Long-term experience with rapid air filtration (6 to 15 air changes per hour) in a K-5 elementary school using HEPA and Do-It-Yourself (DIY) air purifiers during the COVID-19 pandemic

On May 12, 2023 CDC recommended 5 air changes per hour (5 ACH) and in July 2021 California (CDPH) recommended 6 to 12 ACH to reduce long-range, aerosol transmission of COVID-19 and other pathogens in classrooms. EPA recommends MERV 13+ DIY air cleaners for temporary use during wildfires, and a recent EPA study reported inconsistent usage in homes due to excessive noise generated by the DIY air cleaners. Questions also remain about wear and tear including how long filters retain their filtration properties and need to be replaced. Herein we report real-world experience from daily usage of 47 HEPA and 60 MERV 16 DIY air cleaners in a California elementary school during two academic school years from spring 2021 through fall of 2023 across 16 classrooms, a library, an auditorium, a lunchroom, and in a hallway. Three to six purifiers were needed in classrooms to meet 6 to 12 ACH. Teachers reported noise generated by MERV 16 DIY purifiers on lowest fan speed as acceptable for classroom use. Filtration efficiency at 0.3 m (most penetrating particle size) for DIY air cleaners with 5" MERV 16 filters used in the classrooms averaged 77% after six months (1st batch in February 2022) compared to 92% for newly installed filters (2nd batch in October 2022). Follow up testing on both batches of filters after an additional academic year (June 2023) showed only slight changes in average filtration efficiency. Portable air cleaners (HEPA and DIY) averaged and estimated 10 ACH (6-15 ACH) across the 16 classrooms demonstrating feasibility and unit economics of meeting CDPH targets per classroom for $200-$650 with DIY versus $600-$12,000 with the HEPA models used. In one 9000 cubic foot classroom with 7 air purifiers, air exchange rate was measured using ambient aerosols at 18 ACH from air purifiers (within 20% of ACH estimated based on CADR of purifiers) and 7 ACH from HVAC for a combined total of 25 ACH. Based on this long-term experience, specific recommendations for enhancing and improving CDCs web page "Ventilation in Buildings" include: (1) recommended operation of MERV 13+ DIY at their low speed for low noise, cost-effective air cleaning (2) electro-mechanical safety especially in relation to power outlets (3) an open-source procedure known as the "spike test" using ambient aerosols to verify ACH in a room, like the Portacount for mask fit testing. Spike testing can become the basis for ACH certification or verification in any room without generating aerosol contaminants (e.g. salt water, smoke, tracers which may be unsafe or disallowed indoors).