Preventing False Safety by Introducing the Hygienic Air Delivery Rate (HADR) for Mobile Air Purifiers

Abstract

Indoor environments can become contaminated with pathogen-laden respiratory droplets exhaled by people, posing a risk of infection to others. To address this issue, indoor spaces are typically cleaned by ventilating—bringing in fresh outside air and expelling contaminated air. Mobile air purifiers can also help mitigate this risk and are widely used, particularly during emergent situations such as pandemics. However, a thorough assessment of how to measure the effective cleaning performance of air purifiers is still needed. Currently, the clean air delivery rate (CADR) is used as a standard metric to evaluate air purifiers. In this protocol, however, we identify steps that may erroneously influence the measured cleaning rate, potentially leading to falsely safe estimates. To address this, we propose a modified protocol to estimate the hygienic air delivery rate (HADR), which more accurately reflects the effective cleaning performance of air purifiers. We also suggest several considerations to enhance the HADR effectiveness of these devices. Additionally, we present case studies of real devices demonstrating that the HADR can be significantly lower than the volume flow rate of the air purifiers. We emphasize that the HADR—rather than the volume flow rate—should be used by both manufacturers and consumers to evaluate and compare devices, in order to prevent a false sense of safety.