Few studies have examined HEPA filtration in a classroom setting. This study aimed to assess whether air purifiers with HEPA filters reduce exposure using a block-randomized crossover study in elementary schools in Los Angeles County that were using existing HVAC systems with MERV 13 filtration due to the COVID-19 pandemic by reducing air pollution in classrooms. In July 2022, prior to placement of portable air purifiers, there was no statistically significant difference between PM concentrations in the treatment and control classrooms, indicating successful randomization relative to baseline PM concentrations. Our results show that in HEPA classrooms, the average annual PM level was 39.9% lower2.5 as non-HEPA classrooms (0.887 µg/m).3 compared to 1.468 µg/m3) in the 2022-2023 school year. PM outdoors2.5 Infiltration was lower in the HEPA classrooms than in the non-HEPA classrooms.
A previous study using a similar study design was conducted from April to December 2021 in Mengzhou City, Henan Province, China [15]. PM2.5 Concentrations were measured in classrooms, living rooms and outdoors during the study period. MicroPEM samplers were also used in the students’ living room. The living room and classroom interventions contributed to a 42.31% and 21.34% reduction in personal PM, respectively2.5 Exposure or participants with air purification measures in the living room and classroom had the lowest PM values2.5 with an average of 45.9 ± 44.4 µg/m3followed by participants with living room intervention only (62.0 ± 51.5 µg/m ).3), participants with only teaching intervention (73.4 ± 54.1 µg/m3) and participants without intervention (89.0 ± 61.4 µg/m).3). PM2.5 The levels in this study were much higher than the levels in Los Angeles.
Another air filtration intervention study conducted from 2015 to 2020 in urban elementary schools in the northeastern United States used a factorial randomized trial with a four-arm design [16]. This study examined treatment with and without air purifiers with HEPA filters and with and without school-wide integrated pest management (IPM). The middle PM2.5 Classroom exposure at baseline was 5.5 µg/m3 with HEPA filtration and without IPM and 6.1 µg/m3 with control filtration. After the procedure, the mean PM was2.5 in the classroom the exposure was 3.1 µg/m3 and 5.3 µg/m3 for HEPA or control filtration. These PM values are closer to the values measured in the current study, and the control classrooms were 70% higher than the HEPA treatment classrooms, similar to what we observed (65%).
A third study also examined urban elementary schools in the northeastern United States from 2013 to 2014 using a randomized controlled pilot study [17]. Treatment classrooms received HEPA filtration, while control classrooms had their filters replaced with a noise machine to mimic the noise of air filtration. Before randomization, average classroom PM levels at baseline were reported2.5 were 6.3 μg/m3 with no statistically significant differences between the control and treatment classes. In the control group meant PM2.5 The concentrations fell from 6.4 μg/m3 at baseline to 4.8 μg/m3 and 5.0 μg/m3 at the first or second follow-up appointment. In the treatment group means PM2.5 The concentrations fell from 6.2 μg/m3 at baseline to 2.4 μg/m3 and 2.6 μg/m3 at the first or second follow-up appointment. There were greater reductions in PM in the intervention group2.5 compared to the control group, representing a reduction of 49% and 42%, respectively. This is similar to the 29.7% decrease observed in the current study, even though the PM took place in the classroom2.5 The values we observed were much lower. Like the previous results, portable HEPA filter air purifiers were effective in improving short-term air quality in classroom environments. It is unclear whether the classrooms in any of the three previous air purifier studies already had HVAC filtration.
Compliance limits the effectiveness of the air purifiers. Because our study used an intention-to-treat analysis, treatment effect estimates are more conservative (i.e., they are likely to be underestimated when treatment tasks are not followed perfectly, such as when teachers install their own HEPA filters in non-HEPA air purifiers install). Although our study team found that most air purifiers remained on with the correct filters at the end of the first six months, we were unable to monitor and ensure their proper use throughout the year when replacing the filters, making ITT analysis the most appropriate choice . However, heterogeneity of results is more likely when noncompliant and compliant data are mixed in the final analysis. Another limitation is that the outdoor monitors were installed prior to this study, so we do not have calibration data for these monitors. The indoor monitors were assembled and the agreement was confirmed before they were placed in the classrooms.
The interpretation of our results should be done with some consideration. During the study period, air quality was generally good, ranging between 7.8 and 10.1 µg/m³, with no wildfire smoke events. Therefore, we cannot assess the performance of additional air filtration during extreme pollution episodes. Additionally, the indoor levels were very low due to the existing HVAC systems with MERV 13 filters in operation. Although there are statistically significant reductions in PM levels with HEPA portable air purifiers, we cannot say whether these small reductions have significant health benefits for students or teachers.
In summary, our results are consistent with previous studies showing that portable air purifiers with HEPA filters are effective in removing PM when properly maintained2.5 from classrooms [15,16,17]. Schools in our study used MERV 13 filters in their HVAC systems throughout the duration of our intervention study due to the COVID-19 pandemic, preventing significant outdoor particulate matter infiltration2.5 into the classrooms. However, infiltration in the treatment rooms with HEPA filters was still lower than in the control rooms. Our results support the use of portable air purifiers with HEPA filters in classrooms to reduce particulate matter, even in classrooms with existing HVAC air filtration.