Using Dilution Factor to Characterize Aerosol Risk Management in Emergency Scenarios

When an emergency scenario arises, such as COVID-19, engineers must have tools and metrics to be able to quickly characterize system performance and risk management under a variety of scenarios, often novel and creative. HVAC systems may be put into service forfunctions that were not the original design intent. This paper proposes the metric of exhaled breath dilution factor (EBDF) to characterize such risks and mitigations. EBDF is simply defined as the volumetric flow of dilution air to the volumetric flow of the human breath. EBDF can be considered in any space, or in a system. The paper will present a few examples of system configurations, system options, and how the EBDF metric can be useful for engineers to communicate system capabilities. To simplify communications, it is recommended that engineers should memorize, in whatever units they work with most, the followingfact: The normal human breathing rate is P= 0.36 m3/ h, 0.2 CFM, 6 L/min. With that figure committed to memory, room ventilation can be characterized in terms of Exhaled Breath Dilution Factor (EBDF) where EBDF is the ratio of the room air to the exhaled breath. This paper includes both a table of risk reductions based on EBDF, and the method to calculate those.