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

Devabhaktuni Srikrishna, Devabhaktuni Srikrishna

Abstract

Public health departments such as CDC and California Department of Public Health (CA-DPH) advise HEPA-purifiers to limit transmission of SARS-CoV-2 indoor spaces. CA-DPH recommends air exchanges per hour (ACH) of 4-6 air for rooms with marginal ventilation and 6-12 in classrooms often necessitating multiple HEPA-purifiers per room, unaffordable in under-resourced community settings. Pressure to seek cheap, rapid air filtration resulted in proliferation of lower-cost, Do-It-Yourself (DIY) air purifiers whose performance is not well characterized compared to HEPA-purifiers. Primary metrics are clean air delivery rate (CADR), noise generated (dBA), and affordability ($$). CADR measurement often requires hard-to-replicate laboratory experiments with generated aerosols. We use simplified, low-cost measurement tools of ambient aerosols enabling scalable evaluation of aerosol filtration efficiencies (0.3 to 10 μm), estimated CADR, and noise generation to compare 3 HEPA-purifiers and 9 DIY purifier designs. DIY purifiers consist of one or two box fans coupled to single MERV 13-16 filters (1″-5″ thick) or quad filters in a cube. Accounting for reduced filtration efficiency of MERV 13-16 filters (versus HEPA) at the most penetrating particle size of 0.3 μm, estimated CADR of DIY purifiers using 2″ (67%), 4″ (66%), and 5″ (85%) filters at lowest fan speed was 293 cfm ($35), 322 cfm ($58), and 405 cfm ($120) comparable to best-in-class, low-noise generating HEPA-purifier running at maximum speed with at 282 cfm ($549). Quad filter designs, popularly known Corsi-Rosenthal boxes, achieved gains in estimated CADR below 80% over single filter designs, less than the 100% gain by adding a second DIY purifier. Replacing one of the four filters with a second fan resulted in gains of 125%-150% in estimated CADR. Tested DIY alternatives using lower-efficiency, single filters compare favorably to tested HEPA-purifiers in estimated CADR, noise generated at five to ten times lower cost, enabling cheap, rapid aerosol removal indoors.

Keywords: ACH; Aerosol; Air filtration; Airborne bacteria; Airborne virus; Box fan-filter; DIY air cleaner; HEPA purifier; Pandemic; SARS-CoV-2.

Conflict of interest statement

Declaration of competing interest I have no relevant interests to declare. I am not associated with any of the manufacturers mentioned in this research.

Copyright © 2022 The Author. Published by Elsevier B.V. All rights reserved.

Figures

Graphical abstract
Graphical abstract
Fig. 1
Fig. 1
Comparison of CADR (estimated by airflow × filtration efficiency) and cost of HEPA purifiers and DIY box fan-filters e.g., 3 × 2 means three filters were used with two box fans.
Fig. 2
Fig. 2
Do-it-yourself box fan and filters (single, quad filter designs with one fan and triple filter design with two fans). Blue arrows indicate direction of airflow.
Fig. 3
Fig. 3
Estimated CADR (cfm) and filtration efficiency (%) for DIY Air Purifiers (filters × fans) @ speed #1 e.g., 3 × 2 means three filters were used with 2 box fans.

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Source: PubMed

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