The air inside most school buildings is worse than the air outside, and in many classrooms it is bad enough to affect how well students think, how often they get sick, and how much school they miss. A mix of stale exhaled air, fine particles, cleaning-product fumes, and sometimes mold creates an invisible burden on the millions of children and staff who spend six or more hours a day in these spaces. The science connecting classroom air to health and learning outcomes has grown substantially over the past decade, and the gap between what researchers recommend and what most school buildings actually deliver remains wide.
How Stale Air Affects Thinking
Carbon dioxide is harmless at low concentrations, but it serves as a useful proxy for how much fresh air a room is getting. When a classroom full of students breathes without adequate ventilation, COâ‚‚ levels climb quickly, often reaching 1,500 parts per million or higher within a single class period. Outdoor air sits around 420 ppm. A cluster-randomized crossover study in European classrooms found that while elevated COâ‚‚ did not reduce the total amount of work students completed on concentration tests, it significantly increased their error rate.
1PubMed. Effect of classroom air quality on students’ concentration: results of a cluster-randomized cross-over experimental studyMore recent work at a university tracked cognitive test scores alongside real-time COâ‚‚ measurements across a range of roughly 440 to 1,630 ppm. Higher peak COâ‚‚ concentrations during class were consistently associated with lower scores, with the strongest effects appearing at the highest spikes rather than the average level over the session.
2Journal of Exposure Science & Environmental Epidemiology. Associations between indoor air exposures and cognitive test scores among university students in classrooms with increased ventilation rates for COVID-19 risk managementThe practical takeaway is not that COâ‚‚ itself is toxic at these levels. It is that high COâ‚‚ signals inadequate ventilation, meaning every other pollutant in the room is also building up. The cognitive effects researchers measure are likely driven by the combined cocktail of stale air, not COâ‚‚ alone.
More Fresh Air, Fewer Sick Days
One of the clearest and most consistent findings in this field is the link between ventilation rates and student absenteeism. A prospective study across California elementary schools found that for each additional liter per second of outdoor air supplied per student, illness-related absence fell by about 1.6%. The authors estimated that bringing California classrooms from their average ventilation rate up to the state standard would cut illness absence by roughly 3.4% and increase attendance-linked school funding by about $33 million a year, at a cost of only $4 million.
3PubMed Central. Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schoolsA separate study in Midwestern elementary schools confirmed the direction of the effect: each one-liter-per-second-per-person increase in ventilation was associated with about 5.6 fewer days with absences per year across the monitored classrooms.
4PubMed. Associations between illness-related absences and ventilation and indoor PM(2.5) in elementary schools of the Midwestern United StatesThe mechanism is straightforward. Better ventilation dilutes airborne pathogens. A scoping review of ventilation and infection risk in schools found that mechanically ventilated classrooms reduced the relative risk of SARS-CoV-2 infection by at least 74% compared with naturally ventilated rooms, and at higher airflow rates the reduction reached 80% or more.
5PubMed Central. The Influence of Ventilation Measures on the Airborne Risk of Infection in Schools: A Scoping ReviewWhere the Particles Come From
Fine particulate matter, the tiny particles collectively called PMâ‚‚.â‚…, is especially relevant for children because their lungs are still developing and they breathe more air relative to their body weight than adults do. Research in inner-city schools identified secondary pollution and motor-vehicle emissions as the largest outdoor contributors to classroom PMâ‚‚.â‚…, together accounting for close to 60% of total indoor concentrations. The remaining share came from calcium-rich particles, biomass burning, soil dust, and marine aerosols, depending on geography and season.
6PubMed Central. Classroom indoor PM 2.5 sources and exposures in inner-city schoolsIndoor sources matter too. A study of primary schools estimated that about 47% of classroom PMâ‚‚.â‚… was generated indoors, mainly from continuous resuspension of soil particles and a mixed category of skin flakes, clothing fibers, and chalk dust. Schools with unpaved playgrounds saw mineral dust contributions jump by about 5 to 6 micrograms per cubic meter compared with those that had paved surfaces.
7PubMed. Sources of indoor and outdoor PM2.5 concentrations in primary schoolsChalk use is a surprisingly significant factor. Research comparing schools that still use chalkboards with those that have switched to whiteboards found that chalk-based classrooms had higher net indoor PMâ‚‚.â‚… emission rates. Public schools using chalkboards generated indoor particle emissions more than 2.5 times higher than private schools using the same medium, likely reflecting differences in classroom density, cleaning frequency, and ventilation.
8Atmospheric Pollution Research. Quantifying indoor PM2.5 levels and its sources in schools: What role does location, chalk use, and socioeconomic equity play ?Cleaning Products and Chemical Fumes
Volatile organic compounds are chemicals that evaporate at room temperature and accumulate indoors. In schools, the biggest indoor source is cleaning products. A study across urban schools found that cleaning products contributed 41% of total indoor VOC concentrations, exceeding the share from vehicle exhaust outdoors. Art and craft activities and air fresheners made up much of the remaining indoor contribution.
9Atmospheric Environment. Volatile Organic Compounds: Characteristics, distribution and sources in urban schoolsPassive sampling in schools confirmed this pattern, with conventional schools showing VOC concentrations tied to cleaning chemicals, while vocational schools had profiles reflecting their workshop activities.
10PubMed. Presence of emerging organic contaminants and solvents in schools using passive samplingAverage total VOC levels in monitored classrooms in southern Italy were generally below guideline thresholds, but cleaning activities and high occupancy created spikes well above levels considered harmless for children and staff.
11Air Quality, Atmosphere & Health. Outdoor trends and indoor investigations of volatile organic compounds in two high schools of southern ItalyThe practical implication is that switching to low-VOC cleaning products, scheduling heavy cleaning for evenings when classrooms are unoccupied, and storing chemicals in ventilated closets rather than inside classrooms can dramatically reduce exposure without costing much.
Mold, Moisture, and Respiratory Problems
Damp buildings are a known trigger for respiratory symptoms, and schools are no exception. In Finnish schools with documented moisture problems, children had significantly more wheezing (16% versus 6%) and cough (21% versus 9%) than children in control buildings.
12Acta Paediatrica. Asthma and respiratory infections in school children with special reference to moisture and mold problems in the schoolWhen moisture problems existed at both the child’s home and their school, the combined effect was striking: the frequency of asthma reached 21%, and the combined frequency of asthma and wheezing reached 43%.
13PubMed. Moisture and mould problems in schools and respiratory manifestations in schoolchildren: clinical and skin test findingsMold and bacterial endotoxin levels in many classrooms are high enough to cause symptoms in allergic occupants, and the data suggest that even children without a prior asthma diagnosis may develop chronic cough or recurrent wheezing from prolonged exposure.
14Reviews in Medical Microbiology. The role of airborne microbes in school and its impact on asthma, allergy, and respiratory symptoms among school childrenTemperature, Humidity, and Their Interaction with Air Quality
Air quality does not exist in isolation from thermal comfort. Warmer classrooms cause students to exhale more COâ‚‚ and shed more skin cells, raising both pollutant concentrations and perceived stuffiness. Field measurements in classrooms showed that at an indoor temperature of 27°C, visitors entering the room rated odor intensity significantly higher and air-quality acceptability significantly lower than at 24°C. The researchers calculated that students’ COâ‚‚ emission rate increased by about 0.54 liters per hour per person for every one-degree rise in temperature, meaning warmer rooms need proportionally more ventilation just to maintain the same air quality.
15Building and Environment. Association of indoor temperature and air quality in classrooms based on field and intervention measurementsHumidity matters as well. In controlled experiments with undergraduates, performance peaked at around 40% relative humidity and 24°C. At 20% relative humidity, fatigue increased by about 23%, reading speed dropped by roughly 12%, and distraction rose by about 61% compared to the 40% condition. The researchers noted that low humidity dries the eyes and airways, creating discomfort that draws attention away from the task at hand.
16Case Studies in Thermal Engineering. Influence of indoor air temperature and relative humidity on learning performance of undergraduatesWhat Filtration Can Actually Do
When improving ventilation is impractical because of building age, budget, or outdoor pollution, filtration is the main alternative. Portable HEPA air cleaners deployed in classrooms during a randomized crossover trial reduced average PMâ‚‚.â‚… levels by about 40% compared with control classrooms over an entire school year.
17Journal of Exposure Science & Environmental Epidemiology. Classroom air quality in a randomized crossover trial with portable HEPA air cleanersFor ultrafine particles, which are especially concerning near airports and busy roads, HEPA filters performed even better. In five schools affected by roadway and aircraft pollution, portable HEPA units removed an estimated 83% of outdoor-origin ultrafine particles infiltrating into classrooms. Before the filters were deployed, roughly half of outdoor particles made it inside; afterward, only about one-tenth did.
18PubMed Central. Indoor Air Quality Intervention in Schools: Effectiveness of a Portable HEPA Filter Deployment in Five Schools Impacted by Roadway and Aircraft Pollution SourcesUpgrading the filters in a building’s central HVAC system can be even more effective. In near-roadway schools in Las Vegas, improved HVAC filtration reduced black carbon concentrations by 74 to 97% inside classrooms relative to outdoor levels. Even the original, lower-grade filters achieved 31 to 66% reductions, showing that any filtration is better than none.
19PubMed. Filtration effectiveness of HVAC systems at near-roadway schoolsModeling work across 13 U.S. cities found that improved filtration used about 31% less energy than the alternative strategy of pumping in 100% outdoor air, while also lowering modeled infection probability by 29%. That combination of lower energy costs and better health outcomes makes filtration upgrades one of the most practical interventions available.
20Journal of Building Engineering. Modeling impacts of ventilation and filtration methods on energy use and airborne disease transmission in classroomsThe Wildfire Problem
Wildfire smoke is creating a new kind of dilemma for schools that rely on opening windows for ventilation. Research during a wildfire smoke event in 2023 found that schools without central HVAC systems could only achieve adequate air-exchange rates with their windows open. During smoke events, that means choosing between letting in wildfire particulates or sealing the building and watching COâ‚‚ climb. Schools with central HVAC systems were better protected: the presence of a central system was negatively associated with indoor-to-outdoor PM ratios during smoke events, meaning less outdoor smoke got in.
21Environmental Pollution. Associations between classroom ventilation rates and school characteristics with indoor air in classrooms during wildfire smoke eventsOlder buildings performed worse. Building age was positively associated with how much outdoor PM penetrated indoors, likely because of leakier envelopes, worn seals, and outdated ductwork. This is a growing equity issue as wildfire seasons lengthen: the schools least equipped to handle smoke are often the oldest, lowest-funded buildings serving the most vulnerable communities.
Who Gets the Worst Air
Air quality in schools is not distributed evenly. A large Italian study of nearly 457,000 eighth-graders linked geocoded PMâ‚‚.â‚… data to standardized test scores and found no socioeconomic gradient in which students were exposed to worse air, meaning poor and wealthy students attended schools in equally polluted or clean areas. But the effect of that pollution was sharply unequal. PMâ‚‚.â‚… exposure had a small but significant negative effect on math scores, and that effect was concentrated almost entirely among lower-socioeconomic-status students. Higher-SES students were largely unaffected by the same exposure levels.
22IDEAS/RePEc (Max Planck Institute for Demographic Research). Poor air at school and educational inequalities by family socioeconomic statusThe researchers suggested that wealthier families may buffer the effects of poor school air through better nutrition, healthcare access, tutoring, or less-polluted home environments. Whatever the mechanism, it means that improving school air quality is not just a health issue but an equity intervention, with the largest academic gains going to the students who need them most.
Why So Many Schools Fall Short
Ventilation standards exist in most countries, but compliance is another matter. A comprehensive study of Swedish primary-school classrooms found that every classroom relying on natural ventilation or simple exhaust fans failed to meet Swedish ventilation requirements. Even among classrooms with balanced mechanical ventilation, the best-case scenario, about half still fell short.
23Building and Environment. Ventilation strategies and indoor air quality in Swedish primary school classroomsSweden has relatively stringent building codes and a cold climate that motivates tight building envelopes. If half of mechanically ventilated Swedish classrooms do not meet the standard, the situation in countries with older building stock or looser enforcement is likely worse. In warmer climates, the tension between ventilation and energy use intensifies further: natural ventilation strategies in hot or cold extremes lead to worse indoor air quality and thermal comfort simultaneously.
24Energy and Buildings. Energy implications of meeting indoor air quality and thermal comfort standards in Mediterranean schools using natural and mechanical ventilation strategiesPart of the problem is historical. Health-focused ventilation rates of 30 cubic feet per minute per person were common in American building codes by the 1920s, originally motivated by tuberculosis prevention. But in the 1970s energy crisis, the standard-setting body ASHRAE slashed those rates by half or more, explicitly pivoting from health-based targets to odor-control-based targets.
25PubMed Central. Recommitting to Ventilation Standards for Healthy Indoor Air QualityMany school buildings constructed between the 1970s and the 2000s were designed to meet those reduced standards. Retrofitting them is expensive, and school capital budgets routinely lose out to more visible spending priorities.
When HVAC Systems Create New Problems
Mechanical ventilation systems are not without trade-offs. Fan noise from HVAC systems raises background noise levels in classrooms, and the movement of air alters the room’s acoustic properties. Both effects reduce speech intelligibility, which is the percentage of spoken words a listener can accurately hear.
26Archives of Acoustics. The Impact of HVAC Systems on Speech Intelligibility in University ClassroomsFor young children still developing language skills, or for students learning in a second language, even a modest reduction in speech clarity can translate into lost instruction. This is an area where engineering choices matter: duct design, fan placement, sound insulation, and diffuser type all influence how much noise a system generates for a given airflow. The cheapest HVAC installation is rarely the quietest, and noise complaints are one of the most common reasons teachers close vents or turn systems off, defeating the purpose entirely.
Low-Cost Monitoring as a First Step
One reason school air quality has historically been overlooked is that it is invisible. You cannot see COâ‚‚ accumulating, and most particulate matter is too small to notice. Real-time monitoring changes that dynamic. The SAMHE initiative in the U.K. developed a co-designed web platform that lets students and teachers interact with live sensor data from their own classrooms, turning air quality into both a learning tool and an accountability mechanism.
27Developments in the Built Environment. Schools’ air quality monitoring for health and education: Methods and protocols of the SAMHE initiative and projectInexpensive COâ‚‚ monitors now cost as little as $30 to $80 and can be placed in any classroom. When teachers see the number climb past 1,000 ppm on a wall-mounted display, opening a window or adjusting the HVAC becomes a concrete, motivated action rather than an abstract guideline. Several school districts that deployed monitors during the COVID-19 pandemic found that the simple act of making the problem visible led to measurable behavioral changes, even before any infrastructure upgrades occurred.
Voice Problems and Staff Health
Students are not the only people affected. Teachers spend far more cumulative hours in classrooms than any individual student, and their vocal health is directly tied to air quality. A longitudinal study spanning a decade of Finnish schools found that hoarseness prevalence among teachers rose from 10% to 34% over the follow-up period. Indoor air factors correlated significantly with hoarseness even after controlling for age, gender, pet ownership, and job satisfaction. In school buildings where remediation of the building envelope and ventilation was completed, hoarseness prevalence decreased.
28PubMed Central. The Association Between Work-related Stress, Indoor Air Quality and Voice Problems Among Teachers – Is There a Trend?Voice disorders are among the most common occupational health complaints for teachers worldwide. Dry air, airborne irritants, and the need to project over background HVAC noise all compound the strain. Addressing indoor air quality in schools is therefore also a workplace health issue, one that affects teacher retention, substitute-teacher costs, and the continuity of instruction students receive.