Air Quality in Schools: A Guide for Parents & Staff

Quick answer: Ventilation matters more than anything else for classroom air. A classroom holds roughly 15 to 20 times as many people per square foot as a typical home, often in a building that is decades old. When the fresh-air supply can't keep up, every other problem gets worse. A carbon dioxide (CO₂) monitor is the cheapest way to check: readings that stay above about 1,000 ppm during class usually mean the ventilation isn't keeping pace with the number of people in the room.

Why a classroom is harder to manage than a house

Many parents assume that if the air is fine at home, it's fine at school. Three things make a classroom a different problem.

Occupant density. A typical home holds three or four people in about 2,000 sq ft, or roughly 1.5 to 2 people per 1,000 sq ft. A classroom holds 25 to 30 people in about 900 sq ft, or roughly 28 to 33 people per 1,000 sq ft. That is about 15 to 20 times the density of a home. Every person adds carbon dioxide, moisture, and particles to a fixed volume of air. Ventilation needs scale with people, not floor area, so a building that feels fine in an empty hallway can be under-ventilated in a full classroom.

Building age and deferred maintenance. A 2020 report from the U.S. Government Accountability Office found that an estimated 41% of school districts needed to update or replace HVAC systems in at least half their schools, about 36,000 schools nationwide (GAO-20-494). Ventilation systems in that condition rarely fail in an obvious way. They deliver less outdoor air than they were designed to, quietly, for years.

Nobody in the room controls it. A homeowner with a stuffy room opens a window. A teacher may work in a building with sealed windows, a unit ventilator they can't adjust, and a maintenance request queue that takes months. The same lack of control drives sick building syndrome complaints in offices. In a school, the people in the room are children, who rarely describe stale air as a problem.

None of this means schools are unsafe. It means classroom air depends almost entirely on decisions made at the building level.

Start with CO₂: it's cheap and tells you the most

At the levels found in classrooms, carbon dioxide is not the pollutant to worry about. Its value is that people exhale it at a steady, predictable rate. That makes it a good stand-in measure for how much fresh air reaches the room compared with how many people are in it.

CO₂ reading (occupied room)	What it suggests
400–600 ppm	Close to outdoor air. Very well ventilated, or the room is nearly empty
600–800 ppm	Good ventilation for an occupied classroom
800–1,000 ppm	Acceptable but at the upper end; keep an eye on it
1,000–1,500 ppm	Ventilation is not keeping up with occupancy; rooms often feel stuffy
Above 1,500 ppm	Well under-ventilated; have the system checked

These bands, including the 1,000 ppm line, are widely used rules of thumb, not regulatory limits. Ventilation standards such as ASHRAE Standard 62.1 are written as outdoor air delivered per person and per square foot, not as a CO₂ ceiling. ASHRAE has also published a position document on indoor carbon dioxide that treats CO₂ as a ventilation indicator and calls for more research on how to use it.

A classroom CO₂ monitor with an NDIR sensor (the type that measures CO₂ directly with infrared light) costs roughly $100 to $200 and needs no expertise to read. It turns an argument about whether a room "feels stuffy" into a number a facilities department can act on. If a district can afford one thing in this article, make it monitors.

Use them well. A single spot reading tells you little. Log readings across a full school day, in the same room, with normal attendance. The pattern to look for is a reading that climbs through each class period and never drops back down. That room is losing the race between fresh air and occupancy.

The most common problems, in rough order

1. Not enough outdoor air. This is the biggest issue. Systems get set to minimum outdoor air to save energy, dampers stick shut, unit ventilators sit behind furniture or get switched off because they're noisy, or the room now holds more students than the design assumed.

2. Moisture and mold. Roof leaks, plumbing failures, condensation on cold exterior walls, and portable classrooms are the usual sources. Porous materials that stay damp grow mold, and the fix is a building repair. Keeping indoor humidity in the 40–60% range helps prevent condensation-driven mold (the EPA recommends 30–50%, and below 60%). Materials that stay wet from a leak will grow mold at any room humidity.

3. Exposure to cleaning and disinfecting products. Schools use a lot of cleaning chemicals. A study of state surveillance data published in the Journal of Occupational and Environmental Medicine linked 12% of confirmed work-related asthma cases to cleaning products, with janitors and cleaners the largest job group and schools among the most common settings (Rosenman et al., 2003, via CDC). The concern is products applied in occupied rooms, or shortly before students arrive. That is a scheduling and ventilation issue as much as a product choice.

4. Materials inside the room. New furniture, carpet, and renovation materials release gases as they age. Art supplies, science-lab chemicals, and buses idling near air intakes at pickup time add more.

5. Settled dust and allergens. Dust, pet dander carried in on clothing, and other allergens collect in carpet, soft seating, and reading corners. Ventilation and filtration reach this layer least well, since neither moves air through a carpet. Regular deep cleaning of soft furnishings is the main tool here.

If you're a parent: what to ask

Curiosity works better than confrontation. Facilities staff usually know what's wrong with their buildings. What they often lack is budget and a documented reason to put one repair ahead of another. A parent asking specific, answerable questions can supply the second.

Six questions worth asking, and what a good answer sounds like:

"Does the school monitor CO₂ in classrooms?" Good answer: yes, with logging monitors in a sample of rooms. Also fine: no, but we'd consider it, since monitors are inexpensive.
"When was the HVAC system last inspected and serviced, and what filters does it use?" Good answer: a date within the last year and a specific MERV rating. Concerning: nobody knows.
"Are there known moisture or mold issues, and how are they tracked?" Good answer: a written process for reporting and fixing leaks. Openness matters more than a spotless record, because every building leaks somewhere.
"When are cleaning and disinfecting products applied?" Good answer: after hours, with ventilation running.
"Does the district use the EPA's IAQ Tools for Schools program, and who leads indoor air quality work?" The EPA's Indoor Air Quality Tools for Schools program helps districts set up an ongoing indoor air management plan. Good answer: yes, and they can name the person responsible. This one question tells you a lot about how seriously a district takes the topic.
"How is ventilation handled in the portable classrooms?" Portables are a known weak point and deserve their own answer.

What to do with the answers: raise them through the PTA or the school board rather than with one teacher, who almost certainly can't fix any of it. Ventilation upgrades are capital projects, and capital projects move when requests are documented, organized, and repeated.

[CTA: Download the one-page question checklist to take to your next PTA or school board meeting]

If your child has asthma or allergies

Ask a few practical questions about your child's own classroom. Where does your child sit relative to the unit ventilator or air supply? Is there carpet or upholstered seating in the room, and how often is it deep-cleaned? Are classroom pets kept in the room? Share your child's care plan with the school nurse and teacher, and talk to your child's doctor about what matters most for them. These are classroom-management and medical questions, separate from any building-wide fix.

If you run the building: the priority order

The list below is ordered by impact per dollar, which is not the order most vendors will present.

Free or near-free:

Check that outdoor air dampers open. Stuck or manually closed dampers are common and cost nothing to fix.
Clear unit ventilators. Furniture, storage boxes, and displays in front of intakes and outlets are common and directly cut the air delivered.
Move cleaning and disinfecting to unoccupied hours, with ventilation running.
Flush the building before occupancy. Run systems at maximum outdoor air for a period before the building fills, and again after it empties.
Fix leaks when you find them, not at the next budget cycle. Damp materials become a far more expensive problem within weeks.

Low cost:

CO₂ monitors in a representative set of rooms, logged, so you find the worst rooms instead of guessing.
Better filters. Use the highest MERV rating your system can handle without restricting airflow, and check the equipment specifications before you upgrade. A filter that is too dense for the system cuts the outdoor air delivery you are trying to protect.
Portable HEPA purifiers in the worst rooms, sized by the room's volume rather than the coverage area on the box, as a stopgap while you fix ventilation. They filter particles but bring in no fresh air, so CO₂ readings won't change.
Review cleaning product choices with respiratory exposure in mind, especially for custodial staff, who get the most exposure.

Capital:

Repair or replace the ventilation system, or add an energy recovery ventilator (ERV). An ERV brings in fresh outdoor air and transfers heat and moisture between the incoming and outgoing airstreams, which reduces the energy cost of ventilating. Capital work is the fix for the underlying problem, and nothing cheaper substitutes for it. Everything above either measures the problem or eases it.

Where funding is the barrier, say so plainly in budget requests. A logged CO₂ record from your worst rooms makes a stronger case than a general request for HVAC money.

Daycares and preschools

Daycares face the same physics in smaller rooms, often in converted homes, churches, or retail spaces that were never designed for a group of children. Children nap, eat, and play in the same rooms for long days, so ventilation matters as much as it does in a school.

Licensing rules vary by state and usually cover ventilation, temperature, and sanitation in general terms rather than setting air quality numbers. The national guidance most states draw on is Caring for Our Children. Its ventilation standard calls for as much fresh outdoor air as possible in rooms children use, ventilation rates of 15 to 60 cubic feet per minute per person depending on the activity, opening windows when weather and outdoor air allow, keeping indoor air free of unnecessary chemicals and fragrances, and avoiding ionizers and ozone-producing air cleaners.

For a director, the practical checklist is short: a logging CO₂ monitor in the nap room, windows that open, HVAC filters changed on schedule, cleaning done after children leave, no air fresheners or plug-in scents, and a portable HEPA purifier for rooms without mechanical ventilation. For a parent touring a center, ask about each of those items. A director who can answer them quickly is paying attention.

Where continuous surface maintenance fits

One more layer is worth knowing about, and it belongs after everything above, not in place of any of it.

Many schools respond to cleanliness concerns by disinfecting more often. In an occupied building that approach has limits: a disinfectant works at the moment it is applied, surfaces pick up new dust and residue within hours, and staff and students absorb the repeated exposure. We make the general case in over-sanitization. In schools the cleaning is more frequent and the occupants are younger, so the trade-off deserves more thought.

The biological alternative works through the HVAC system. A device connected to the ductwork releases beneficial Bacillus bacteria into the supply air. They settle on surfaces and consume the organic residue that odor-causing bacteria and mold feed on, continuously and between cleanings. The system adds no ozone, VOCs, or chemical residue. The probiotic formula is MADE SAFE certified and EPA registered as a bacteriostat and fungistat; the registration covers the formula, not the HVAC device.

Before EnviroBiotics sold home units, the company operated as BetterAir, and this probiotic technology was installed in school buildings and assisted living facilities under that name. We mention it as installation history only and make no claims about outcomes in those buildings. Facilities teams can read about the HVAC-integrated probiotic system and find a local contractor through our HVAC applications page.

Its limits matter more in a school than anywhere else:

It does not ventilate. It will not change a CO₂ reading. A building short on outdoor air is still short on outdoor air.
It does not filter. Removing airborne particles is a filtration job.
It does not fix leaks or remove mold. Water intrusion needs a building repair, and existing mold needs proper cleanup.
We make no claims about illness, attendance, or academic results. Be skeptical of any vendor in this space who does. Its role is surface maintenance between cleanings, and everything else in this article comes first.

The bottom line

Classroom air quality comes down mostly to ventilation, and ventilation comes down to maintenance and capital budgets. That can feel unsatisfying to a parent who wants to act this week. The useful step is to find out whether the ventilation is adequate, and a $150 CO₂ monitor answers that reliably when its readings are logged over full school days.

For administrators, the sequence is plain and dependable: confirm outdoor air is arriving, clear whatever blocks it, move chemical cleaning out of occupied hours, fix water right away, filter as well as the system allows, and keep pushing for the capital fix. For parents, the most useful step costs nothing: ask the six questions above, in the room where budgets are decided, more than once.

Frequently asked questions

How do I know if my child's classroom has poor air quality? The most reliable indicator is a CO₂ reading taken during normal class time. Readings that stay above about 1,000 ppm usually mean ventilation isn't keeping pace with the number of people in the room, and a monitor costs roughly $100 to $200. A stuffy or warm room and lingering musty smells are worth asking about, but a reading logged across a full school day is what moves a facilities department.

What CO₂ level is acceptable in a classroom? Below about 800 ppm during class suggests good ventilation, 800 to 1,000 ppm is acceptable but worth watching, and readings that stay above 1,000 ppm suggest ventilation isn't matching occupancy. These numbers are rules of thumb, not legal limits; ventilation standards such as ASHRAE 62.1 set outdoor air per person and per square foot instead. At these levels CO₂ is useful as a measure of fresh-air supply, not as a pollutant.

Do air purifiers help in classrooms? Portable HEPA purifiers remove airborne particles and are a reasonable stopgap in the worst rooms, but they don't bring in fresh air. They clean the air already in the room and do nothing about CO₂, humidity, or outdoor air supply, so a room with a purifier can still be under-ventilated. Size them by the room's volume rather than the coverage claim on the box.

What are the signs of mold in a school building? Common signs include persistent musty odors, stains on ceiling tiles or walls, warped or bubbling surfaces, and a history of roof or plumbing leaks. Portable classrooms are especially prone to moisture problems. Mold grows where there is water, so cleanup fails unless the moisture source is found and fixed.

What can parents do about poor air quality at school? Ask specific questions: whether CO₂ is monitored, when the HVAC system was last serviced and with what filters, how leaks are tracked, when cleaning products are applied, and whether the district uses the EPA's IAQ Tools for Schools program. Raise them through the PTA or school board rather than with individual teachers, who can't approve building work. Ventilation upgrades are capital decisions and respond to documented, organized, repeated requests.

Is disinfecting classrooms frequently a good idea? Frequent chemical disinfection has trade-offs. It works only at the moment of application, surfaces collect new residue within hours, and occupational health research has linked regular exposure to some cleaning and disinfecting products with work-related asthma among the staff who use them. Applying products after hours with ventilation running reduces much of that exposure, whichever products a school uses.

Are daycare centers held to air quality standards? Licensing rules vary by state and usually cover ventilation, temperature, and sanitation in general terms rather than setting specific air quality numbers. The national Caring for Our Children guidance recommends as much fresh outdoor air as possible, 15 to 60 cubic feet per minute of ventilation per person, and avoiding ionizers and ozone-producing air cleaners. Parents can ask a center how it ventilates rooms, how often filters are changed, and when cleaning products are used.

Air Quality in Schools and Daycares: What Parents and Administrators Should Know