Rethinking building ventilation and outdoor airflow rate–Can we engineer clean indoor air?

Clean air is a human right, but providing clean air in the built environment amid climate change presents challenges. The conventional wisdom to develop efficient, low-carbon mechanical ventilation methods primarily relying on dilution remains difficult. Meanwhile, the pandemic raises promises and questions about how we view and apply indoor purification from portable air cleaners. This paper attempts to rethink the connotation of “clean indoor air” by categorizing its sources into four groups, i.e., filtered and unfiltered outdoor air, and purified and unpurified indoor air. A systematic review and analysis were conducted based on literature, market surveys, and standards. Three indices, i.e., Rout, Rin, and Re, were proposed to evaluate the cleanness considering outdoor pollution, indoor pollution, and induced risks, respectively. Air change rates, cleaning efficiency, and infiltration or transfer air were identified as key factors, and a sensitivity analysis was performed to assess their impacts. The feasibility of combining different “clean air” types is explored, and a qualitative analysis was proposed to outline a composite clean air roadmap. Clean air needs are first identified and categorized into three layers using the “3W” framework: target pollutants (What), occupants (Who), and scenarios (Where). Clean air strategies are recommended using a four-dimensional (4D) evaluation method, which assesses cleaning efficiency, energy consumption, flexibility, and manageability. This study provides insights into optimizing clean air systems by focusing on purified indoor air, aiming to enhance system efficiency, resilience, and energy savings, thereby addressing the increasing and diverse clean air demands in human-centered environments.