Journal Articles: 

Absenteeism

Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schools (Mendell et al. 2013) 

Summary: Investigated the relationship between ventilation rate and illness absence over two school years in 162 classrooms in 28 schools in three school districts in California. The study found a 3.4% decrease in illness absence from increasing the ventilation rates from 4 L/s-person (8.5 cfm/person) to 7.1 L/s-person (15 cfm/person). This decrease in illness absence corresponds to a $33 million increase in attendance-linked funding to a school district. 

Cost Effectiveness:

• Increasing from 4 L/s-person to 7.1 L/s-person would increase annual energy costs by $4 million, but yield benefits by increasing attendance linked funding to schools by $33 million and decreasing family costs from caregiving by $80 million, adding to a total benefit of $113 million across a school district.
• Increasing from 4 L/s-person to 9.4 L/s-person (20 cfm/person) would increase energy costs by $7.3 million, but increase attendance-linked funding by $66 million and decrease family costs by $160 million, adding to a total benefit of $226 million across a school district.

The Effect of Air Purifiers in Schools (Bonan et al. 2025)

Summary: Air purifiers were randomly assigned to 95 classrooms in 5 primary schools across Milan to evaluate the impact of portable air purifiers on indoor air quality and student outcomes. The intervention reduced indoor PM_2.5 concentration by 32% and decreased student absenteeism by 12.5% (1.34 fewer missed days per student per year). The intervention cost $12 per avoided absence day and yielded a cost-benefit ratio of 1:10. 

Cost Effectiveness:

• The cost of each air purifier was €3,070 with a 10 year lifespan (€307 or $350 per year). The average classroom size was 21.6 students, meaning the cost of each air purifier was €14.20 or $16 per student per year. 
• The addition of air purifiers resulted in 1.34 fewer missed days per student per year meaning that the cost per avoided absence day was €10.60 or $12 per student. The societal cost of a single school day missed due to influenza is estimated to be €102 or $116 (includes costs related to childcare, lost parental productivity, and healthcare). This yields a benefit-cost ratio of 1:9.6.

Student Performance

Providing better thermal and air quality conditions in school classrooms would be cost-effective (Wargocki and Wyon 2012)

Summary: This study synthesizes research on the effects of increasing ventilation rate and optimizing temperature in classrooms in Denmark and Sweden. The study finds that ventilation rates in many schools are below recommended levels, leading to poor indoor air quality and elevated temperatures in classrooms. Evidence shows that doubling ventilation rates increased task performance (measured by speed) by 14%. 

Cost Effectiveness: 

• The total cost of improving indoor environmental conditions in schools is less than $1 per pupil per day, while delivering large learning benefits.

Air Filters, Pollution, and Student Achievement (Gilraine 2020 and Gilraine 2025)

Summary: After the Aliso Canyon gas leak, the largest gas leak in U.S. history, Southern California Gas Company placed air filters in every school within a 5-mile radius of the leak. Air filters were delivered to 18 schools in the region. At an annual cost of ~$1020 per classroom (~$33 per student), the presence of air filters was associated with an increase in math scores by 0.20 standard deviations and English scores by 0.19 standard deviations (equivalent to an 8% increase in standardized test scores).

Cost Effectiveness:

• Installing and maintaining air filters costs approximately $1,000 per classroom per year, yet yields large improvements in academic performance that are substantially more cost-effective than many traditional education interventions (e.g., class size reduction, tutoring, or early childhood programs). 
• For example, based on this study, a 0.20 standard deviation (8%) increase in test scores can be achieved by decreasing class size by 7 students, which would require on average hiring an additional teacher for every 2 classrooms in a school. Hiring additional teachers costs on average about $7,000 per student, making air filters ~210 times more cost-effective. 

The Ventilation Problem In Schools: Literature Review (Fisk 2017)

Summary: Literature review that summarizes 28 studies related to ventilation in schools. The review shows that classroom ventilation rates are frequently below recommended standards and higher ventilation rates are linked to better academic performance, reduced respiratory symptoms, and lower student absenteeism. 

Cost Effectiveness:

• Estimates that improving ventilation in schools costs up to $10 per student, which is less than 0.1% of typical public spending on elementary and secondary education in the United States, while delivering measurable improvements in health, attendance, and academic performance. 

Indoor air quality and academic performance (Stafford 2014)

Summary: Indoor air quality related renovations (mold remediation, ventilation improvements, roof repairs) were conducted in 66 out of 74 elementary schools in a Texas school district. Improving ventilation and removing mold in schools led to meaningful gains in English and math scores as well as pass rates. 

Cost Effectiveness:

• Mold remediation: Costs ~$500,000 per project. Resulted in 0.15 standard deviation (6%) increase in math test scores, 0.14 standard deviation (5.6%) increase in English test scores.
• Ventilation: Costs ~$300,000 per project. Resulted in 0.07 standard deviation (2.8%) increase in math test scores, 0.11 standard deviation (4.4%) increase in English scores. 

The Long-Run Economic Consequences of High-Stakes Examinations: Evidence from Transitory Variation in Pollution (Ebenstein et al. 2016) 

Summary: Study explores how variations in PM_2.5 exposure during high-stakes matriculation exams in Israel affected student performance and long-term outcomes. The study found that short-term exposure to air pollution during exam days significantly reduces test scores, and that these effects persist over time, leading to lower rates of postsecondary enrollment and reducing earnings a decade later. 

Cost Effectiveness:

• Every ~11 µg/m³ increase in PM_2.5 (about 1 standard deviation in this sample) reduces test scores by 0.04 standard deviations (0.93-point decline in exam scores). 
• Long-term outcomes show that an additional ~11 µg/m³ of PM_2.5 was associated with 0.15 fewer years of university education and ~$35 less in monthly earnings a decade later. 

Reducing Indoor Particulate Air Pollution Improves Student Test Scores: A Randomized Double-Blind Crossover Study (Xu et al. 2024) 

Summary: A randomized, double-blind crossover trial among 162 college students in Tianjin, China to test whether reducing indoor particulate matter improves academic performance. Students took standardized English tests on two consecutive weekends in classrooms equipped with either active air purification or sham filtration, with the intervention switched between rooms across test days. Air purification reduced  PM_2.5 from ~40 µg/m³ to ~13 µg/m³ and was associated with a significant increase in combined English test scores. 

Cost Effectiveness:

• A ~27 µg/m³ reduction in PM_2.5 from air purification increased test scores by 0.11 standard deviations (~4% improvement). 

Health – Respiratory Disease

Mask Use and Ventilation Improvements to Reduce COVID-19 Incidence in Elementary Schools - Georgia, November 16-December 11, 2020 (Gettings et al. 2021)

Summary: Investigated the association of mask requirements and ventilation improvements with COVID-19 incidence in 169 elementary schools in Georgia during November 16–December 11, 2020. The study found that improved ventilation was associated with a 39% lower COVID-19 incidence. Of the 123 schools that made improvements in ventilation, COVID-19 incidence was 35% lower in schools that used dilution only (opening doors, windows, using fans) and 48% lower in schools that combined dilution with filtration, with or without ultraviolet germicidal irradiation. 

Cost Effectiveness: 

• Improvements in ventilation through low or no cost dilution methods (opening doors, opening windows, or using fans) were associated with a 35% lower incidence of COVID-19, showing that relatively simple interventions can reduce respiratory pathogen transmission. 

Health – Asthma

Effect of School Integrated Pest Management or Classroom Air Filter Purifiers on Asthma Symptoms in Students With Active Asthma: A Randomized Clinical Trial (Phipatanakul et al. 2021)

Summary: The largest school-based RCT of HEPA filtration for asthma. Conducted across 41 urban elementary schools in the northeastern US (2015–2020), 236 students with active asthma were randomized to classrooms with active HEPA purifiers or identical sham units, with outcomes assessed every 2 months over a 10-month school year. HEPA filtration did not significantly reduce asthma symptom-days. The authors noted that 98% of classrooms had detectable mouse allergen at baseline, suggesting that in schools where indoor allergens—rather than airborne particles—are the dominant trigger, HEPA filtration alone may be insufficient.

HEPA Filtration Improves Asthma Control in Children Exposed to Traffic-Related Airborne Particles (James et al. 2020)

Summary: A double-blind, placebo-controlled crossover RCT of 43 children with asthma living near high-traffic areas in Cincinnati. A HEPA air cleaner or sham unit was placed in the home for four weeks each. Among children with poorly controlled asthma at baseline, HEPA filtration significantly improved asthma control scores (ACQ: 1.3 → 0.9, p = 0.003) and quality of life. Note: this was a home-based, not school-based, intervention.

Cost-Effectiveness of School Integrated Pest Management and Air Filtration in Students with Asthma (Socolovsky et al. 2025)

Summary: Cost-effectiveness analysis based on the SICAS-2 randomized trial (Phipatanakul et al. 2021), which tested school-based integrated pest management (IPM), classroom HEPA filtration, and a combination of both among students with asthma across 41 urban elementary schools. Socolovsky et al. conducted the cost-effectiveness analysis on 154 of those participants. In a post-hoc analysis of the trial, IPM reduced asthma symptoms by 63% in fall/winter and significantly reduced missed school days; HEPA filtration alone did not significantly improve asthma outcomes. 

Cost Effectiveness:

• Per-student cost: $4.94 for HEPA alone, $7.27 for IPM, and $12.21 for IPM + HEPA combined. The combined strategy had an incremental cost-effectiveness ratio of $19,667 per Quality-Adjusted Life Year (QALY) — well below the conventional U.S. willingness-to-pay threshold of $50,000–$100,000 per QALY.
• The HEPA intervention was dominant in terms of QALY gains (0.423 QALYs) given its low cost.

Additional Studies:

Classroom air quality in a randomized crossover trial with portable HEPA air cleaners (Simona et al. 2025) 

Summary: A large block randomized crossover trial across 350 classrooms in 17 elementary schools in Los Angeles County to evaluate the effect of portable HEPA air cleaners on indoor air quality. Classrooms receiving HEPA filtration experienced substantial reductions in particulate matter, with approximately 40% lower average annual PM_2.5 concentrations compared to control classrooms. 

Cost Effectiveness:

• HEPA air cleaners are an effective, scalable strategy for reducing particulate matter, even in classrooms with existing HVAC air filtration. 

Coupled indoor air quality and dynamic thermal modelling to assess the potential impacts of standalone HEPA filter units in classrooms (Burridge et al. 2024)

Summary: A coupled indoor air quality and dynamic thermal modelling study assessed the impacts of standalone HEPA filter units in naturally ventilated UK classrooms representative of both modern and Victorian-era construction.

Cost Effectiveness:

• HEPA filters reduced indoor PM_2.5 by 40–60% and airborne viral RNA by 30–50%, while the energy cost of operating the filters was only 1–2% of classroom heating costs.

Other Reports and Resources:

• Johns Hopkins Center for Health Security: School Ventilation: A Vital Tool to Reduce COVID-19 Spread
• U.S. Green Building Council Center for Green Schools: School Research Highlights: PM_2.5 (plus the School Health Research Library); Boston Public Schools Case Study
• Center for American Progress: School Air Filtration and Ventilation Strategies To Improve Health, Education, Equity, and Environmental Outcomes
• ASHRAE: Design Guidance for Education Facilities: Prioritization for Advanced Indoor Air Quality
• Carrier: Achieving Cost-Effective Indoor Air Quality Improvements In Schools
• Colorado Department of Public Health and Environment: Clean Air for Schools
  • Summary: Clean Air for Schools campaign across Colorado plans to provide Colorado K-12 classrooms with HEPA portable air cleaners and air quality monitors. It is estimated reducing indoor air pollution with PACs would cost $65/student/classroom/year – “cheaper than a textbook”
• Technical University of Denmark: Socio-Economic Consequences Of Better Air Quality In Primary Schools

Limitations and Evidence Gaps

Readers should interpret the findings in this summary with the following considerations:

• Study design limitations. Most studies summarized here are observational, quasi-experimental, or based on natural experiments rather than large-scale randomized controlled trials. Causal inference is therefore limited in several cases, and findings should be interpreted accordingly.
• Context-dependence of effect sizes. The magnitude of benefits reported across studies varies considerably by region, baseline pollution levels, building age and construction type, ventilation starting point, and student population. For example, Bonan et al. (2025) found that the absenteeism benefit of air purifiers was no longer statistically significant when outdoor PM2.5 exceeded 10 μg/m³, suggesting diminishing returns in highly polluted environments. Studies conducted in high-pollution cities (Milan, Tianjin, Los Angeles) or under exceptional circumstances (a pandemic) may not be directly generalizable to lower-pollution settings.
• Costs and cost projections. Cost estimates cited across studies reflect specific hardware, labor, and energy markets at the time of each study and may not reflect current procurement costs in all districts. Cost projections also vary depending on assumptions about school size, occupancy, and ventilation starting conditions.
• Publication and selection bias. As with any literature summary focused on a specific intervention, studies reporting null or negative results are less likely to be published and may therefore be underrepresented here. The evidence base should be understood as reflecting positive findings in the literature, not necessarily the full distribution of results from all interventions attempted.

Future Research Needs   

Priority areas for future research include:  

• Conduct cluster-randomized trials that demonstrate health protection from deploying clean indoor air technologies in schools. 
• Develop cost-benefit analysis frameworks that measure costs and benefits of deploying clean indoor air technologies in schools. Costs include installation costs, ongoing maintenance costs, and deployment risks, and benefits such as fewer infections, reduced absenteeism, and lower healthcare costs.
• Create deployment guidance that will give school districts and other implementers best practices for adopting clean indoor air technologies.