Improving air quality in schools comes down to three things: moving more clean air through classrooms, filtering out harmful particles, and monitoring conditions so problems get caught early. Most U.S. classrooms fall short on all three. Studies of schools in Texas, Washington, and Idaho found that 45 to 66% of classrooms had CO2 levels above 1,000 ppm, a sign of inadequate ventilation that directly affects how well students think and how often they get sick.
Why Classroom Air Quality Matters
Poor ventilation does more than make a room feel stuffy. A controlled study published in Environmental Health Perspectives found that cognitive function scores dropped 21% for every 400 ppm increase in CO2. At around 945 ppm, scores were 15% lower than in well-ventilated conditions. At 1,400 ppm, scores dropped by 50%. For reference, well-ventilated rooms in the study sat around 560 to 610 ppm. Many classrooms routinely exceed the levels where performance starts to decline.
Ventilation also affects health directly. A study of elementary schools in the Midwest, published in Environment International, found that every 1 liter per second per person increase in classroom ventilation was associated with 5.59 fewer days of illness-related absences per year. That adds up quickly across a school with hundreds of students.
Hit the 5 Air Changes Per Hour Target
The CDC recommends aiming for at least 5 air changes per hour (ACH) in occupied spaces. This means the total volume of air in a room is replaced with clean air five times every hour. Five ACH won’t guarantee perfectly safe air, but it significantly reduces exposure to viral particles, allergens, and other airborne contaminants.
Most schools rely on mechanical HVAC systems to deliver this, but older buildings often can’t reach 5 ACH through their existing ductwork alone. That gap can be closed with a combination of upgraded HVAC settings, portable air purifiers, and, when outdoor air quality permits, opening windows. The key is measuring your actual ACH and supplementing where it falls short.
Upgrade HVAC Filters
The filter in your HVAC system determines what stays in the air and what gets removed. Filters are rated on the MERV scale. A MERV 13 filter captures at least 50% of particles in the 0.3 to 1.0 micron range, which includes many viruses, bacteria, and fine particulate matter. A MERV 16 filter captures at least 95% of those same particles.
MERV 13 is the minimum recommended by ASHRAE for schools during smoke events, and it’s a reasonable year-round target for any school that can accommodate it. Before upgrading, have a technician confirm the HVAC system can handle the increased airflow resistance of a higher-rated filter. Installing a filter that’s too restrictive for the system can reduce airflow, which makes things worse, not better.
Add Portable Air Purifiers Where Needed
Portable purifiers with HEPA filters are the fastest way to boost clean air delivery in individual classrooms, especially in buildings where HVAC upgrades are slow or expensive. To size a purifier correctly, you need to match its Clean Air Delivery Rate (CADR) to the room.
The formula is straightforward: multiply the room’s length, width, and height in feet to get the volume. Multiply that volume by 6 (for 6 air changes per hour). Divide by 60 to convert to cubic feet per minute. That final number is the CADR you need. A typical 30-by-30-foot classroom with 9-foot ceilings has a volume of 8,100 cubic feet, which means you’d need a combined CADR of about 810 cfm. Most commercial HEPA purifiers deliver 140 to 300 cfm, so a single unit won’t cover a full classroom. You’d likely need two or three units, or one high-capacity unit.
DIY Corsi-Rosenthal Boxes
For schools with tight budgets, the Corsi-Rosenthal box is a surprisingly effective option. Built from a box fan and four MERV 13 furnace filters taped together in a cube shape, these DIY units deliver clean air delivery rates of 600 to 850 cfm depending on fan speed. That substantially exceeds the performance of most commercial HEPA purifiers. Testing published in Aerosol Science and Technology found that even on low speed, the Corsi-Rosenthal box outperformed every U.S. Energy Star certified air cleaner for CADR. A single unit can come close to covering a standard classroom on its own, at a cost of roughly $60 to $100 in materials.
Monitor CO2 as a Ventilation Proxy
CO2 monitors are one of the most cost-effective tools available. Because CO2 builds up when people exhale in an enclosed space, it serves as a reliable indicator of how well a room is ventilated. A reading under 800 ppm generally signals adequate fresh air exchange. Readings consistently above 1,000 ppm mean the room needs more ventilation or filtration. Monitors cost $100 to $250 each and can be moved between rooms to identify problem areas.
During wildfire smoke events or high outdoor pollution days, the EPA recommends pairing CO2 monitors with low-cost PM2.5 sensors. These measure fine particulate matter inside the building versus outside, so you can tell whether your filtration is actually keeping smoke out. Schools in wildfire-prone areas should have at least one PM2.5 sensor per building as part of a smoke readiness plan.
Prepare a Wildfire Smoke Plan
Schools in the western U.S. and increasingly across the country face regular wildfire smoke events that can send outdoor PM2.5 levels to hazardous levels. The EPA and ASHRAE recommend a 10-element smoke readiness plan that includes:
- Upgrading filters to MERV 13 or higher before fire season, and purchasing spares
- Weatherizing the building by sealing gaps around doors and windows and limiting the number of entrances in use during smoke events
- Keeping portable air cleaners on hand to create cleaner air spaces within the building
- Reducing indoor PM2.5 sources like cooking, vacuuming, and printing during smoke days
- Adding supplemental filtration at HVAC intake vents where possible
- Maintaining positive building pressure so air flows outward through gaps instead of pulling smoky air in
The goal is to keep windows and doors closed while still running the HVAC system with enhanced filtration. A well-prepared school can maintain indoor PM2.5 levels far below outdoor levels even during severe smoke events.
Stay on Top of HVAC Maintenance
Even the best HVAC system degrades without regular upkeep. The EPA’s Indoor Air Quality Tools for Schools program provides maintenance schedules that break tasks into daily, weekly, monthly, semiannual, and annual frequencies. The single most important task is changing filters on schedule. A missing or clogged filter leads to plugged coils, reduced airflow, and eventually expensive repairs or full system replacement.
Beyond filter changes, maintenance should include regular damper inspections to confirm outdoor air intakes are actually open, coil cleaning to maintain heat exchange efficiency, and checking that ductwork connections are sealed. Many schools discover that dampers have been closed for years, cutting off fresh air entirely. A full system inspection before each school year is the minimum starting point, with monthly filter checks during the academic year.
Funding for Air Quality Upgrades
The U.S. Department of Energy’s Renew America’s Schools program provides $500 million in grants for energy improvements at K-12 public schools, with indoor air quality upgrades specifically listed as a priority. The program is structured for consortiums that include a local education agency partnered with nonprofits, for-profits, or community organizations. The first round of funding was released in 2022, and a second round of selectees was announced in August 2024.
Schools can also pursue state-level funding, utility rebate programs for energy-efficient HVAC equipment, and in some cases, FEMA hazard mitigation grants for buildings in wildfire-prone areas. Starting with low-cost interventions like CO2 monitors, Corsi-Rosenthal boxes, and filter upgrades gives schools immediate improvements while they pursue larger capital projects through grant funding.