Practical Implications
There is substantial evidence that contaminated air can result in disease spread, and that the combination of air filtration and recirculation can reduce this risk. Observational and animal studies suggest that air recirculation alone may result in the airborne transmission of pathogens.
Waiting for a contaminant to decay naturally in the building is a slow process. Hence, engineering alternatives like mixing indoor air with the outside air and recirculating it back to the room and pressurization control are utilized. There are two types of general air systems: (1) single pass and (2) recirculating. The single pass air system supplies air from the outdoors, and after passing through the room air it is then exhausted 100% back to the outdoor environment. This system does not, however, take advantage of the embodied energy required for heating/cooling of discharged air. In the recirculating system, however, a limited portion of the air is exhausted to the outdoors and replaced by fresh air. Recirculating systems are typically designed for significantly larger flow rates compared to single pass systems as they utilize the power of contaminant dilution. Recirculation could be used as long as air is filtered through high efficiency filters or is cleaned by disinfection techniques. Recirculation of air can be problematic under two conditions: (1) when it leads to the delivery of insufficient fresh air, and consequently excessive CO2exposure, or (2) when the return air is not properly filtered.
The combination of HEPA filtration and air recirculation has been shown to be extremely effective in many space functions such as ORs, AIIRs, and PEs. These systems clean air by simultaneously removing (i.e., filtering) and diluting (through recirculation) contaminants from the space. They have shown significant reduction in the number of bacterial colonies, and surrogate particulate matters previous studies, and they are expected to show a similar efficiency for the COVID-19 pandemic.
The coronavirus outbreak has severely hit the healthcare settings. Notably, in hospitals in the epicenter of the pandemic during the spring of 2020 (New York, Connecticut, and Boston), the surge capacity exceeded standard room configurations and SARS-CoV-2 positive patients were placed in general patient wards and converted ICUs without negative pressure capacity. These spaces likely frequently lack sufficient filtration requirements, ventilation rates, and negative pressurization capacity. Therefore, one short-term solution seems to be substituting the existing filter in those rooms with higher efficiency filters (i.e., HEPA filters). Though this seems like a logical decision at the first glance, it may result in unfavorable outcomes such as increasing the pressure drop when replacing a lower rate filter with HEPA filters. When the pressure drop increases and the fan speed remains constant, the amount of air supplied to the space will decrease. This can result in an imbalanced system that can increase the distribution to spread disease.
