Effects of portable air cleaners and A/C unit fans on classroom concentrations of particulate matter in a non-urban elementary school

Alexandra Azevedo; Jahred Liddie; Jason Liu; Jessica E Schiff; Gary Adamkiewicz; Jaime E Hart

doi:10.1371/journal.pone.0278046

. 2022 Dec 1;17(12):e0278046. doi: 10.1371/journal.pone.0278046

Effects of portable air cleaners and A/C unit fans on classroom concentrations of particulate matter in a non-urban elementary school

Alexandra Azevedo ^1,^‡,^#, Jahred Liddie ^1,^‡,^#, Jason Liu ^1,^‡,^#, Jessica E Schiff ^1,^‡,^*,^#, Gary Adamkiewicz ^1,^‡,^#, Jaime E Hart ^1,^2,^‡,^#

Editor: MARIA LUISA ASTOLFI³

PMCID: PMC9714748 PMID: 36454721

Abstract

Given the increased use of air cleaners as a prevention measure in classrooms during the COVID-19 pandemic, this study aimed to investigate the effects of portable air cleaners with HEPA filters and window A/C fans on real-time (1 minute) concentrations of PM less than 2.5 microns (PM_2.5) or less than 1 microns (PM_1.0) in two classrooms in a non-urban elementary school in Rhode Island. For half of each school day, settings were randomized to “high” or “low” for the air cleaner and “on” or “off” for the fan. Descriptive statistics and linear mixed models were used to evaluate the impacts of each set of conditions on PM_2.5 and PM_1.0 concentrations. The mean half-day concentrations ranged from 3.4–4.1 μg/m³ for PM_2.5 and 3.4–3.9 μg/m³ for PM_1.0. On average, use of the fan when the air cleaner was on the low setting decreased PM_2.5 by 0.53 μg/m³ [95% CI: -0.64, -0.42] and use of the filter on high (compared to low) when the fan was off decreased PM_2.5 by 0.10 μg/m³ [95% CI: -0.20, 0.005]. For PM_1.0, use of the fan when the air cleaner was on low decreased concentrations by 0.18 μg/m³ [95% CI: -0.36, -0.01] and use of the filter on high (compared to low) when the fan was off decreased concentrations by 0.38 μg/m³ [95% CI: -0.55, -0.21]. In general, simultaneous use of the fan and filter on high did not result in additional decreases in PM concentrations compared to the simple addition of each appliance’s individual effect estimates. Our study suggests that concurrent or separate use of an A/C fan and air cleaner in non-urban classrooms with low background PM may reduce classroom PM concentrations.

Introduction

Airborne transmission of the novel respiratory virus SARS-CoV-2 has prompted increased scrutiny of indoor air quality (IAQ), especially in settings where many people gather such as schools, offices, and restaurants [1–3]. To further improve indoor air quality and make indoor environments healthier, the Biden Administration launched an effort in March 2022 to improve ventilation and reduce the spread of COVID-19 in buildings, including school buildings [4]. In schools, where attendance is typically mandatory and classrooms can reach a high occupant density, environmental health experts recommend advanced air ventilation and filtration to protect students and teachers [5]. While it is possible to achieve high ventilation rates in classrooms by opening doors and windows, filtration presents a set of challenges. Many schools do not have windows that can be opened for safety or energy efficiency reasons. Heating, ventilation, and air-conditioning (HVAC) systems in older schools are typically equipped with low-grade filters such as minimum efficiency reporting value (MERV) 8, which captures approximately 84% of PM 3–10 μm in diameter, 20% of PM 1–3 μm in diameter, and is not rated for capture efficiency of particles smaller than 1.0 μm [6]. The Center for Disease Control and Prevention (CDC) recommends upgrading to more efficient filters such as MERV 13 and higher, especially in buildings that recirculate air within the same room or same local ventilation zone [6, 7]. However, the mechanical ventilation systems in many school districts are too old to be compatible with newer filters [8, 9]. For school districts that are structurally capable of adding MERV 13 filters, costs associated with installation, operations, and maintenance can easily exceed millions of dollars [9]. Given the challenges of installing new HVAC systems or filters in school buildings, portable air cleaners and fans have become popular, less expensive alternatives to reduce PM concentrations. Air cleaners with high-efficiency particulate air (HEPA) filters, which are at least 99.97% efficient at capturing particles 0.3 μm and larger in size, offer the possibility of substantially reducing SARS-CoV-2 viral particles as well as improving overall IAQ [6, 8].

PM is one of the most common pollutants that could potentially degrade air quality in classrooms [10]. Indoor PM levels are influenced by several factors, such as ambient air pollution levels, air exchange rates, occupancy, type and intensity of indoor activities, and particle sizes [10–12]. One study in Munich, Germany found that PM concentrations in classrooms are about six times higher than outdoor concentrations [13]. Children are particularly vulnerable to potential health consequences related to PM exposure due to their immature respiratory and immune systems and greater breathing rates per body weight [16]. Beyond the current context of the COVID-19 pandemic, improved air ventilation is associated with lower school absenteeism, better performance on cognitive function tests, and fewer respiratory symptoms, such as those related to asthma, lung inflammation, and allergies [14–16]. PM_2.5 exposure in children has been associated with asthma incidence (OR = 1.10, 95% CI:1.01, 1.20), prevalence of asthma symptoms (OR = 1.08, 95% CI:1.02, 1.16), and rhinitis (OR = 1.15, 95% CI: 1.05,1.26) [17]. A review study of 33 articles further provided evidence that exposure to particulate air pollution has adverse impacts on children’s respiratory health, with stronger negative effects seen among children with asthma [18].

While portable air cleaners are commonly used to reduce PM concentrations, especially in locations near roadways and other sources of air pollution, less is known about the use of conventional fans. Several studies have tested the use of portable air cleaners in school settings, characterizing their effectiveness in reducing exposure to PM_2.5 alone [19]; PM₁₀ alone [17]; PM_2.5 and PM₁₀ [12, 20]; airborne allergens [21]; ultrafine particles (UFPs), black carbon, PM_2.5 and PM₁₀, and volatile organic compounds (VOCs) [16]; as well as their effect on indoor NO₂ and CO₂ concentrations [22]. Only one study has evaluated the performance of fans in mitigating PM levels in classrooms [23]. The objective of this study is to provide data on the effectiveness of portable air cleaners and fans operating in tandem to reduce PM_2.5 and PM_1.0 concentrations in occupied classrooms.

Materials and methods

Study location

The study was conducted in a public elementary school in a small town in Rhode Island, USA. Two classrooms of similar size, occupancy, age of students, and ventilation conditions were selected for air monitoring. Both classrooms were rectangular, with areas of approximately 140 m² and 152 m² (volume 427 m³ and 463 m³) for classroom A and B, respectively (S1 Fig). Neither classroom had windows that open, except for the windows in which the A/C units were located. Classroom A was located on the first floor and classroom B was located on the second floor of the same building.

Prior to the start of the study, each classroom was equipped with a portable air cleaner with an H13-grade True HEPA filter (Medify Air Model MA-112) and a window A/C unit (LG 15000 BTU Model LW1516ER) with separate cooling and fan functions. The volume flow through the air cleaner can be adjusted in five increasing settings: “sleep,” 1, 2, 3 and 4. According to the manufacturer, the air cleaner has a reported maximum clean air delivery rate (CADR) rating of 950 m³/h and can cover an area of 762 m² with two air changes/h. While it is recommended that HEPA filters be cleaned regularly and replaced every six months, the school’s maintenance schedule was unknown. However, as the air cleaners had been installed after the beginning of the COVD-19 pandemic, we can assume that they were under six months old at the time of sampling. The fan function on the A/C unit could be adjusted in three increasing settings: 1, 2, and 3. Due to school policies, the air cleaners were operated continuously during the school week (24 h, M-F) throughout the study period, while the A/C fans were operated only during school hours (8:30am to 3:45pm, M-F). The A/C cooling function was not used during the study period.

Sampling methods

Two personal aerosol monitors (SidePak Model AM510, TSI Inc., Shoreview, MN, USA), one measuring PM_2.5 and one measuring PM_1.0, were placed side-by-side across the room (approximately 12 m away) from the air cleaner and A/C unit in each of the classrooms. The same units were used in the same classrooms throughout the sampling period. The inlets of the aerosol monitors were located about 1–1.2 m above ground level. Additionally, one HOBO data logger (HOBO U12 Temp/RH/Light/External Data Logger, Onset Computer Inc., Pocasset, MA, USA) was placed next to the aerosol monitors in each classroom to measure temperature and relative humidity.

Samples were collected at one-minute intervals during the school week (M-F) from 3/31/2021 to 4/16/2021, for a total of eleven days. Different sets of conditions were used in each classroom each half-day. The HEPA cleaner settings (“high” = setting 4 or “low” = setting 1) and fans (“fan on” = setting 1 or “fan off” = 0) for classroom A and classroom B were assigned randomly prior to the start of sampling using R version 4.1.2 [24]. The schedules of half-day settings were distributed to teachers at the start of sampling, as well as log sheets to record deviations from the schedule and times of events that may impact PM concentrations (e.g., bus arrivals, times during which all students typically leave the classroom). We defined noon (12pm) as the cutoff for each trial given that teachers typically transferred from one trial to the next between noon and 12:30pm on each day. Temperature and relative humidity (RH) for each classroom, measured at one-minute intervals, were matched to the PM data.

Quality assurance and quality control procedures

The SidePak aerosol monitors were factory-calibrated to the respirable fraction of standard ISO 12103–1, A1 Test Dust prior to the start of data collection. Prior to data collection, all monitors were co-located within a controlled environment and operated for several hours to ensure all monitors functioned properly. The monitors were deemed to have an acceptable level of precision if the average percent difference in PM concentrations between the co-located instruments was ≤ 10%. If this threshold was not met, correction factors were calculated to ensure that differences in the collected data were not due to differences in the instruments used. Exceedances of this threshold were reviewed within the context of testing data, given that greater percent error was possible for these monitors at lower concentrations.

Statistical analysis

The raw data were trimmed to standard-length school days (8:30am to 3:45pm) and data pertaining to days on which there was no school (i.e., weekends and holidays) were removed. Summary statistics, box plots, and histograms were first generated for PM_2.5 and PM_1.0 at the half-day and minute level. With our dataset of half-day trial averages, we utilized one-way analysis of variance (ANOVA) to determine if there were significant differences between trial averages.

We also developed linear mixed models using concentration data at the one-minute level. We defined three fixed effects: fan (1 = on, 0 = off), air cleaner (1 = high, 0 = low), and fan*air cleaner, and one random effect: half-day (1 = morning, 0 = afternoon). We included a fixed effect for classroom (0 = classroom B (second floor), 1 = classroom A (first floor)) to control for time-invariant differences between the two classrooms. Given the potential for high autocorrelation in minute-level data, we also used a covariance structure following autoregressive 1 (AR 1) in the linear mixed models. Given large changes in relative humidity over our study period, a secondary analysis considered relative humidity in each classroom.

All descriptive statistics and analyses were performed using R. Statistical significance was set using ɑ = 0.05.

Results

A total of 12 half-day trials of fan off / air cleaner low and fan off / air cleaner high; and 11 half-day trials of fan on / air cleaner low and fan on / air cleaner high were completed in the classrooms. We removed one full day of sampling for one classroom due to deviations from the scheduled settings. Given the observed percent differences in the pre-sampling data, correction factors were applied to all measurements from both monitors in one of the classrooms. The mean half-day concentrations ranged from 3.4–4.1 μg/m² for PM_2.5 and 3.4–3.9 μg/m² for PM_1.0 (Table 1). Mean temperatures were similar across days, with temperatures ranging from 69.6–70.9 ℉. However, mean relative humidities varied more widely, with a range of 31.5–37.6%. The lowest mean relative humidity was observed in the fan on / air cleaner high trials, while the greatest was observed in the fan off / air cleaner low trials.

Table 1. Distributions of PM2.5, PM1.0, temperature and relative humidity under different scenarios of HEPA cleaner and air conditioning unit fan use in elementary school classrooms in Rhode Island, USA.

		Fan off, filter low		Fan off, filter high		Fan on, filter low		Fan on, filter high
		Half-day	Minute-level	Half-day	Minute-level	Half-day	Minute-level	Half-day	Minute-level
PM2.5	Concentration [μg/m3]
	Min	2.6	1.0	2.0	1.0	2.0	1.0	2.5	2.0
	Median	3.8	4.0	3.9	4.0	3.6	3.62	3.3	3.6
	Mean ± SD	4.1 ± 0.9	4.0 ± 1.7	3.7 ± 1.1	3.8 ± 1.4	3.4 ± 0.8	3.4 ± 1.2	3.7 ± 0.8	3.7 ± 1.1
	Max	5.6	53	5.8	29	4.4	20	5.1	18
	N	12	2637	12	2625	11	2370	11	2338
PM1.0	Concentration [μg/m3]
	Min	2.5	1.6	2.3	0.5	2.1	1.03	2.3	2.0
	Median	3.4	3.0	3.8	3.86	3.5	3.35	3.2	3.1
	Mean ± SD	3.9 ± 1.6	3.9 ± 1.7	3.9 ± 1.1	4.0 ± 1.3	3.8 ± 1.5	3.8 ± 1.7	3.3 ± 0.8	3.3 ± 0.9
	Max	7.5	27	5.9	17.8	6.7	33	4.7	9.0
	N	12	2637	12	2432	11	2313	11	2337
	Temperature [F]
	Mean ± SD	70.5 ± 2.4	70.5 ± 2.3	70.9 ± 2.5	70.8 ± 2.4	69.5 ± 3.2	69.6 ± 3.05	69.6 ± 1.9	69.6 ± 1.8
	N	11	2,412	9	1,965	8	1,725	6	1,290
	Relative humidity [%]
	Mean ± SD	37.6 ± 7.7	37.6 ± 7.3	37.5 ± 7.8	37.5 ± 7.5	36.5 ± 6.0	36.4 ± 5.6	31.6 ± 4.9	31.5 ± 4.6
	N	11	2,412	9	1,965	8	1,725	8	1,290

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Note: All data pertain to school hours (8:30am to 3:45pm). Lower sample is shown for temperature, humidity, and PM1.0 data (compared to PM2.5 data) data due to missingness. Differences between half-day and minute-level averages are due to rounding errors.

No significant differences between half-day averaged PM_2.5 and PM_1.0 concentrations were observed between the four trials (Table 2). The linear mixed model results are shown in Table 3. The differences in N included are due to missing humidity information. When the air cleaner was on low, use of the fan was associated with an average 0.53 μg/m³ [95% CI: -0.64, -0.42] decrease in PM_2.5 concentrations compared to having the fan off. When the fan was off, use of the air cleaner on a high setting decreased concentrations, on average, by 0.10 μg/m³ [95% CI: -0.20, 0.005] compared to low setting. For PM_1.0, when the air cleaner was on low, use of the fan decreased concentrations, on average, by 0.18 μg/m³ [95% CI: -0.36, -0.01]. When the fan was off, use of the air cleaner on the high setting decreased PM_1.0 concentrations, on average, by 0.38 μg/m³ [95% CI: -0.55, -0.21] compared to the low setting. The product terms for fan on * air cleaner high were statistically significant and positive for PM_2.5 and null for PM_1.0, indicating no additional decreases in either size fraction when both appliances were used simultaneously. Results were generally similar when adjusting for relative humidity.

Table 2. One-way unadjusted analysis of variance (ANOVA) comparing four scenarios of HEPA cleaner and air conditioning unit fan use in elementary school classrooms in Rhode Island, USA.

Size fraction	Degrees of freedom	F value	Pr (f > F)
PM2.5	3	0.95	0.43
PM1.0	3	0.45	0.72

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Note: ANOVAs include comparisons of averaged concentrations (over half-days) from each of the four trials (air cleaner on high, fan on; air cleaner on low, fan on; air cleaner on high, fan off; air cleaner on low, fan on) in two classrooms.

Table 3. Trial results from different scenarios of HEPA cleaner and air conditioning unit fan use in elementary school classrooms in Rhode Island, USA.

	Particulate matter size fraction [ug/m ³ ]
	PM2.5	PM1.0	PM2.5	PM1.0
	(1)	(2)	(3)	(4)
Fan on	-0.53^***	-0.18^**	-0.91^***	-0.57^***
Fan on	(-0.64, -0.42)	(-0.36, -0.01)	(-1.03, -0.78)	(-0.77, -0.38)
Filter high	-0.10^*	-0.38^***	0.05	-0.35^***
Filter high	(-0.20, 0.005)	(-0.55, -0.21)	(-0.07, 0.16)	(-0.54, -0.15)
Fan on * filter high	0.39^***	0.04	0.26^***	0.17
Fan on * filter high	(0.24, 0.54)	(-0.21, 0.28)	(0.08, 0.45)	(-0.13, 0.47)
Classroom 2	-1.03^***	1.77^***	-1.13^***	1.77^***
Classroom 2	(-1.11, -0.95)	(1.64, 1.89)	(-1.23, -1.04)	(1.62, 1.93)
Relative humidity			-0.02^***	0.04^***
Relative humidity			(-0.03, -0.02)	(0.03, 0.05)
Constant	4.43^***	3.16^***	5.38^***	1.73^***
Constant	(4.17, 4.70)	(2.85, 3.46)	(5.06, 5.70)	(1.24, 2.23)
Observations	9,970	9,719	7,392	7,143

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*p < 0.1;

**p<0.05;

***p<0.01

Note: Results are from linear mixed models including a random intercept for half-day (morning or afternoon). Model results shown include one-minute level data with AR1 correlation structure. Differences in the number of observations are due to missing relative humidity data (column 3 and 4).

Discussion

This was one of the first studies to investigate the effects of different low-cost indoor air quality interventions on PM_2.5 and PM_1.0 concentrations in a classroom in a non-urban elementary school in the Northeastern United States. The mean half-day concentrations ranged from 3.4–4.1 μg/m³ for PM_2.5 and 3.4–3.9 μg/m³ for PM_1.0. On average, addition of the fan (with the air cleaner on low) decreased PM_2.5 by 0.53 μg/m³ [95% CI: -0.64, -0.42] and use of the filter on high (compared to low) alone decreased PM_2.5 by 0.10 μg/m³ [95% CI: -0.20, 0.005]. For PM_1.0, addition of the fan (with the air cleaner on low) decreased concentrations by 0.18 μg/m³ [95% CI: -0.36, -0.01] and use of the filter on high (compared to low) alone decreased concentrations by 0.38 μg/m³ [95% CI: -0.55, -0.21]. We observed that the lowest classroom PM_2.5 and PM_1.0 concentrations occurred when both the fan and portable HEPA air cleaner were used simultaneously. However, for PM_2.5, the lowest concentrations were seen when the fan was on and the air cleaner was on low because most of the benefit was through the use of the fan. Overall, the trends observed in this study give insight into non-urban classroom air quality and how different commercially available, cost-effective IAQ interventions could be incorporated into classrooms to effectively reduce PM concentrations.

Our results were consistent with previous studies on the effects of ventilation systems and portable air cleaners on indoor PM_2.5 and PM_1.0 levels in various settings. However, it is important to note that these studies did not evaluate fan use and occurred in areas with higher ambient air pollution. The application of a new mechanical ventilation system with a fine F8 (MERV 14) filter reduced classroom PM_2.5 concentrations by 30% compared to baseline in an elementary school in Amsterdam with average background PM_2.5 ranging from 11.2 to 17.9 μg/m3 [25]. Barkjohn et al. 2020 also demonstrated that the use of HEPA fitted portable air cleaners in homes in Beijing, where outdoor PM_2.5 ranged from 25 to >200 μg/m3, significantly reduced indoor/outdoor PM_2.5 ratios by 72% when windows were closed [26]. In a crossover study within two residential homes, Hart et al. 2011 demonstrated that a portable air cleaner can be efficient in reducing particulate matter concentrations by >60% for PM_2.5 and PM_1.0 in homes that used wood stoves as a primary heating source [27]. A pilot study in four kindergarten classrooms in Poland found that air quality in classrooms with a HEPA air cleaner was on average almost 40–50% better than in those without any procedures to decrease air pollutant concentrations [19]. These studies support our findings that use of air cleaners indoors in a classroom reduces PM_2.5 and PM_1.0 concentrations, although we observed smaller decreases of approximately 13% and 2% in PM_2.5 concentrations when the fan was on and the air cleaner was on high, respectively. In addition, they do not provide an explanation for why the greatest reduction in PM_2.5 concentrations occurred with the low HEPA air cleaner setting and not the higher setting, which filters a greater volume of air per hour.

We speculate that using a lower CADR setting could provide a greater reduction in PM_2.5 concentrations in classrooms with low background PM_2.5 due to particle removal efficiency. A lab study demonstrated that increased media velocity was associated with a decrease in HEPA filter efficiency from 99.999%, although the efficiency rate did not decrease below 99.977% [28]. A higher CADR setting increases the volume of air filtered during a given period, resulting in greater air exchange. However, as air is pulled through the HEPA filter more quickly, the filter may be less effective at removing particles, especially at low concentrations. A setting that reduces the rate of air filtration would theoretically move the air through the filter more slowly, allowing for more effective removal of particles. One study found that using portable HEPA air cleaners at home on “Auto” mode significantly reduced indoor PM_2.5 concentrations compared to homes that manually selected the air cleaner settings [29]. The authors concluded that auto-mode filtration provided the most efficient results, especially when indoor sources (e.g., cooking) were present. While the air cleaners used in this study did not have an “Auto” mode, it is possible the lower CADR setting was more efficient for PM_2.5 given the low background concentrations and possible settling on indoor surfaces. However, we were not able to address the influence of classroom occupancy and activity on indoor PM levels. It is possible the higher CADR setting may have been more efficient at removing PM_1.0 since smaller particles are more likely remain suspended. Croxford et al. reported that use of an electrostatic air cleaner in an office setting in London reduced smaller particles (PM_2.0) more effectively than larger particles (PM_2-10) [30]. Similarly, Park et al. 2020’s study of 34 elementary schools in Korea found that air cleaners appeared to remove PM_2.5 more effectively than PM₁₀ [12]. Meanwhile, a crossover study of five elementary schools found that the use of electrostatic air cleaners in school classrooms substantially decreased airborne indoor PM concentrations but not indoor surface dust levels [31]. Our findings might imply that, at low concentrations, larger particles are removed more effectively with a lower CADR setting while smaller particles are removed more effectively with a higher CADR setting since they are less likely to settle on indoor surfaces.

This study also evaluated A/C fan ventilation; however, research is limited on the effect of fan use on indoor classroom PM concentrations. A study of five classrooms in Hong Kong, a region with high ambient concentrations of PM, suggested that even in the presence of ventilation (air conditioning or ceiling fans) indoor PM₁₀ concentrations mirrored outdoor PM₁₀ concentrations [32]. If we expect that classroom PM_2.5 concentrations may mirror outdoor PM_2.5 concentrations in areas with high pollution levels, this may also be the case in a city with very low ambient PM_2.5 concentrations. If a location has low background PM_2.5 concentrations, ambient PM from natural ventilation may dilute indoor PM concentrations. During the study period, there were four days with average daily ambient PM_2.5 concentrations that were lower than the average daily classroom concentrations (see Fig 1). Therefore, natural ventilation with ambient air may have contributed to lower classroom PM concentrations. The presence of a window A/C unit could have also introduced small quantities of outdoor air into the classroom if not sealed properly, or through leaks in the A/C unit. If ambient air that had lower PM_2.5 concentrations was introduced into the classroom, the classroom PM_2.5 concentrations could be diluted, resulting in lower indoor PM2.5 concentrations.

We speculate the dual effect of the air cleaner running on “low” and the fan “on” may result from low background PM concentrations compounded by the combined filtration and air circulation from both the air cleaner and the A/C unit. The fans were located approximately one meter above ground level and therefore could have introduced diluted air into the classroom without disturbing settled particles on the ground. Background PM concentrations in the classrooms may have further influenced the effectiveness of the air cleaners. As detailed above, low concentrations of PM may not be effectively filtered by higher CADR settings due to increased flow rate and potential settling of coarser particles. We were not able to evaluate flow dynamics between the A/C fan, the air cleaner, and the mechanical ventilation systems in each classroom, so it is difficult to say whether the additional air circulation provided by the A/C fan contributed to resuspension of such particles. Additionally, the filter in the A/C unit, even if crude, could have removed some larger particles. Further, the concentrations observed in this study could relate to the condition of the HEPA filters. The air cleaner machines were purchased for the classrooms as a response to the COVID-19 pandemic, however, it is unknown when (or whether) the HEPA filters in the air cleaners were last cleaned or changed. If the filters were clogged with debris, filtration efficiency may have been affected.

Our study was strengthened by use of the same classrooms over a two-week period in the same elementary school. The classrooms were of approximately equal size (area: 140 m² and 152 m²; volume: 427 m³ and 463 m³) and layout, and there was a consistent number of students (14–16) in each classroom over the sampling period (S1 Fig). This limited potential confounding in our data by changes in classroom size and occupancy. The use of a randomized schedule further limited potential sources of unmeasured confounding. Another strength of this study is that we evaluated two fractions of particulate matter to determine whether air quality interventions differed by PM particle size.

In contrast, a major limitation of this study was our inability to evaluate the classrooms under the condition of air cleaner “off” due to COVID-19 and health guidelines, eliminating the possibility to compare the effects of HEPA filter settings to a baseline without the air cleaner. We also did not evaluate the overall school filtration system, which could have influenced PM concentrations in the classrooms and could have attenuated the impacts of the air quality interventions seen in this study. Correction factors calculated from baseline data were applied to two of the aerosol monitors to adjust for differences between the units in measuring PM concentrations, which could have influenced the results.

Furthermore, given the nature of the COVID-19 pandemic and visitor restrictions in place, we were unable to be present in the classrooms during the study period and were not able to gather real-time CADR values or evaluate the sound level of the air purifier and A/C unit at different levels. Sound at different levels was also not evaluated in this study as the primary purpose was to evaluate PM concentrations. Additionally, the teachers were required to operate air purifiers while the classroom was occupied to reduce risk of COVID-19 exposure, however, teachers had the ability to run the purifiers at a variety of settings and we selected only a few settings to study as part of our intervention. It should also be noted that we did not receive noise complaints from the teachers who volunteered their classrooms for the study and the sound was not loud enough to interfere with speaking or listening in the classroom. Even though the use of the air purifier and fan introduces additional noise into the classroom, the devices improve indoor air quality and health. Lastly, despite the classrooms being of approximately equal length, width, and height, the air cleaners in the two classrooms were positioned at different heights (one on the floor and another elevated approximately 0.5 m). However, we were able to include a fixed effect to correct for baseline classroom differences in our mixed model analyses.

Despite the limitations, this study is impactful because it adds to the limited body of literature surrounding PM_2.5 and PM_1.0 concentrations in classroom settings in the United States. This study provides a foundation for future research in the field, especially since the Biden administration initiated a new focus on indoor air quality with an emphasis on schools. Though we did not consider the effects of seasonality given the small sampling period, seasonality should be considered in future studies. Classroom PM concentrations have been shown to vary by season, with higher concentrations of PM occurring in the winter compared to the summer [33]. Occupant behavior (e.g., window opening, time/activity spent indoors) would also vary significantly by season. It would also be worthy to more closely evaluate the impacts of occupancy—the number of teachers and students per classroom—on PM concentrations. Multiple studies have shown that mechanical forces are a primary driver of classroom air quality [34]. Although the teachers were asked to track times of events that may impact PM concentrations (e.g., bus arrivals, times during which all students typically leave the classroom), the data was not sufficient to make any conclusions about the influence of activity on PM concentrations in our study. Therefore, it is important for future studies to look at how the number of occupants per classroom, and student activity, influences PM concentrations and the effectiveness of different air quality interventions.

Our study only involved two classrooms because of availability of equipment and could have been strengthened if more classrooms were evaluated. As a result, the findings from two classrooms in one school limit the generalizability of the study findings even though we included multiple day measurements. Furthermore, to better evaluate the effectiveness of air quality intervention, air cleaner devices should be placed in the same location and height in each classroom to ensure consistency in sampling. Monitoring the CADR levels at different operating levels would help contextualize the impact of using the air cleaners at different settings. It is also important to note that this study was conducted in a non-urban area with low ambient PM concentrations and is not reflective of how these air quality interventions may work in urban settings or areas with higher ambient PM concentrations.

Conclusion

This study demonstrated that the lowest average classroom PM concentrations occurred when the A/C fan was on in the classrooms. The highest concentrations of both PM_2.5 and PM_1.0 concentrations occurred when the filter was used on low without concurrent use of the fan. Therefore, the use of a fan in a non-urban classroom with low background PM concentrations may help to reduce overall classroom PM concentrations. Specifically, within the context of this study for a school with a low background PM_2.5 concentration, the use of an air cleaner on the “low” setting instead of a “high setting” could be used to supplement classroom ventilation and filtration while also lowering operating costs. In settings where ambient air quality is already at an acceptable level, using air cleaners at a higher setting may not be as efficient and may not significantly improve air quality compared to a lower setting.

Supporting information

S1 Fig. Schematic of classroom A and classroom B.

(TIF)

Click here for additional data file.^{(401KB, tif)}

Acknowledgments

The authors would like to thank Sara Gillooly and Jordana Leader for their technical and study design support throughout the duration of this project. Additional support was appreciated from the Superintendent, School Principal, teachers, and staff at the elementary school in RI who made this study possible.

Data Availability

All relevant data are within the paper and its Supporting information files.

Funding Statement

Jahred Liddie, an author on this project, was supported by a Training Grant in Environmental Epidemiology (T32 E007069) from the National Institute of Environmental Health Sciences. Funds from the Department of Environmental Health, Harvard TH Chan School of Public Health and the Harvard Chan NIEHS Center (NIH/NIEHS P30 ES000002) were used to purchase and maintain the equipment used for the study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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PLoS One. doi: 10.1371/journal.pone.0278046.r001

Decision Letter 0

MARIA LUISA ASTOLFI

4 May 2022

PONE-D-22-05922Effects of portable air cleaners and A/C unit fans on classroom concentrations of particulate matter in a non-urban elementary schoolPLOS ONE

Dear Dr. Schiff,

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Furthermore we recommend that the consent statement is revised to indicate that the head teachers of the school provided in parentis loco consent to conduct this study.

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“Jahred Liddie, a co-author, was supported by a Training Grant in Environmental Epidemiology (T32 E007069) from the National Institute of Environmental Health Sciences. No other specific funding was received for this work by any of the co-authors. Funds from the Department of Environmental Health, Harvard TH Chan School of Public Health and the Harvard Chan NIEHS Center (NIH/NIEHS P30 ES000002) were used to purchase and maintain the equipment used for the study.”

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Reviewer #2: Yes

Reviewer #3: Partly

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Abstract:

The structure of the abstract must be changed. Start with the problem you have found that triggers your research and finalize with the objective, methodology and conclusions.

This is the second sentence of the abstract: “Our objective was to determine if use of air cleaners with HEPA filters and air conditioning (A/C) units were associated with changes in particulate matter (PM) air pollution concentrations in a real-world environment.”

Could you change the order and include the sentence as the main goal of the article? What is the method to achieve that goal?

Introduction:

Lines 101-103: “To the best of our knowledge, this is the first study to provide data on the effectiveness of portable air cleaners and fans operating in tandem to reduce PM2.5 and PM1.0 concentrations in occupied classrooms.” This could be the main objective of the article. I suggest not to mention “To the best of our knowledge, this is the first study…” Just state the objective of the article.

Please, unify the format of the tables.

Figure 1. Please explain the equation and the parameter “r”.

Conclusions:

Use some of the numerical results to reinforce the statements.

Include some limitations you might have found in the methodology and future improvements.

All the data stated in the abstract should have comments in the conclusion section:

“The mean half-day concentrations ranged from 3.4 - 4.1 µg/m 2 for PM 2.5 and 3.4 - 3.9 µg/m 2 for PM 1.0 On average, use of the fan decreased PM 2.5 by 0.53 µg/m 3 [95% CI: -0.64, -0.42] and use of the filter on high (compared to low) decreased PM 2.5 by 0.10 µg/m 3 [95% CI: -0.20, 0.005]. For PM 1.0 , use of the fan decreased concentrations by 0.18 µg/m 3 [95% CI: -0.36, -0.01] and use of the filter on high (compared to low) decreased concentrations by 0.38 µg/m 3 [95% CI: -0.55, -0.21].”

Reviewer #2: The manuscript titled ‘Effects of portable air cleaners and A/C unit fans on classroom concentrations of particulate matter in a non-urban elementary school’ documents the effects of air cleaner in the low/high mode and A/C fan in the on/off mode using linear mixed regression models. Interestingly, the manuscript does not discuss potential impacts of students’ activity on classroom PM and on the effects of air cleaner. Because student activity is an important factor affecting classroom PM concentrations and effect of air cleaners on classroom PM, this needs to be discussed in Discussion. The manuscript may also consider the following comments to improve clarity and flow of the manuscript.

Major comments:

1. Lines 44-47: It should be mentioned that the effect of fan was adjusted for air cleaner; likewise, the effect of air cleaner was adjusted for A/C fan.

2. Lines 48-51: General statement (lines 48-49) of the study findings and the conclusion (lines 50-51) don’t seem to agree in the current writing. It looks like that the conclusion should be modified to more specific one because the effects of concurrent use of an A/C fan and air cleaner on reducing the PM were dependent of the mode of the air cleaner and PM size. The proper conclusion would have implication in saving energy.

3. Lines 48-49: Does the ‘additional decreases in PM concentrations’ mean no interaction effect between fan on and air cleaner on high? But the interaction was only significant for PM2.5 but not for PM1.0. If this statement was from the interaction model outputs, it should be also specific to PM size.

4. Lines 79-84: The sentences are about health effects of PM, which has nothing to do with use of air cleaner or improved ventilation that seemed to be the theme for the previous sentences within the paragraph. Thus, it would flow better if these were moved to the next paragraph (after the last sentence of the next paragraph that is describing PM health effect).

5. Wondering if the authors examined interaction between classroom humidity and fan or between humidity and air cleaner in the regression models?

6. Lines 198-205: It seemed that fan effect was bigger for PM2.5 then PM1.0, but air cleaner effect was bigger for PM1.0 than PM2.5. Isn’t this worth to discuss further in Discussion?

7. The title of Table 2 should state ‘Unadjusted ANOVA’ if the ANOVA models were not adjusted for anything.

8. Lines 235-243: The studies discussed are all home studies. Are there any studies of evaluating the effect of air cleaners in classrooms? Unless discussion is strictly limited to HEPA air cleaners, there are some studies examining effects of air cleaners in classrooms (e.g., Wargocki et al., HVAC R Res. 14 (2008): 327-344; Mattsson and Hygge, 2005 Indoor Air Conference Proceedings, pp 1111-1115; Park et al., Building and Environment 167, 2020: 106437), which should be discussed.

9. Lines 256-257: The discussion doesn’t agree with the current study finding. It was reported in Results that the air cleaner on high mode reduced PM1.0 concentrations more than PM2.5. Thus, your argument in this sentence about ‘less effective at removing particles for smaller size PM by air cleaner on high mode’ is not supported by your own finding.

10. Line 268: ‘….some average daily ambient …’. Specify ‘some’ in this sentence because there are only two days when ambient PM2.5 concentrations were lower than classroom PM2.5.

11. Lines 261-273: This paragraph was not clear. What is the main discussion point of the paragraph? The paragraph may need to be modified for clarification.

12. In the limitation section, the small number of classrooms in the study also needs to be mentioned as a limitation. Findings from only two classrooms in one school may not be generalizable although the study has multiple day measurements.

Reviewer #3: Dear Author,

Your manuscript follows a very interesting approach. However, important information is missing to be able to assess the results and evaluate them for a school. For example, a room sketch is missing, as well as information about the ceiling height, with the coordinates for the air purifier, A/C unit and also the measuring points. Furthermore, you do not address the CADR values that are realised at different levels of the air cleaner. The sound level at the different levels is also not mentioned, although this is a critical factor for use in classrooms. The same applies to the A/C unit. I read online that a sound pressure level of 57 dB(A) is generated at the "Low" level, which is clearly too high and would in turn have an impact on the students' ability to concentrate. In my view, when you address the specific case in schools, you have to take these points into account.

From my point of view, a major revision is necessary so that a general gain in knowledge emerges from the manuscript.

**********

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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PLoS One. 2022 Dec 1;17(12):e0278046. doi: 10.1371/journal.pone.0278046.r002

Author response to Decision Letter 0

10 Aug 2022

Response to Editors’ Comments

Furthermore, we recommend that the consent statement is revised to indicate that the head teachers at the school provided in parentis loco consent to conduct this study.

Response: Please see the PDF file “IRB Exemption”, categorized as “other”

3. Thank you for stating the following financial disclosure:

If this statement is not correct you must amend it as needed.

Please include this amended Role of Funder statement in your cover letter; we will change the online submission form on your behalf.

Response: Thank you for this comment. We have added the provided statement to the funding section of the paper and included a Role of Funder statement in the cover letter.

4. Thank you for stating the following in the Acknowledgments Section of your manuscript:

Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement. Currently, your Funding Statement reads as follows:

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

Response: These statements were moved to the funding section.

We will update your Data Availability statement to reflect the information you provide in your cover letter.

Response: All the data necessary for the analyses have been uploaded. There are three datasets.

Comments to the Author

Reviewer #1: Abstract:

The structure of the abstract must be changed. Start with the problem you have found that triggers your research and finalize with the objective, methodology and conclusions.

Could you change the order and include the sentence as the main goal of the article? What is the method to achieve that goal?

Response:

Abstract (Reorganized)

Given the increased useing of air cleaners in classrooms during the COVID-19 pandemic as a prevention measure, this study aimed to investigate the the effects of portable air cleaners with HEPA filters and window A/C fans on real-time (1 minute) concentrations of PM less than 2.5 microns (PM2.5) or less than 1 microns (PM1.0) in two classrooms in a non-urban elementary school in Rhode Island. For half of each school day, settings were randomized to “high” or “low” for the air cleaner and “on” or “off” for the fan. Descriptive statistics and linear mixed models were used to evaluate the impacts of each set of conditions on PM2.5 and PM1.0 concentrations. The mean half-day concentrations ranged from 3.4 - 4.1 µg/m3 for PM2.5 and 3.4 - 3.9 µg/m3 for PM1.0 On average, use of the fan alone decreased PM2.5 by 0.53 µg/m3 [95% CI: -0.64, -0.42] and use of the filter on high (compared to low) alone decreased PM2.5 by 0.10 µg/m3 [95% CI: -0.20, 0.005]. For PM1.0, use of the fan alone decreased concentrations by 0.18 µg/m3 [95% CI: -0.36, -0.01] and use of the filter on high (compared to low) alone decreased concentrations by 0.38 µg/m3 [95% CI: -0.55, -0.21]. In general, simultaneous use of the fan and filter on high did not result in additional decreases in PM concentrations compared to the simple addition of each appliance’s individual effect estimates. use of either appliance individually. Our study suggests that concurrent or separate use of an A/C fan and air cleaner in non-urban classrooms with low background PM may reduce classroom PM concentrations.

Introduction:

Response: The objective of this study is to provide data on the effectiveness of portable air cleaners and fans operating in tandem to reduce PM2.5 and PM1.0 concentrations in occupied classrooms.

Please, unify the format of the tables.

Response: We have adjusted the formatting and file types of the tables to be unified.

Figure 1. Please explain the equation and the parameter “r”.

Response: We have added a caption within the figure describing both these components.

Conclusions:

Use some of the numerical results to reinforce the statements.

Response: Please see the tracked changes document that demonstrate the use of numerical results to reinforce the statements.

Include some limitations you might have found in the methodology and future improvements.

Response (bold are the changes): This study adds to the limited body of literature surrounding PM2.5 and PM1.0 concentrations in classroom settings in the United States and provides a foundation for future research in the field. Though we did not consider the effects of seasonality given the small sampling period, seasonality should be considered in future studies. Classroom PM concentrations have been shown to vary by season, with higher concentrations of PM occurring in the winter compared to the summer [32]. Occupant behavior (e.g., window opening) would also vary significantly by season. It would also be worthy to more closely evaluate the impacts of occupancy—the number of teachers and students per classroom—on PM concentrations. Multiple studies have shown that mechanical forces are a primary driver of classroom air quality, therefore it is important to look at how the number of students per classroom influences PM concentrations and the effectiveness of different air quality interventions [32]. This study only involved two classrooms as a result of availability of equipment and could have been strengthened if more classrooms were evaluated. As a result, the findings from two classrooms in one school limit the generalizability of the study findings even though the study has multiple day measurements.. Furthermore, to better evaluate the effectiveness of air quality intervention, the air quality device should be placed in the same location and at the height in each classroom to ensure consistency in sampling method. Monitoring the CADR levels at different air quality device operating levels would help contextualize the impact of using the air quality devices at different settings. It is also important to note that this study was conducted in a non-urban area with low ambient PM concentrations and is not reflective of how these air quality interventions may work in urban settings or areas with higher ambient PM concentrations.

All the data stated in the abstract should have comments in the conclusion section:

Response: Thank you - we have added this to the conclusions section.

Reviewer #2:

The manuscript titled ‘Effects of portable air cleaners and A/C unit fans on classroom concentrations of particulate matter in a non-urban elementary school’ documents the effects of air cleaner in the low/high mode and A/C fan in the on/off mode using linear mixed regression models. Interestingly, the manuscript does not discuss potential impacts of students’ activity on classroom PM and on the effects of air cleaner. Because student activity is an important factor affecting classroom PM concentrations and effect of air cleaners on classroom PM, this needs to be discussed in Discussion. The manuscript may also consider the following comments to improve clarity and flow of the manuscript.

Response: Thank you – we have added not being able to record student activity (due to COVID-19 limitations at the time) as a limitation to the study, which can be found in the Discussion. It should be noted that we asked the teachers to track times of events that may impact PM concentrations (e.g. bus arrivals, times during which all students typically leave the classroom), but the data was not sufficient to make any conclusions.

Major comments:

1. Lines 44-47: It should be mentioned that the effect of fan was adjusted for air cleaner; likewise, the effect of air cleaner was adjusted for A/C fan.

Response: Thank you – please see lines 40-46 in the tracked changes document to see the clarification on the adjustments for air clear and the A/C fan.

Response: Simultaneous use of the fan and air cleaner did not result in significant, additional decreases in PM2.5 - as stated in the results section, the interaction term was actually significantly positive. Regarding PM1.0, no significant differences associated with simultaneous use of the air cleaner and fan were found. We have updated the phrasing of the results in the Abstract as well as in the Results section to better reflect this finding.

Response: Thank you for this comment. As clarification and as described in the Result section, the interaction term was significantly for PM2.5 and null for PM1.0. This indicated that concurrent use of the appliances, in our study setting, did not result in significantly lower PM concentrations for either size fraction below the simple addition of each separate effect estimates. In line with previous comment, we have rephrased the results description in the Abstract and Result sections to reflect this more clearly.

Response: Given the challenges of installing new HVAC systems or filters in school buildings, portable air cleaners and fans have become popular, less expensive alternatives to reduce PM concentrations. Air cleaners with high-efficiency particulate air (HEPA) filters, which are at least 99.97% efficient at capturing particles 0.3 µm and larger in size, offer the possibility of substantially reducing SARS-CoV-2 viral particles as well as improving overall IAQ [7,5]. Beyond the current context of the COVID-19 pandemic, improved air ventilation is associated with lower school absenteeism, better performance on cognitive function tests, and fewer respiratory symptoms, such as those related to asthma, lung inflammation, and allergies [9, 10, 11]. PM2.5 exposure in children has been associated with asthma incidence (OR=1.10, 95% CI:1.01, 1.20), prevalence of asthma symptoms (OR=1.08, 95% CI:1.02, 1.16), and rhinitis (OR=1.15, 95% CI: 1.05,1.26) [12]. A review study of 33 articles further provided evidence that exposure to particulate air pollution has adverse impacts on children’s respiratory health, with stronger negative effects seen among children with asthma [13].

PM is one of the most common pollutants that could potentially degrade air quality in classrooms [14]. Indoor PM levels are influenced by several factors, such as ambient air pollution levels, air exchange rates, occupancy, type and intensity of indoor activities, and particle sizes [14, 15, 16]. One study in Munich, Germany found that PM concentrations in classrooms are about six times higher than outdoor concentrations [17]. Children are particularly vulnerable to potential health consequences related to PM exposure due to their immature respiratory and immune systems and greater breathing rates per body weight [11]. Beyond the current context of the COVID-19 pandemic, improved air ventilation is associated with lower school absenteeism, better performance on cognitive function tests, and fewer respiratory symptoms, such as those related to asthma, lung inflammation, and allergies [9, 10, 11]. PM2.5 exposure in children has been associated with asthma incidence (OR=1.10, 95% CI:1.01, 1.20), prevalence of asthma symptoms (OR=1.08, 95% CI:1.02, 1.16), and rhinitis (OR=1.15, 95% CI: 1.05,1.26) [12]. A review study of 33 articles further provided evidence that exposure to particulate air pollution has adverse impacts on children’s respiratory health, with stronger negative effects seen among children with asthma [13].

5. Wondering if the authors examined interaction between classroom humidity and fan or between humidity and air cleaner in the regression models?

Response: We tried this and a model with a three-way interaction - generally, the main effects are almost identical, although there are some significant but small interactions with humidity that vary for the size fractions. However, we decided not to include the result due the difficulty to interpret the impact and the concern that if may distract from results.

6. Lines 198-205: It seemed that fan effect was bigger for PM2.5 then PM1.0, but air cleaner effect was bigger for PM1.0 than PM2.5. Isn’t this worth to discuss further in Discussion?

Response: Thank you - this point has been addressed in the Discussion.

7. The title of Table 2 should state ‘Unadjusted ANOVA’ if the ANOVA models were not adjusted for anything.

Response: Thank you - we have added this distinction.

Response: The studies discussed were limited to HEPA air cleaners. We discussed one classroom study that also used HEPA air cleaners, but acknowledged a general lack of existing literature. However, we have cited more school studies elsewhere in the Discussion, including a few mentioned by the reviewer.

Response: Thank you for this comment. We went to this paragraph and revised our language. We had incorrectly discussed particle size instead of particle concentration since our study was in a classroom that had very low PM concentrations. We also added additional context. Please see lines 365- 398 (in track changes document).

10. Line 268: ‘….some average daily ambient …’. Specify ‘some’ in this sentence because there are only two days when ambient PM2.5 concentrations were lower than classroom PM2.5.

Response: Thank you - we have added this clarification.

11. Lines 261-273: This paragraph was not clear. What is the main discussion point of the paragraph? The paragraph may need to be modified for clarification.

Response: Thank you – The main discussion of the paragraph was to highlight the fact that the study was conducted in a location that has very low ambient PM concentrations and that those low ambient concentrations may have impacted classroom PM concentrations. We edited the paragraph starting at line 404-422 (in the track changes document) to emphasize the low ambient PM concentrations.

Reviewer #3: Dear Author,

Your manuscript follows a very interesting approach. However, important information is missing to be able to assess the results and evaluate them for a school. For example, a room sketch is missing, as well as information about the ceiling height, with the coordinates for the air purifier, A/C unit and also the measuring points. Furthermore, you do not address the CADR values that are realized at different levels of the air cleaner. The sound level at the different levels is also not mentioned, although this is a critical factor for use in classrooms. The same applies to the A/C unit. I read online that a sound pressure level of 57 dB(A) is generated at the "Low" level, which is clearly too high and would in turn have an impact on the students' ability to concentrate. In my view, when you address the specific case in schools, you have to take these points into account.

Response: We added sound level as a limitation in our discussion. We were concerned about the impact of the sound in the classroom; however, the air purifiers and A/C fans were not so loud and did not interfere with the ability to teach in the classroom. Furthermore, internet research indicates that 60 dB is conversation volume. We currently do not have a sketch of the classrooms but we included dimensions of the classrooms and location of the air purifiers in the classroom in the methods section.

Attachment

Submitted filename: Revisions Classroom Air Cleaners 8_10_2022.docx

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PLoS One. doi: 10.1371/journal.pone.0278046.r003

Decision Letter 1

MARIA LUISA ASTOLFI

6 Sep 2022

PONE-D-22-05922R1Effects of portable air cleaners and A/C unit fans on classroom concentrations of particulate matter in a non-urban elementary schoolPLOS ONE

Dear Dr. Schiff,

ACADEMIC EDITOR: The authors improved the manuscript by addressing most of the reviewers' comments. However, some further corrections are required prior acceptance. I ask the authors to respond in detail to Reviewer 3's comments.

Please submit your revised manuscript by Oct 21 2022 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

We look forward to receiving your revised manuscript.

Kind regards,

MARIA LUISA ASTOLFI, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Partly

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Reviewer #1: The authors have adequately addressed the previous comments. Relevant results are stated in the discussion section, not in the conclusion section.

Reviewer #2: I appreciate authors' efforts to address all the comments that I made. I have no further comment and thus believe that the current manuscript is ready for publication.

Reviewer #3: Dear Authors,

Unfortunately, my points were not fully considered.

Noise level: A sound pressure level of 60 dB(A) in a school is totally unacceptable. The American National Standards Institute recommends a maximum level of 35 dB(A) in classrooms, the same applies to the guidelines in Europe, most of which also consider a maximum value of 35 dB(A) to be acceptable, although maximum values of up to 40 dB(A) can be found in guidelines.

I consider this point critical for classrooms, since the use of such devices requires the acceptance of teachers and students. Especially in classrooms, guidelines on noise levels should be discussed.

The measurement setup is not yet sufficiently described.

Nevertheless, I find the approach of this work good and the investigations carried out interesting. The presentation of the results is also very good. The description of the measurements and their structure, which would be important for a deeper understanding and classification, is still missing.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

**********

PLoS One. 2022 Dec 1;17(12):e0278046. doi: 10.1371/journal.pone.0278046.r004

Author response to Decision Letter 1

21 Oct 2022

Response to Editors’ Comments (October 2022)

*** Please see page 6 for the previous responses to the reviewers’ comments that were addressed in the first round of revisions and were accepted and see page 2-3 (response highlighted in green) for our response to the reviewers’ comment that needed to be revised after our prior submission

Comments to the Author

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: (No Response)

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Partly

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

6. Review Comments to the Author

Reviewer #1: The authors have adequately addressed the previous comments. Relevant results are stated in the discussion section, not in the conclusion section.

Reviewer #2: I appreciate authors' efforts to address all the comments that I made. I have no further comment and thus believe that the current manuscript is ready for publication.

Reviewer #3: Dear Authors,

Unfortunately, my points were not fully considered.

The measurement setup is not yet sufficiently described.

Response: We added sound level as a limitation in our discussion. We were concerned about the impact of the sound in the classroom; however, the air purifiers and A/C fans were required to be used in the classrooms for health concerns during the COVID-19 pandemic. Teachers were required to have the air purifiers on but teachers had the ability to choose which settings they wanted to use in their classrooms; some used higher settings than others. During our study we varied the treatment settings but these did not include the air purifier setting ‘off” nor did they include the highest air purifier setting. Furthermore, we asked teachers to report on the sound level throughout the study period and to report if the sound level interfered with their ability to teach in the classrooms. We did not receive any reports of noise interfering with the ability to teach in the classroom. Overall, the purpose of the study was to evaluate the impact of various air purifier and fan settings in a classroom setting on particulate matter concentrations and we did not focus on other environmental exposures, although sound is a consideration especially in schools. The use of air purifiers in the classroom has health benefits even though noise can be a concern, however, in our study noise complaints were not received.

Furthermore, research indicates that 60 dB is conversation volume. According to our research, the CDC notes that 60 dBA is a normal conversation/AC level and that sounds at these levels do not typically cause hearing damage. The CDC further states that 30 dBA is equivalent to a whisper. We also investigated the air purifier used in the classroom and at its maximum level (there are four levels, 1-4) the sound would be 70 dBA, however, in our study the maximum level used was level 3, so less than 70dBA. 70 dBA is the level the CDC notes as causing annoyance by noise, but not hearing damage. We acknowledge that 35 dBA is a recommendation for classrooms, however, it is not a requirement for classrooms and was not realistic in the context of COVID-19 and the requirement for classrooms to use air purifiers.

We also provided a classroom schematic in the supplementary material, depicting the layout and sizes of the classroom. The schematic also includes dimensions of the classrooms and location of the air purifiers in the classroom in the methods section. Dimensions and description of air purifier and air monitor locations in the classrooms can be found in the methods section of the manuscript.

CDC Reference:

https://www.cdc.gov/nceh/hearing_loss/what_noises_cause_hearing_loss.html

Air Purifier Model Technical Specifications (see image below):

https://medifyair.com/collections/air-purifiers/products/ma-112#specs

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

Response to Editors’ Comments (August 2022)