Can 10x cheaper, lower-efficiency particulate air filters and box fans complement High-Efficiency Particulate Air (HEPA) purifiers to help control the COVID-19 pandemic?

Abstract

BackgroundSARS-Cov-2 aerosols have potential to accumulate in airspaces of poorly ventilated, indoor spaces such as classrooms, offices, homes, restaurants, and bars. Separately, toxic aerosol pollution (e.g. wildfires, wood burning) is frequently encountered in these locations with ventilation relying on outside air. Portable high efficiency particulate air (HEPA) air purifiers are useful to remove both types of aerosols without relying on outside ventilation. To meet or exceed recommended 4 to 6 air exchanges per hour (ACH), high price-points for select HEPA air purifier models without incurring excessive noise generation make them unaffordable for many households and communities. Do-it-yourself (DIY) alternatives with box-fans and HVAC filters can be much lower cost but their clean air delivery rate (CADR) and noise generation varies greatly depending on choices of filters, number of filters, and fan speeds.ObjectiveTo compare cost-effectiveness and noise-generation of aerosol filtration by commercially available, portable HEPA air purifiers and do-it-yourself (DIY) alternatives built from box fans and HVAC filters rated at MERV 13 or 14 using single and quad filter designs.MethodsThe comparison is based on three metrics: clean air delivery rate (CADR), the noise generated (dBA), and affordability ($$) using an ISO-certified aerosol measurement device to measure input/output particle filtration of particles sizes ranging from 0.3 microns to 10 microns, airspeed meter to measure airflow, and NIOSH sound meter app to measure noise.ResultsAccounting for reduced filtration efficiency of MERV 13/14 filters compared to HEPA, estimated clean air delivery rate (CADR) of a do-it-yourself (DIY) setup using 2” and 4” filters with a box fan running at fan speed 1 for tolerable noise was 278 cfm ($38) to 371 cfm ($58), comparable or better than a commercial HEPA air purifiers running at maximum speed with low noise generation at 282 cfm ($375). However the quad filter designs, popularly known as the Corsi-Rosenthal box, achieved gains in CADR of only 60% over a single filter design but in contrast to 200-250% gains reported by UC-Davis.ConclusionsDIY alternatives using single 1”, 2” and 4” MERV 13/14 filters compare favorably to commercially available systems in terms of estimated CADR and noise but at five to ten times lower cost. Compared to use of one filter, an improvement in CADR (air flow) was observed with a popular DIY configuration involving quad filter design, popularly known as the Corsi-Rosenthal box, not as high as reported by a recent study by UC-Davis