Less pollution
Particulate matter — from smoke, traffic, and indoor sources like mold — reaches deep into the lungs. Filtration, ventilation, and positive pressure all impact these levels.
Why clean air · Definitions
What is clean indoor air?
Less pollution, fewer airborne germs, and a simple yardstick for both. Here is how clean indoor air is defined, how it is achieved, and what it looks like in a classroom.
Fewer airborne germs
Colds, flu, and other viruses spread through shared air. Filtration, ventilation, and UV air quality interventions lower virus-carrying particles in the room.
A simple yardstick
Air changes per hour (ACH) show how often a room’s air is replaced with clean air. Many classrooms get 0.5–1; interventions can add five or more.
The standard
How is cleaner indoor air defined?
No federal standard governs the air inside schools or child care buildings. A written local standard gives everyone the same target, for both the equipment and the air.
| What the standard covers | Recommended target |
|---|---|
| Delivered clean air | 750 to 1,500 CFM per classroom, counting HVAC and in-room cleaners together. Consistent with ASHRAE Standard 241 for a typical classroom; about five air changes per hour in a 9,000 cubic foot room. |
| Noise | Under 45 dBA with every unit in the room running at its operating setting |
| Filtration | MERV-13 minimum |
| Indoor PM2.5 | At or below the WHO guideline: 15 µg/m³ over 24 hours, 5 µg/m³ annual average |
| Ventilation | A CO2 action level (commonly around 1,000 ppm) that triggers a ventilation check |
| Excluded technologies | No ionizing, plasma, or ozone-generating air cleaners |
Noise belongs in the standard. Loud units were the most common reason pandemic-era air cleaners failed: teachers turned them down or off.
The tools
How is it achieved?
Most buildings need a combination of tools. Filtration is often the strongest first step because it handles particles and germs at once and works without new HVAC.
| Tool | Fine particles | Airborne germs | CO2 and gases | Smoke days | Keep in mind |
|---|---|---|---|---|---|
| Filtration | Strong effect | Strong effect | Little or none | Strong effect | Filters need changing on schedule |
| Ventilation | Partial | Strong effect | Strong effect | Little or none | Costs energy; can bring smoke in |
| Positive pressure | Partial | Little or none | Little or none | Strong effect | Needs working HVAC, sealed building |
| UV air treatment | Little or none | Strong effect | Little or none | Little or none | Germs only; needs safety testing |
In practice
What does this look like in a classroom?
- In-room air cleanersMounted out of the way and sized to the room. Quiet enough to run all day, with nothing asked of teachers.
- Classroom monitorShows PM2.5 and CO2 in the room in real time, and whether cleaning is working.
- Outdoor monitorTracks smoke and pollution so schools can respond on bad-air days.
- Networked dashboardShows that each unit is on and when filters need changing, with a named owner and filter costs in the budget.
- The HVAC you already haveBrings in fresh outdoor air (ventilation) through better filters, and can keep the building at slight positive pressure so smoke stays out.
About $1,000per classroom to put in place
$5–10per student per year to keep running
The field
Who else is doing this work.
Center for Green Schools at the U.S. Green Building Council
Its School Air Quality Fellowship helps 50 district staff a year write indoor air policies, and it co-developed with us the Model RFP and performance criteria for classroom air cleaners.
Air Club and the Global Pledge for Healthy Indoor Air
Co-founded by Dr. Georgia Lagoudas and Bronwyn King, Air Club builds the global movement for healthy indoor air, including a pledge launched at the United Nations in 2025.
American Lung Association
Its Clean Air School Challenge offers schools up to $30,000 for indoor air quality work, in collaboration with the Center for Green Schools fellowship.
The evidence
Randomized trials, natural experiments, and cohort studies — including the inconvenient findings.
Research libraryEarly Round: 2027 Cohort
A small national cohort of cities aiming for cleaner classroom air by fall 2027.
Join the cohortWork with us
Whether you lead a city or a district, fund this work, or want it for your own school.
Get in touch