How Clean Air Shapes Young Minds: What New Research from Harvard, ASHRAE, and the EPA Reveals About IAQ, Cognition, and Student Health

When we talk about student achievement, we usually focus on instruction, curriculum, attendance, and culture. But a fast‑growing body of evidence says there is another lever hiding in plain sight: the air students breathe—minute by minute—inside classrooms.

In 2025, Harvard researchers published a real‑world classroom study linking ventilation-related indoor conditions to measurable changes in cognitive test performance. The U.S. EPA’s school guidance, updated in 2025, connects indoor conditions to asthma, attendance, dropout rates, and test performance. ASHRAE, which sets widely used ventilation and IAQ frameworks, clarifies how to interpret CO₂ and what “good” looks like in school HVAC verification, filtration, and monitoring.

The Harvard finding that changes the conversation: “peak” classroom air quality tracks “peak” learning demands

Harvard’s Healthy Buildings team summarized their 2025 classroom work with three takeaways that administrators can use as a north star. The first is unambiguous: “Lower CO₂ concentrations are associated with higher cognitive test scores, even over the low range of CO₂ exposures measured in the classrooms.”  The second is the most operationally important: “Peak CO₂ exposures, also corresponding with other indoor air pollutants, showed the strongest statistical evidence of associations.”  The third shifts IAQ from a pandemic-only issue to a permanent learning strategy: “Higher ventilation rates appear to have benefit beyond infection risk reduction by reducing indoor air exposures and supporting cognitive performance.”

Under the hood, the peer‑reviewed paper explains what was actually done. Researchers examined “associations between indoor carbon dioxide concentrations, reflective of ventilation and building occupancy, and cognitive test scores” among graduate students in lecture halls with increased ventilation. They used continuous indoor monitoring and post‑class cognitive tests (Stroop and arithmetic tasks) delivered via smartphone, which capture attention, inhibition, speed, and working memory—skills students rely on all day.

“Model estimates show directionally consistent evidence that higher central and peak classroom carbon dioxide concentrations… are associated with lower cognitive test scores” across roughly 440–1630 ppm CO₂. And the “peaks matter” message is explicit: “The effect estimates are strongest for 95th percentile class carbon dioxide concentrations, representing peak class carbon dioxide exposures.” 

That 95th‑percentile phrase is more than a statistic. It points to a leadership insight: it’s not enough for air quality to be “pretty good on average.” If a classroom has repeated spikes—after lunch, at the end of a block period, during assemblies, in portable units, or in overcrowded rooms—those spikes may be when students are asked to do the hardest thinking.

What the EPA says principals should care about: health, attendance, and academics move together

The EPA’s school IAQ page (updated August 15, 2025) reads like a direct response to the “air quality is just comfort” myth. It states: “The presence of dampness and mold increase the risk of asthma and related adverse respiratory health effects… by 30–50 percent.”  It also ties facility condition to district KPIs: “Schools without a major maintenance backlog have a higher average daily attendance (ADA) by an average of 4 to 5 students per 1,000 and lower annual drop out rate by 10 to 13 students per 1,000.” 

Then the EPA connects ventilation directly to performance outcomes: “Children in classrooms with high outdoor air ventilation rates tend to achieve higher scores on standardized tests in math and reading than children in poorly ventilated classrooms.”  That statement matters because it validates what many principals already suspect: if students feel sluggish, headachy, or unfocused in certain rooms, it may not be “motivation”—it may be exposure conditions interacting with learning demands.

The EPA’s broader Tools for Schools reference guide reinforces the practical leadership logic: “Poor IAQ can lead to a large variety of health problems and potentially affect comfort, concentration, and staff/student performance.”  That single sentence bridges the nurse’s office and the classroom: air quality is a health intervention and an academic intervention at the same time.

CO₂ isn’t “the” problem—but it’s the simplest signal of a problem you can fix

A lot of school conversations get stuck on one question: “What CO₂ number is acceptable?” ASHRAE’s technical FAQ helps leaders avoid both extremes—treating CO₂ as harmless (and ignoring it) or treating CO₂ as a toxin (and overreacting).

ASHRAE states: “CO₂ at the concentrations commonly found in buildings is not a direct health risk, but CO₂ concentrations can be used as an indicator of occupant odors (odorous bioeffluents).”  It then ties CO₂ to ventilation per person: “steady-state CO₂ concentrations of about 700 ppm above outdoor air levels indicate an outdoor air ventilation rate of about 7.5 L/s/person (15 cfm/person).” 

Crucially, ASHRAE also warns against treating CO₂ as “overall air quality.” It says: “CO₂ concentration is not a good indicator of… other indoor contaminants, such as volatile organic compounds,” and concludes: “Thus CO₂ concentration is not a reliable indicator of overall building air quality.”  Harvard’s paper makes the same conceptual point from a research angle, noting that cognitive findings are “more consistent when CO₂ concentrations vary due to changes in ventilation rates and building occupancy,” suggesting the driver may be a mixture of indoor pollutants that rise when ventilation is insufficient.

For leadership decisions, this nuance is empowering rather than confusing. You don’t need CO₂ to be “the cause” for it to be useful. You need CO₂ to be a fast, inexpensive proxy for “how well we are ventilating this room for the number of people inside it,” especially during the spikes Harvard found most predictive.

A superintendent-level KPI shift: stop managing “averages,” start managing “peaks”

Because Harvard found the strongest relationships at the “95th percentile” (peak) CO₂ level, it is reasonable for districts to adjust what they ask for in IAQ reporting. If your dashboards only show daily averages, they can hide the reality of a crowded class period that pushes a room into a high‑CO₂, low‑fresh‑air condition for 30–90 minutes.

ASHRAE’s school design guidance reads like a roadmap for exactly that type of governance. It explicitly instructs teams to verify and document ventilation, demand control ventilation (DCV), sensor placement, and calibration—because the control system is often where “peaks” are created or prevented. The document states that districts should “verify operation and conditions of existing systems” and that “this baseline assessment must be performed by a skilled, trained, and certified technician.” This matters for superintendents: without verification, you can spend money and still fail to deliver real classroom outcomes.

What to say (and ask) as a principal or superintendent—without becoming the engineer

If you lead schools, the most powerful move is not to memorize HVAC acronyms; it is to ask for the right evidence and to set expectations that align with the research.

One practical way to frame this for your facilities team is to borrow ASHRAE’s own language: you want a “physical assessment” and you want them to “verify operation” of ventilation and filtration systems. In superintendent terms, that means you are asking for proof that classrooms are receiving the outdoor air and clean air delivery you believe you are paying for.

When your team reports CO₂, you can align it with ASHRAE’s explanation that CO₂ is a ventilation indicator and with Harvard’s focus on peak exposures. Instead of asking for one “average CO₂ number,” ask for the percentage of instructional time each classroom spends above an agreed threshold and ask for the 95th percentile CO₂ during occupied hours. That request is directly consistent with what Harvard found most predictive.

When ventilation increases are proposed, you can anchor the “why” in Harvard’s words: higher ventilation “appear[s] to have benefit beyond infection risk reduction” because it reduces indoor exposures that correlate with cognitive performance. When moisture remediation is proposed, you can anchor it in EPA’s statement that dampness and mold increase asthma risk by 30–50%, and you can connect that to attendance and instructional stability.

Conclusion: The Learning Environment is Literal

Harvard’s 2025 classroom study reports “directionally consistent evidence” that higher central and peak classroom CO₂ concentrations are associated with lower cognitive test scores, with the strongest effects at the “95th percentile” peaks. The EPA states that indoor conditions tied to moisture and maintenance backlog correlate with asthma risk, attendance, dropout rates, and test performance—and it explicitly notes that students in better-ventilated classrooms tend to score higher in math and reading than students in poorly ventilated rooms. ASHRAE clarifies that CO₂ is not “overall air quality,” but it is a valuable signal of ventilation adequacy, and it provides a school-specific framework emphasizing verification, filtration, sensor calibration, and continuous improvement.

So the leadership takeaway is simple and urgent: You can’t separate academic leadership from environmental leadership anymore. The “learning environment” is not a metaphor; it is a measurable set of indoor conditions that can support—or suppress—student thinking and student health.

References & Further Reading

• Harvard Healthy Buildings blog summary of the 2025 classroom study on CO₂ and cognitive test scores in elevated‑ventilation lecture halls.

• Journal of Exposure Science & Environmental Epidemiology article (2025): Associations between indoor air exposures and cognitive test scores… (Dedesko et al.).

• COGfx program overview: multi‑study evidence that enhanced ventilation and green building conditions increase cognitive function.

• U.S. EPA: How Does Indoor Air Quality Impact Student Health and Academic Performance? (evidence on asthma, attendance, ventilation & scores).

• EPA/partner summaries on indoor pollutant levels and school IAQ program gaps.

• ASHRAE‑aligned CO₂ guidance and school IAQ targets (operational perspective for classrooms).

• Literature reviews connecting indoor environmental quality (ventilation, thermal comfort, moisture) to pupil health and academic performance. [mdpi.com]