Indoor air quality in schools: what to measure and why

Indoor air quality in schools comes down to three readings tracked room by room: CO₂ in ppm, PM2.5 in µg/m³, and relative humidity in %. When a classroom's CO₂ climbs past roughly 1,000 ppm, the room has run short of fresh air, whatever the thermostat says.

A facilities team usually hears about this secondhand: a teacher says the room feels stuffy by third period, a parent asks why the windows stay shut, or a board member wants to know what changed since the HVAC upgrade. None of those are readings. A monitor in the classroom is.

This post covers what to measure, the thresholds that matter, where to put a monitor across a building, and what to do once a number crosses a line.

Key Takeaways

• CO₂ above roughly 1,000 to 1,100 ppm in a classroom signals inadequate outdoor air ventilation, per ASHRAE and EPA guidance.
• A good PM2.5 reading is under 9.0 µg/m³ as a daily average, the top of the EPA's Good band for fine particles.
• Relative humidity between 30% and 50% keeps a classroom out of the EPA's range for condensation and mold risk.
• One monitor per occupied classroom, away from doors, vents and windows, gives a reading that reflects what students actually breathe.
• A reading tells a facilities team what is happening. It does not fix ventilation, filtration or humidity on its own.

What indoor air quality in schools actually measures

A single number called "air quality" does not exist. A classroom reading is really four or five readings, each answering a different question:

• CO₂ (ppm): how much stale, exhaled air has built up, which tracks outdoor air ventilation.
• PM2.5 (µg/m³): fine particles from wildfire smoke, nearby traffic, cooking in a shared kitchen, or dust from renovation work.
• Relative humidity (%): too high and surfaces stay damp long enough for mold; too low and a room feels dry and dusty.
• Temperature (°F): comfort, and a sanity check on whether the HVAC system is keeping up.
• The air outside: every indoor reading means more next to the outdoor reference for that day.

The EPA's Indoor Air Quality Tools for Schools framework also covers radon testing and mold inspection, which are building-level checks rather than continuous readings. A monitor complements that work; it does not replace a radon test.

The CO₂ number that says a classroom needs more fresh air

CO₂ is the most useful single reading in a classroom, because it tracks occupancy and ventilation in one number. Outdoor air runs close to 420 ppm. ASHRAE's position is that indoor CO₂ should stay within about 700 ppm of the outdoor level, which puts the practical ceiling at roughly 1,100 ppm. The EPA's school guidance uses the same logic: a CO₂ monitor tells a facilities team when outdoor air ventilation is falling behind, not just when a room feels off.

A full classroom with the HVAC system on a minimum setting can cross 1,000 ppm within an hour of the first period starting. The reading climbs through the morning, drops over lunch when the room empties, and climbs again by the afternoon. That pattern, not a single spot check, is what tells a facilities team whether the ventilation schedule matches the actual school day.

PM2.5 on wildfire smoke days and other particle events

A good PM2.5 reading indoors is under 9.0 µg/m³ averaged over a day, the top of the EPA's Good band for fine particles. Most classrooms sit near that line on an ordinary day. Wildfire smoke season, most common across the West, is where the number moves the most: indoor PM2.5 tracks roughly 55% to 60% of the outdoor level once windows and doors are shut, so a smoky day outside still shows up inside.

A classroom reading during a smoke event tells a facilities team two things a nose can't: whether closing the building actually worked, and whether a portable HEPA room purifier is bringing the number back down or just running. A MERV 13 filter in the HVAC system helps the same way, but only while the fan is actually moving air, which is worth confirming with a reading rather than assuming.

Humidity, temperature and the rest of the picture

The EPA's guidance keeps indoor relative humidity under 60%, with 30% to 50% recommended to avoid the condensation that lets mold take hold. A classroom that runs high in humidity through summer break or a leaking roof section can sit in that range for weeks with nobody noticing until there is a smell or a stain.

Temperature is less a health reading than a comfort and equipment check. A classroom running several degrees warmer than the setpoint, alongside a climbing CO₂ line, usually means the same thing: not enough outdoor air is getting in.

Where to put a monitor across a school building

One monitor per occupied classroom is the standard, not one per building or one per hallway. Air quality varies room by room: a portable classroom on its own HVAC unit, a room next to the kitchen, and a gym with double the ceiling height all read differently even on the same day.

Inside the room, a monitor belongs away from the door, the supply vent and any window, at roughly the height where students are sitting. A monitor tucked next to a vent reads the freshest air in the building, not what the back row is actually breathing. Common areas worth their own reading include the cafeteria during lunch periods, the gym during PE, and any room recently renovated or carpeted, since VOCs off-gas from new paint, flooring and furniture for weeks.

What to do when a classroom reading crosses a threshold

A reading tells a facilities team what is happening. It does not open a window, adjust a damper or dry out a roof leak by itself. What to try depends on which number moved:

• CO₂ climbing: increase outdoor air through the HVAC system, or open windows when the outdoor air is clean, then watch whether the line actually drops.
• PM2.5 climbing: confirm windows and doors are shut on a smoke day, run a HEPA room purifier in the affected room, and check that the HVAC fan is running enough for the filter to do anything.
• Humidity climbing: look for a moisture source before reaching for a dehumidifier. A reading that stays high after the source is fixed means the fix didn't work.

The tradeoff is the same for every reading: a monitor turns "the room feels off" into a number a maintenance request can point to, but somebody still has to act on it.

FAQ

What is a good CO₂ level for a classroom? A good CO₂ level for a classroom stays under roughly 1,000 to 1,100 ppm, the range ASHRAE and the EPA treat as a sign that outdoor air ventilation is keeping up with occupancy. Above that, expect the room to read as stuffy well before anyone checks a number.

How often should schools test indoor air quality? Continuous monitoring, not periodic testing, is what catches the patterns that matter: a CO₂ line that climbs every afternoon, or a humidity reading that stays high for weeks after a leak. A one-time spot check only ever shows a single moment.

What causes poor indoor air quality in schools? The most common causes are outdoor air ventilation that hasn't kept pace with a full classroom, wildfire smoke or traffic particles getting in through gaps around doors and windows, and humidity from a roof or plumbing leak that goes unnoticed. Each shows up as a different reading.

Does an air purifier fix a classroom's air quality? A HEPA room purifier lowers PM2.5 while it is running, but it does nothing for CO₂, since CO₂ is a ventilation problem, not a particle problem. A reading in the room is the only way to tell which fix a given classroom actually needs.

Is indoor air quality monitoring required in schools? Requirements vary by state and district; some now require CO₂ monitoring or set target levels in code, while others leave it to district policy. The EPA's IAQ Tools for Schools framework is a voluntary federal framework, not a nationwide mandate, so check state and local rules directly.

The next step for a facilities team

Indoor air quality in schools is not one number a building either passes or fails. It is CO₂, PM2.5 and humidity, read room by room, against the outdoor air outside that day. A classroom running near 1,100 ppm of CO₂ needs more outdoor air. A smoke day pushing PM2.5 past 9.0 µg/m³ needs a closed building and a running purifier. A room stuck above 50% humidity needs whoever finds the moisture source, not just a fan.

A facilities team that wants that evidence before the next parent question or board meeting, not after, can start with a short form on Measured Air's page for commercial buildings. Measured Air replies within a business day.