After reading my September column about using the heart as a guide for energy efficient HVAC design, a forward-thinking ES reader asked about using the respiratory system to think about air handling strategies. We could clearly use some help with this topic as the debate rages on about the key metric of indoor air management. Do we prioritize low-energy consumption with a bow to “human comfort” or do we prioritize human health (not merely comfort) and hope that we do not raise building fuel consumption in the process? Let’s ask Mother Nature about how she manages gas exchange in the most energy-efficient way.
In mammals (this includes humans), life–supporting gas exchange occurs in the innermost part of our lungs, in tiny air sacs called the alveoli. The ultimate job of each alveolus is to allow inhaled oxygen to get into our blood stream, and simultaneously remove excess carbon dioxide, the waste product of our metabolism. This gas exchange occurs through passive diffusion, so the air and blood sides of the alveoli can only be separated by one thin membrane.
Think about the challenges our lungs face. Only a delicate cell membrane separates the perilous air-world filled with particles, microbes, and gases that would ultimately kill us if they reached our blood. Only a few bacteria crossing into our circulation can lead to sepsis, a potentially fatal infection. Clearly, our alveolar membranes need additional protection.
How does our upper respiratory system protect our lungs, and can any of the strategies inform HVAC design to support occupant health while still conserving energy? The answer is a resounding “Yes!” Our bodies take in unrefined air that is then carefully processed for gas exchange in our lungs. First, the mucus in our nose and throat must be sticky, yet not too thick (gross). Tiny hairs known as cilia line the cells from our throat to lungs and continually sweep upwards, carrying particulate debris away from the delicate alveoli. For this cleaning to occur, inhaled air velocity is first reduced while air hydration is increased.
You may be asking yourself, “What does this have to do with HVAC systems and hospital IAQ?”
Hospitals are currently the second most energy-intensive building type in the United States, where heating, cooling, and ventilation account for 52% of their energy use. High room air change (RAC) rates in patient care spaces contribute significantly to this high energy use, yet studies comparing volume exchange ventilation with patient infection rates have not documented any correlation.
In fact, there is evidence associating both dry and turbulent air with transmission of infectious diseases. This makes sense because infectious aerosols originating from breathing, speaking, coughing, toilet flushing, vomiting, and diarrhea shrink to become tiny droplet nuclei in air below 40% rh. When the upward velocity of room air with circulating droplets of aerosol exceeds the settling velocity of the aerosols, they remain suspended in the air for prolonged periods. The relationship between the settling and resuspension rate of particles depending on their size is mathematically defined by Stokes law.
The design lesson is: by maintaining indoor air hydration between 40-60% rh, and reducing RAC rates, occupant health and patient outcomes are improved while energy efficiency is increased. Thanks to the elegant modeling by mammalian lungs, the previously diverse goals of physicians and HVAC engineers are now united, and the hospital bottom line will improve.