key: cord-0792164-98wyj3r5 authors: Srikrishna, D. title: Price-performance comparison of HEPA air purifiers and lower-cost MERV 13/14 filters with box fans for filtering out SARS-Cov-2 and other particulate aerosols in indoor community settings date: 2021-12-05 journal: nan DOI: 10.1101/2021.12.04.21267300 sha: aa9267a870422ed63198d835ad67d09eb6f62514 doc_id: 792164 cord_uid: 98wyj3r5 Background: SARS-Cov-2 has the potential to accumulate in the airspace of poorly ventilated, indoor spaces such as classrooms, offices, homes, restaurants, and bars because it has been detected in aerosols from exhaled breath. Separately, toxic aerosol pollution such as from wildfires, wood burning, and other sources is frequently encountered in these same locations especially with ventilation using outside air. Portable high efficiency particulate air (HEPA) air purifiers are useful to remove both types of aerosols without relying on ventilation from outside the room. However in order to meet or exceed the recommended 4 to 6 air exchanges per hour (ACH), high price-points and filter replacement costs for select HEPA air purifier models without incurring excessive noise generation make them unaffordable for many households and communities. Objective: To compare the cost-effectiveness of aerosol filtration by commercially available, portable air cleaning devices with HEPA filters 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. Methods: The 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. Results: Accounting for reduced filtration efficiency of MERV 13/14 filters compared to HEPA, the 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 to 371 cfm, comparable or better than a commercial HEPA air purifier running at maximum speed with low noise generation at 282 cfm. The upfront cost of the components of the DIY setup were $35-$58 compared to the commercial HEPA air purifier at $375. However the quad filter design, popularly known as the Corsi-Rosenthal box, did not achieve gains in CADR of more than 60% over a single filter design which is in sharp contrast to the 200-250% gains reported by UC-Davis. Conclusions: DIY alternatives using single 1", 2" and 4" MERV 13/14 filters compare favorably to commercially available systems in terms of estimated CADR and dBA 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. Recent studies have demonstrated the presence of live SARS-Cov-2 in micron [1] [2] and submicron [3] aerosols from the exhaled breath of infected people. Virus-carrying aerosols have the potential to accumulate in the airspace of poorly ventilated, indoor spaces such as classrooms, offices, homes, restaurants, and bars and other community settings, and if inhaled may result in COVID-19 infections [4] . Separately, toxic aerosol pollution such as from wildfires, wood burning, and other sources can be encountered in these same locations with ventilation using unfiltered, outdoor air. Portable air filtration in the form of high efficiency particulate air (HEPA) air purifiers is useful to remove both types of aerosols without relying on ventilation from outside the room. Although in healthcare settings there are minimum specifications for ventilation and air filtration up to 15 air exchange per hour (ACH), in community settings the US-CDC does not specify a target but recommends increasing air filtration as high as possible [5] . Whereas experts recommend 4 to 6 ACH in community settings [6] . For example, in a typical classroom of size 30' x 30' classroom with 8' foot ceilings, a HEPA air purifier rated at a CADR of 300 cubic feet per minute needs 24 minutes to cycle the air once (2.5 ACH) [7] [8]. This classroom would therefore require two to three such air HEPA purifiers with sufficiently low noise generation typically costing $400-$500 each (with periodic $40-$80 filter replacements) to achieve the target ACH exceeding 4 to 6 ACH. For an upfront cost of approximately $1000-$1500 per classroom, this level of air filtration may be outside the budget of many schools in the US and worldwide. Approaching 15 ACH as in hospitals and passenger aircraft [9] would require further HEPA units putting it further out of reach. If portable HEPA air purifiers of sufficiently low noise generation with sufficiently high clean air delivery rate (CADR) are unaffordable for many communities, how else can they achieve target ACH while addressing [10] [11] both cost and noise generation concerns? Lower cost alternatives to HEPA air purifiers have been investigated for air filtration and safety. For example, a Minimum Efficiency Rating Value-13 (MERV-13) filter attached to a box fan using duct tape or a bungee cord has been demonstrated to effectively filter wildfire PM2.5 and submicron particles [12] . California Air Resource Board recommends never leaving such devices unattended, and only using box fans manufactured in or after 2012 because these fans will have a fused plug to prevent electrical fires if the device is inadvertently knocked over [13] . Chemical Insights, a subsidiary of Underwriter Laboratories, recently tested five different electric box fan models (approximately 20" × 20" in size) with attached air filters and concluded that all measured temperatures fell below the maximum acceptable thresholds defined by the market safety standard for electric fans (UL 507) [14] . These lower-cost alternatives have not yet been directly compared to commercially available HEPA air purifiers in terms of filtration efficiency (%), air flow and clean air delivery rate (CADR), and noise generation (dBA). Below, we compare the cost-effectiveness of aerosol filtration by commercially available, portable air cleaning devices with HEPA filters and do-it-yourself (DIY) . CC-BY-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted December 5, 2021. ; https://doi.org/10.1101/2021.12.04.21267300 doi: medRxiv preprint alternatives built from box fans and HVAC filters rated at MERV 13 or 14. The comparison is based on three metrics: clean air delivery rate (CADR), the noise generated (dBA), and affordability ($$). The test devices include 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. We also evaluate a DIY configuration involving quad filter design, popularly known as the Corsi-Rosenthal box, that has been in use to enhance the airflow and CADR over a single filter design. . CC-BY-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted December 5, 2021. ; https://doi.org/10.1101/2021.12.04.21267300 doi: medRxiv preprint Methods Aerosol filtration efficiency. For each HEPA air purifier or box fan filter, counts at each particle size (0.3, 0.5, 0.7, 1.0, 2.5, 5, and 10 in microns) were tested using an ISO-certified particle counter (Temtop Particle Counter PMD 331) with the filter turned 'on' and 'off' consecutively. Five counts were recorded and averaged on the particle counter when its input was placed directly at the output of the air purifier/filter for 30 seconds. The efficiency at each particle size was estimated by the formula, efficiency = 1 -on / off. Air flow. For each HEPA air purifier or box fan filter, the airspeed was measured using an anemometer (BTMETER BT-100 Handheld Anemometer) held at the output and also input (if possible). An average of four airspeed measurements (feet per minute) were multiplied by the area of the input or output to estimate the airflow (cubic feet per minute). Noise. Noise measurements were taken for each air purifier/filter using an iPhone app maintained by National Institute for Occupational Safety and Health (NIOSH) [15] . Three noise measurements were taken at a 9" distance perpendicular to the direction of the output airflow and averaged. Commercially available HEPA air purifiers. Three models evaluated include the Coway Airmega 300, Coway AP-1512HH, and Airthereal AHG550. 20" x 20" x 2" 13 $15 20" x 20" x 4 14 $38 . CC-BY-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted December 5, 2021. ; https://doi.org/10.1101/2021.12.04.21267300 doi: medRxiv preprint Commercial HEPA air purifiers. The results for the commercial air purifiers show particle filtration efficiencies near or exceeding 90% at all particle sizes ranging from 0.3 to 10 microns. At their maximum fan speed, the noise generated by the Coway Airmega 300 was lowest and most (subjectively) tolerable of the three models. To achieve similar tolerability levels both the Coway AP-1512 and the Airthereal AGH550 had to be set at fan speed 2 which cut their estimated airflow (CADR) roughly in half. . CC-BY-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted December 5, 2021. ; https://doi.org/10.1101/2021.12.04.21267300 doi: medRxiv preprint Box fan plus single MERV 13/14 filter. The filtration efficiency of the MERV 13/14 filters varied by thickness with highest observed for 4" filters compared to 1" filters, and also at higher particle sizes above 1 micron exceeding 90%. The box fan was (subjectively) tolerable only at fan speed one (out of three). Compared to 282 cfm for the Coway Airmega 300 ($375) at fan speed 3, at fan speed 1, the box fan has estimated CADR of 371 cfm at for 70% filtration efficiency with the 4" MERV 14 filter ($58) and 278 cfm at 60% efficiency for the 2" MERV 13 filter ($38), 4 . CC-BY-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted December 5, 2021. ; https://doi.org/10.1101/2021.12.04.21267300 doi: medRxiv preprint Box fan plus quad MERV 13/14 filters. The quad filter design, popularly known as the Corsi-Rosenthal box, showed significant improvement over single filters with using comparably rated filters and fan speeds. With the quad 1" MERV 14 filters at fan speed 1, the output airflow was 617 cfm versus 386 cfm with a single filter at that fan speed for an improvement of 60%. Similarly with 2" MERV 13 filter the improvement was only 35%. These results are in contrast to approximately 200%-250% improvement between single filter and quad filter designs (also with shroud and 20" Lasko box fan) reported in a study by UC-Davis [19] . Notably, there was almost no difference in output airflow when the 1" air filters were attached with duct tape (sealed airtight) or velcro (loose fitting) suggesting any leaks created by a loose fit are likely insignificant. quad . CC-BY-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted December 5, 2021. ; https://doi.org/10.1101/2021.12.04.21267300 doi: medRxiv preprint Accounting for reduced filtration efficiency of MERV 13/14 filters compared to HEPA, the 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 to 371 cfm, comparable or better than a commercial HEPA air purifier running at maximum speed with low noise generation (Coway Airmega 300) at 282 cfm. Yet the upfront cost of the components of the DIY setup were 5 to 10 times less ($35-$58) than the commercial HEPA air purifier ($375). The DIY setup also did not require tape but could be assembled with loose-fitting velcro or simply attached by the vacuum of the fan. However the quad filter design, popularly known as the Corsi-Rosenthal box, did not achieve gains in CADR of more than 60% over a single filter design which is in sharp contrast to the 200-250% gains reported by UC-Davis [19] . The reason why there is a discrepancy between the results reported here and by UC-Davis remains to be reconciled. Anecdotally, we observed that the airflow was non-uniform across all four filters which may be responsible for this. Given the simplicity and low-cost of the box-fan design using single 2" or 4" MERV 13/14 filters, form-factor improvements may include a more stable frame so it is not so easy to knock over with visual designs such as wallpaper to make it aesthetically pleasing. Also a mosquito screen on the front of the fan may help prevent curious fingers from contacting the fan blade which may be especially important for classroom and home applications when young children are present. Viral Load of SARS-CoV-2 in Respiratory Aerosols Emitted by COVID-19 Patients while Breathing, Talking, and Singing Infectious SARS-CoV-2 in Exhaled Aerosols and Efficacy of Masks During Early Mild Infection Epub ahead of print The size and culturability of patient-generated SARS-CoV-2 aerosol Epub ahead of print Scientific Brief: SARS-CoV-2 Transmission Ventilation in Buildings Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission Safety of air travel during the ongoing COVID-19 pandemic The Impact of Large Mobile Air Purifiers on Aerosol Concentration in Classrooms and the Reduction of Airborne Transmission of SARS-CoV-2 The effect of a mobile HEPA filter system on 'infectious' aerosols, sound and air velocity in the SenseLab. Build Environ Impact of Wildfire Smoke Events on Indoor Air Quality and Evaluation of a Low-cost Filtration Method We would like to thank Hugh Churchill, Dan Jaffe, Amalavoyal Narsimha Chari, and Robert Freedman for their helpful discussions and advice.