Indoor Air Quality Monitoring Guidance for Schools

Your school should identify one or more reasons for monitoring its air.  Below are possible reasons to consider.

• Establish an IAQ baseline 
• Assess classroom ventilation 
• Screen for elevated levels of specific indoor pollutants, such as particulate matter (PM2.5), carbon dioxide (CO2), carbon monoxide (CO), volatile organic compounds (VOCs), and/or radon 
• Investigate occupant complaints 
• Monitor the impacts of outdoor air pollution events (e.g. wildfire smoke, dust events) 
• Assess improvements after corrective action
• Other

There are numerous manufacturers and different air quality monitors on the market. When considering which monitors are best for your school, there are several considerations.³

1. Choose CO₂ monitors that meet with ASHRAE Standard 62.1-2025, Ventilation for Acceptable Indoor Air Quality minimum performance specifications.
2. Prioritize NDIR (Non-Dispersive Infra-Red) sensors for CO₂ monitoring, as they provide direct measurements instead of estimating CO₂ levels.
3. Choose CO monitoring equipment that complies at minimum with UL 2034 Ed. 5-2024 - Single and Multiple Station Carbon Monoxide Alarms - Published Date: August 7, 2024
4. Verify the frequency of data collection, how many data points the device can store, how the sensors store data, how the data is reported, and whether the data can be downloaded/exported.
5. Consider connectivity options to the building automation system.
6. Consider data communication options (e.g. cellular, Wi-Fi, PoE).
7. Consider the typical lifespan of a monitor.  Monitors have a limited lifespan and can become less reliable over time.  Manufacturers may provide an estimate of how long their product can be operated before it needs to be serviced or replaced.  
8. Consider the limits of detection, accuracy, and precision of the monitor you are selecting.  (Accuracy is a measure of closeness to what is actually in air.   Precision is a measure of how well each air reading repeats.)
9. Consider sensor measurement range, operating environmental conditions, calibration, and maintenance requirements.
10. Think about the upfront cost of the monitors, but also ongoing costs like batteries and cloud storage. 

"The cost of a monitor does not necessarily indicate how well it will perform. The cost of most monitors vary due to their additional features such as display quality, housing, the number and type of sensors used in the monitor, power source, internet connectivity, and/or how the information received from the sensor is processed."¹

Most monitors sold in the U.S. are designed to detect gases or particles in the air such as PM, radon, CO, CO₂, formaldehyde, VOCs, or environmental factors such as temperature and humidity. Deciding which parameters to monitor is not easy or straightforward.*There are no national standards or thresholds for many indoor air pollutants.

Below is a list of possible pollutants and environmental factors (aka parameters) to consider. Please see the table, Indoor Air Monitoring in Schools Guidance Parameters, Prioritization, Thresholds, Interpretation, Recommended Actions, and Reporting, for a list of prioritized parameters, along with an explanation of each, thresholds, action levels and recommendations, along with how to document in reports.

• Carbon Dioxide (CO₂)
• Carbon Monoxide (CO)
• Differential Pressure
• Formaldehyde
• Outdoor Particulate Matter (PM_2.5)
• Particulate Matter (PM_2.5)
• Radon
• Relative Humidity (RH)
• Temperature
• Total Volatile Organic Compounds (TVOC)**

*ASHRAE identifies CO₂, humidity, PM_2.5, temperature, and other parameters as potential components of an educational-facility monitoring system, but specifically cautions that VOC monitoring should be applied carefully.

**Do not use TVOC as a measure of "overall chemical safety." A TVOC number does not tell a school which VOCs are present, the source of the VOC, if that VOC is harmful to human health, or whether a particular chemical concentration exceeds a health-based limit.

There is no universally established national requirement such as "one monitor per classroom." ASHRAE's Design Guidance for Education Facilities: Prioritization for Advanced Indoor Air Quality recommends 10% of classrooms plus central air-handling locations as a base-minimum approach for developing a building IAQ profile.⁴  Use this easy formula to determine the number of monitors needed for your classrooms:

Total number of classrooms x 0.10 = Number of monitors needed for classrooms

In selecting this representative coverage, consider this tiered approach: 

Tier 1 — Monitor First

• Classrooms with IAQ complaints 
• Rooms with unusually high occupancy 
• Special education classrooms 
• Portable classrooms 
• Rooms served by different HVAC systems 
• Older buildings 
• Rooms affected by wildfire smoke 
• Rooms near pollutant sources 
• Rooms with known HVAC problems  

Tier 2 — Select Representative Classrooms

• Different floors of the school building
• Different wings 
• Different HVAC zones
• Rooms with exterior versus interior exposure 
• Different building ages 
• Different orientations 
• Different occupancy levels

Tier 3 — Other Spaces

• Administrative areas 
• Art rooms 
• Auditorium 
• Cafeteria 
• Copier room
• Gymnasium 
• Health office 
• Industrial Arts rooms
• Library 
• Music rooms 
• Science rooms

Always follow the manufacturer’s instructions for monitor placement. There are specific and different recommendations for the time of year, building conditions, and placement of radon measurement equipment.   For more information on radon measurement in schools, see the Lung Association’s Radon Measurement Guidance for Schools. Use the following EPA guidance on monitor placement to ensure accurate monitoring results: 

• Approximately 3–6 feet above the floor, in the occupied “breathing zone”
• In a location with unrestricted airflow (ideally 270° unobstructed flow at sensor; no less than 180°)
• Away from walls/corners when practical 
• Away from direct sunlight 
• Away from doors and windows 
• Away from HVAC supply and return vents 
• Away from fans 
• Away from portable air cleaners 
• Away from obvious pollutant sources 
• Where students cannot easily tamper with the device 

Consider placing one or more monitors outdoors in an area that can provide information about background conditions or outdoor contaminant sources that may impact the IAQ in the school. For example, you can compare indoor air PM_2.5 with outdoor PM_2.5 to provide clues (not causation) of the source. Airnow.gov may also provide useful information regarding local ambient air quality.  Thorough review of comparable data gathered over many days and conditions is the preferred approach for interpretation, here are some examples for illustration only: 

There is currently no recommended measuring interval from either EPA or ASHRAE. The New York School Environmental Health Program Indoor Air Quality Monitoring Tool Program recommends that schools monitor IAQ twice a year, at a minimum, in both the heating (October 1 to March 31) and cooling (April 1 to September 30) seasons for at least five consecutive occupied school days.  Additional monitoring may be needed in response to pollution events or occupant concerns. 

For most school applications, measuring every 5, 10, or 15 minutes may be a good balance between useful information and data volume.   Do not rely solely on single, instantaneous readings.  A single reading such as CO₂ = 1,450 ppm (parts per million) is much less informative than CO₂ averaged at 1,450 ppm for 60 minutes while the classroom was occupied.

ASHRAE's Design Guidance for Education Facilities:   Prioritization for Advanced Indoor Air Quality recommends the following.⁴ 

To establish a baseline: 

For schools establishing a comprehensive building IAQ baseline, measure for 6 months or more. This allows for:

• When the building is both occupied and unoccupied   

• Different days of the week 
• Different weather conditions 
• Normal occupancy
• HVAC operating cycles  

During outdoor pollution events to: 

• Monitor the infiltration of wildfire smoke and other outdoor pollutants into occupied school spaces. 
• Assess the effectiveness of HVAC filtration and air-cleaning interventions during pollution events. 
• Enable rapid, evidence-based operational decisions to reduce occupant exposure. 
• Document indoor environmental conditions to support health protection, risk communication, and continuous improvement of school IAQ management practices. 
• Provide data-driven communication to school administrators, families and staff about indoor air conditions and the effectiveness of measures taken to protect students and staff.  

 After corrective action:  

Continue to monitor for at least 2–4 weeks after an intervention, preferably during comparable occupancy and weather conditions.

"There are currently no widely accepted performance criteria used to standardize how measurements are made by low-cost air pollutant monitors, or what threshold limits are used to determine the alert cues used for the various low-cost air pollutant monitors. It is important to note that air pollutant concentrations do not necessarily directly translate to health outcomes and that for most pollutants there are no widely accepted indoor health-based standards or thresholds at this time to use in comparison."¹ Levels that trigger an alert of a potential problem are determined by the manufacturer of the monitor.

There are no nationally accepted indoor air concentration limits for CO, CO₂, PM_2.5, or VOCs; however, there are for radon, temperature, and relative humidity.  The following summarizes the recommended action level for radon and ranges of temperature and relative humidity. Refer to Indoor Air Monitoring in Schools Guidance Parameters, Prioritization, Thresholds, Interpretation, Recommended Actions, and Reporting for an explanation of CO, CO₂, indoor PM_2.5, outdoor PM_2.5, radon, relative humidity, temperature, and VOCs, reason for monitoring, action thresholds, interpretation, recommended action levels, how to document in reports, and example report documentation. 

Radon Action Level for Indoor Environments

The average concentration of radon in outdoor air is 0.4 pCi/L (picoCuries per liter).  EPA and the Lung Association recommend homes and buildings be fixed if the radon level is 4.0 pCi/L or more.  Because there is no known safe level of exposure to radon, EPA and the Lung Association recommend considering fixing your home or building for radon levels between 2.0 pCi/L and 4.0 pCi/L.⁵   

Recommended Ranges of Temperature and Relative Humidity⁶

Recommendations apply for people clothed in typical summer and winter clothing, at light, mainly sedentary activity.

There are currently no widely accepted air concentration limits for most pollutants indoors. Therefore, levels that trigger an alert of a potential program are determined by the manufacturer of the monitor. These are sometimes indicated like a stoplight: Green, Yellow, and Red. The American Lung Association does not recommend using a color-based system for screening or action thresholds because there is not standardization between or definition of what these color-based systems. Rather, the American Lung Association recommends utilizing actual measurement readings for interpretation, action, and documentation.

EPA recommends that every school establish data quality assurance and quality control for their monitoring.³  Below is guidance for your school to consider.  

1. Write a quality assurance/quality control plan. Include:

• Who will be trained to set up the monitors, conduct maintenance, and data management procedures?
• Who will document the data from the monitors?
• How frequently will the monitors be calibrated, if recommended by manufacturer?
• How and when will the batteries be replaced?

 2. Address “sensor drift” which is the gradual change in monitor readings over time, even when the actual concentration of the pollutant has not changed. This can occur due to aging of sensor components, exposure to contaminants, extreme temperature and humidity over time, and/or physical wear and tear of the device.  Sensor drift can lead to false alarms, unnecessary HVAC adjustments, missed IAQ problems, incorrect assessment of ventilation performance, or inaccurate long-term trend data.  Sensor drift can be managed through routine calibration against a reference instrument, periodic replacement of sensors, and co-location studies where field monitors are periodically compared against higher-grade reference monitors. 

3. What is the monitor device maintenance schedule? Below is a suggested schedule your school may wish to consider. 

Before deployment

• Record manufacturer and model
• Record serial number
• Record sensor type
• Record firmware version
• Record installation date
• Record location
• Record room number
• Record HVAC zone
• Confirm time/date settings
• Confirm Wi-Fi/data connection
• Confirm measurement units
• Before deploying numerous monitors, place all monitors together in the same location for 24-72 hours.  Look for unusually high/low readings, missing data, and inconsistent responses. This can help identify sensor-to-sensor differences. 

Monthly

• Check monitor connectivity
• Check missing data
• Inspect physical condition of sensors
• Check for tampering
• Check battery/power
• Review unusual readings 

Quarterly

• Review sensor performance
• Review trends
• Inspect placement
• Verify room/HVAC information
• Review dashboard alerts 

Annually

• Review manufacturer's maintenance requirements
• Consider deploying all monitors together in the same location for 24-72 hours.  Look for unusually high/low readings, missing data, and inconsistent responses.  This can help identify sensor-to-sensor differences.   
• Evaluate sensor replacement needs
• Review monitoring locations
• Review thresholds
• Review the overall monitoring program

Schools should ensure there is sufficient data before using monitor results to make decisions. EPA recommends:  

• For daily reports, at least 75% of the expected data should be available for the results to be considered representative.³  
• For long-term reporting, schools should aim for at least 80% of expected data.³ 
• For Immediately Dangerous to Life or Health (IDLH) exposures, even brief periods of high levels are enough to warrant immediate action.  IDLH is defined as acute or short-term exposures to high concentrations of some airborne chemicals, such as CO, that have the ability to quickly overwhelm students and staff, resulting in a wide spectrum of undesirable health outcomes that may include irritation of the eyes and respiratory tract, severe irreversible health effects, impairment of the ability to escape from the exposure environment, and, in extreme cases, death.
• Missing data: If a monitor has too much missing or unreliable data—for example, only 42% of the expected data is available—the dashboard should clearly state "Insufficient data — do not interpret." Schools should not calculate or report an average when there is insufficient valid data.

ASHRAE recommends that schools have a monitoring dashboard and that it contains three levels.⁴ However, the frequency of public reporting of school monitor data should be determined by your schools.

A note about public dashboards: A key consideration is determining how much real-time information should be shared with the public. For example, if air quality monitor data are displayed live, people may see a spike in readings before staff have had an opportunity to investigate the cause, determine whether it poses a concern, and take any needed corrective action. 

Some results may also require explanation or context to be properly understood. Without that context, data can be misinterpreted, particularly if it is shared or discussed by others. In addition, monitoring equipment and data systems can occasionally produce inaccurate readings, so unusual results should be verified before conclusions are drawn. 

Because inaccurate data can create unnecessary confusion or concern, it is important to establish a process for reviewing and validating monitoring data before it is released to the public. This process should ensure that reasonably reliable information is shared in a timely manner. To promote transparency, information about decisions to temporarily withhold data and the outcome of those reviews should also be made publicly available. 

Consider including this disclaimer on all dashboards, which is consistent with EPA’s current explanation of the limitations of monitors:

Indoor air quality monitors provide information about certain pollutants and environmental conditions. They do not measure every indoor air pollutant and cannot determine on their own whether a space is safe or unsafe for any individual. Monitor readings can be influenced by factors such as placement, temperature, humidity, the number of people in a space, and building ventilation. Elevated readings can help identify conditions that may need further investigation or corrective action.

A monitor is not a substitute for testing by properly trained staff/professionals using calibrated professional-grade hand-held devices. Your school should consider hiring a professional when: 

• An IAQ emergency is declared
• Persistent high readings cannot be explained
• Multiple classrooms show the same problem
• There is suspected combustion exposure
• There is a suspected chemical release
• There is suspected mold/moisture damage, depending on magnitude (See Mold in Schools and Commercial Buildings | US EPA)
• There are persistent occupant complaints
• HVAC problems cannot be corrected by routine maintenance
• Screening testing shows elevated radon levels or radon testing is required by state law
• Formaldehyde/VOC exposure is suspected
• A major renovation/construction project is involved
• The school needs exposure assessment rather than screening

1. U.S. Environmental Protection Agency. (2025). Low-cost air pollution monitors and indoor air quality. EPA Low-Cost Air Pollution Monitors and Indoor Air Quality
2. New York State School Environmental Health Program. (2026, September). IAQ monitoring tool program. https://www.health.ny.gov/environmental/indoors/healthy_schools/iaqprogram
3. U.S. Environmental Protection Agency. (2026, July 20). Air Sensor Toolbox. https://www.epa.gov/air-sensor-toolbox
4. American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2023). Design guidance for education facilities: Prioritization for advanced indoor air quality, Version 2.0. ASHRAE Design Guidance for Education Facilities, Version 2.0
5. U.S. Environmental Protection Agency. (2025, June 12). What is EPA's action level for radon and what does it mean? https://www.epa.gov/radon/what-epas-action-level-radon-and-what-does-it-mean 
6. American Society of Heating, Refrigerating and Air-Conditioning Engineers. (1992). Thermal environmental conditions for human occupancy (ANSI/ASHRAE Standard 55-1992). Accuris Standards Store. https://store.accuristech.com/ashrae/standards/ashrae-55-1992?product_id=10001 
7. Minnesota Department of Health. (n.d.). Carbon dioxide (CO₂) in indoor air.  https://www.health.state.mn.us/communities/environment/air/toxins/co2.html 
8. U.S. Environmental Protection Agency. (2026). Carbon monoxide (CO) pollution. https://www.epa.gov/co-pollution 
9. World Health Organization. (2021). WHO global air quality guidelines: Particulate matter, ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. World Health Organization. https://www.who.int/publications/i/item/9789240034228 
10. National Institute for Occupational Safety and Health. (2024). Carbon monoxide (CO): NIOSH pocket guide to chemical hazards. Centers for Disease Control and Prevention. https://www.cdc.gov/niosh/npg/npgd0105.html 
11. Underwriters Laboratories. (2021). Guidance on the use of integrated indoor air quality sensors (UL Environment Standard 2906). UL Standards & Engagement. https://www.shopulstandards.com/ProductDetail.aspx?productId=ULE2906_1_S_20210927