral infections and auto-immune diseases. Studies done in 2019 by Dr. Akiko Iwasiki with Yale University’s immunobiology department, showed that interferon production by airway cells of genetically engineered mice exposed to influenza was impaired when the ambient relative humidity levels were 20%. Conversely, mice in relative humidities of 50% had healthy interferon levels and significantly decreased influenza disease. 

Once again, we are reminded of the power of IAQ to promote health or cause harm. The relationships between ambient humidity, interferon levels, and COVID-19 disease reinforce the need for a holistic understanding of the role of the indoor environment in our health and in personalized medicine.