Indoor air quality is measured using sensors and monitors that detect specific pollutants including carbon dioxide (CO2), particulate matter, volatile organic compounds (VOCs), radon gas, and relative humidity (RH), with readings interpreted against established health and building standards.
This guide covers the key pollutants to monitor, the sensor types used to measure each, how to read results against recognised thresholds, and when professional testing is warranted instead of a consumer device.
Key Takeaways
- Indoor air quality monitoring covers at least five distinct pollutant categories, including CO2, particulate matter (PM2.5), VOCs, radon, and relative humidity, each requiring a different sensor type and sampling methodology.
- The Air Quality Index is an outdoor measurement framework tracking five ambient pollutants at federal monitoring stations. It does not apply to indoor environments, where pollutant sources, concentrations, and health thresholds differ significantly.
- The WHO 2021 Global Air Quality Guidelines set a PM2.5 annual mean guideline of 5 µg/m³ and a 24-hour mean of 15 µg/m³. The Environmental Protection Agency’s NAAQS sets a 24-hour PM2.5 standard of 35 µg/m³.
- Radon testing requires a minimum 90-day long-term sampling period per EPA protocol to produce reliable results. A single short-term reading is not sufficient for a remediation decision.
- Electrochemical sensors used in most consumer CO and NO2 monitors have a typical operational lifespan of two to three years, after which sensor drift can cause inaccurate readings even when the device appears functional.
- Relative humidity in the 40 to 60% RH range is the ASHRAE-recommended target for occupied indoor spaces. Humidity outside this range affects particulate suspension, mold and mildew growth potential, and static electricity generation.
What Indoor Air Quality Measurement Actually Covers
IAQ measurement is distinct from outdoor air quality monitoring, and conflating the two leads to measuring the wrong things with the wrong tools. The two frameworks exist for different purposes, use different reference standards, and respond to different pollutant sources.
Why Outdoor AQI Does Not Apply Indoors
The air quality index (AQI) is a public communication tool the Environmental Protection Agency uses to report on five outdoor pollutants tracked by federal monitoring stations: ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and airborne particles.
Its 0 to 500 scale is calibrated to ambient outdoor concentrations. Indoor environments can contain pollutants with no outdoor equivalents, including off-gassing VOCs from building materials, radon from soil, and CO2 from occupant respiration. Using the AQI as an indoor benchmark is technically misleading because the index was never designed to reflect indoor air pollutant concentration or the sources that drive it.
The Five IAQ Variables Most Monitoring Programmes Measure
A standard IAQ monitoring programme covers five core variables, each present for a distinct reason.
- Carbon dioxide: A proxy for ventilation adequacy. Rising CO2 signals that fresh air exchange is insufficient for the occupant load.
- Particulate matter (PM2.5 and PM10): Airborne particles linked to respiratory health. PM2.5 particles are the size most associated with health impact.
- Volatile organic compounds: Off-gassed from building materials, adhesives, finishes, and cleaning products. Some compounds, including formaldehyde exposure sources such as pressed wood and adhesives, are acutely toxic at sustained concentrations.
- Radon gas: A naturally occurring radioactive gas that enters buildings through foundation gaps. The primary concern is long-term cumulative exposure.
- Relative humidity: Affects how particles behave in air, supports or inhibits mold growth, and contributes to static electricity at low RH levels.
Consumer IAQ monitors detect temperature and humidity levels that support mold growth. They do not detect mold spores directly.
Pollutants to Measure and the Thresholds That Matter
Identifying a pollutant is only useful if a reading can be interpreted against a threshold that triggers action. The following covers each core IAQ variable across four consistent dimensions: primary indoor sources, the air quality sensor type used to measure it, the named standard or guideline threshold, and what action that threshold should prompt.
Carbon Dioxide and Ventilation Adequacy
CO2 is not directly toxic at typical indoor concentrations, but elevated carbon dioxide levels and ventilation adequacy indicate that the ventilation system is not supplying adequate fresh air for the occupant load. ASHRAE 62.2 uses 1,100 ppm as a ventilation design target for residential spaces. Readings above 1,000 ppm indoors are commonly associated with reduced cognitive performance in occupants, based on peer-reviewed research published in occupational health literature.
- Primary sources: Occupant respiration. No building material source.
- Sensor type: Non-dispersive infrared (NDIR). This is the most accurate sensor type for CO2 and the standard in quality indoor air quality monitors.
- Threshold: ASHRAE 62.2 design target of 1,100 ppm. Readings above 1,000 ppm are a ventilation adequacy flag.
- Action: Increase mechanical fresh air supply or investigate HVAC performance. A HEPA air purifier does not reduce CO2.
Particulate Matter, Humidity, and Mold Risk
PM2.5 particles are the size most associated with respiratory health risk because they penetrate deep into lung tissue. Relative humidity directly affects particulate behaviour: elevated humidity causes hygroscopic particle growth, while low humidity allows particles to remain suspended longer. Both conditions create different but real risks.
- Primary sources (PM2.5): Combustion, cooking, candles, outdoor infiltration, and secondary particle formation.
- Sensor type: Laser particle counter.
- Thresholds: WHO 2021 guidelines set an annual mean of 5 µg/m³ and a 24-hour mean of 15 µg/m³ for PM2.5. EPA NAAQS 24-hour standard of 35 µg/m³.
- Action: At or above the EPA 24-hour standard, identify and eliminate the combustion source. Improve ventilation and filtration.
- Relative humidity thresholds: 40 to 60% RH is the commonly cited target range for occupied spaces. ASHRAE Standard 55 sets an upper comfort-related humidity bound but notably declines to specify a lower one, though conditions below approximately 30% RH are associated with skin drying, irritation, and increased static electricity risk. Above 60%, mold and mildew growth risk increases.
VOCs, Radon, and When Thresholds Require Professional Action
VOCs off-gas from building materials, finishes, adhesives, and cleaning products. No single federal indoor VOC standard exists. The WELL Building Standard v2 Feature 05 sets a total VOC (TVOC) limit of 500 µg/m³. Consumer metal oxide sensors (MOS) for VOCs detect aggregate TVOC but cannot identify individual compounds or their concentrations, which limits their usefulness for source identification.
- Primary sources (VOCs): Paints, adhesives, pressed wood, flooring, cleaning agents. Formaldehyde exposure is specifically associated with pressed wood and insulation.
- Sensor type: Photoionisation detector (PID) or MOS. PIDs are more accurate; MOS sensors are common in consumer devices.
- Threshold: WELL Building Standard v2: 500 µg/m³ TVOC. No EPA federal indoor standard.
- Action above threshold: Increase ventilation. Identify the off-gassing source. Professional testing needed to identify specific compounds.
- Radon gas: EPA action level is 4 picocuries per litre (pCi/L). EPA Home Buyer’s and Seller’s Guide to Radon recommends remediation above this level.
- Carbon monoxide: A carbon monoxide detector should be installed per NFPA 720. CO is acutely dangerous. Real-time monitoring, not periodic testing, is required.
Sensor Types and How They Work
Each pollutant category requires a different measurement principle. Understanding the sensor behind a reading explains why accuracy varies between devices and why no single consumer product measures everything reliably.
- NDIR sensors measure CO2 by detecting how much infrared light a gas sample absorbs at a specific wavelength. This is the most accurate method for CO2 at the concentrations relevant to occupied spaces. NDIR sensors perform consistently across temperature ranges but require periodic calibration to maintain accuracy.
- Laser particle counters count particles by size as they pass through a laser beam. They provide reliable PM2.5 readings but are sensitive to humidity. At relative humidity above 70%, hygroscopic particle growth can cause overcounting.
- PID and MOS sensors both detect volatile organic compounds but through different mechanisms. A PID ionises gas molecules with UV light and measures the resulting current. An MOS sensor measures resistance changes when VOCs interact with a metal oxide surface. PIDs are more selective and accurate. MOS sensors are cheaper but susceptible to cross-sensitivity from humidity and temperature fluctuations, which can produce false readings.
- Electrochemical sensors are standard in carbon monoxide detectors and NO2 monitors. They generate a current proportional to gas concentration. Their key limitation is lifespan: most electrochemical sensors degrade after two to three years. A device may appear operational while producing inaccurate readings if the sensor has drifted past its rated lifespan.
- Capacitive sensors measure relative humidity and temperature by detecting changes in electrical capacitance as moisture is absorbed. These are stable and inexpensive but require placement away from direct airflow, condensation risk, and sources of heat.
- Alpha track detectors for radon gas capture the particle trails left by radon decay. They are passive devices requiring no power source and must be deployed for 90 days to one year to produce statistically reliable results.
Why Multi-Pollutant Monitors Have Accuracy Trade-offs
Most consumer indoor air quality monitors combine three or more air quality sensor types on a single circuit board. Temperature gradients, humidity fluctuations, and elevated TVOC concentrations can interfere with adjacent sensors, producing readings that reflect the cross-interference rather than the actual pollutant level.
This is why the EPA’s guidance on low-cost sensors notes significant performance variability in multi-pollutant devices compared to reference-grade instruments. You should treat consumer monitor readings as directional indicators rather than certified measurements when precision is required.
Calibration, Sensor Age, and When Readings Become Unreliable
Consumer devices leave the factory with calibrated sensors, but that calibration degrades over time through field drift. An electrochemical CO sensor in a two-year-old device may display a reading and show a functional indicator light while its accuracy has fallen below the threshold required for health-relevant decisions.
Checking the manufacturer’s stated sensor lifespan, and replacing or recalibrating the device at that interval, is not optional maintenance. It is the baseline requirement for any monitoring programme to remain meaningful.
How Building Type and Construction Era Affect What You Should Measure First
Building construction type changes which pollutants are most likely to be present and at what concentrations. A generic pollutant list applies to every building in theory but guides no building in practice. Three profiles cover the most common situations.
Why Tight Buildings Often Show Higher CO2 and VOC Readings
Post-2010 construction built to energy efficiency standards reduces air infiltration, which improves thermal performance but reduces natural dilution of indoor-generated pollutants. Carbon dioxide levels rise faster in tightly sealed spaces under the same occupant load because the ventilation system must supply all fresh air mechanically.
ASHRAE standards 62.2 sets mechanical ventilation requirements specifically to compensate for the reduced natural infiltration in modern construction. Modern finishing materials, including low-VOC paints and adhesives, still off-gas TVOC above the WELL 500 µg/m³ threshold during and after installation, making a baseline VOC measurement worthwhile in newly completed spaces.
Older Buildings and Priority Pollutants
Pre-1980 construction is more likely to present radon entry through foundation cracks, VOC off-gassing from original adhesives and insulation materials, and low indoor humidity from inefficient heating systems. Radon long-term sampling and a VOC baseline measurement are the appropriate starting point before investing in real-time continuous monitoring.
Buildings constructed between 1980 and 2000 more commonly present concerns from synthetic carpet off-gassing and inadequate HVAC capacity as systems age. In these spaces, PM2.5 and TVOC monitoring alongside a CO2 baseline provides a useful first picture of air pollutant concentration.
Short-Term vs. Long-Term Sampling: Matching the Method to the Pollutant
Different pollutants require fundamentally different sampling durations. A single snapshot reading is meaningful for some contaminants and meaningless for others.
Why a Single Radon Reading Is Insufficient for a Remediation Decision
Radon gas concentrations fluctuate with seasonal pressure changes, weather conditions, and soil moisture. A single short-term test taken during an atypical weather event can produce a reading that is not representative of average annual exposure. The EPA’s long-term protocol using an alpha track detector runs 90 days to one year and averages out these fluctuations.
This averaged result is the basis the EPA uses for the 4 pCi/L action level recommendation. A 48-hour short-term test is considered preliminary; it cannot support a remediation decision on its own.
Continuous Monitoring vs. Periodic Testing: A Practical Decision Framework
The appropriate sampling methodology follows the nature of the pollutant’s risk and variability.
- CO2 and relative humidity: Continuous monitoring. Both fluctuate meaningfully with occupancy cycles and HVAC operation. Real-time air quality data from a continuous monitor captures peaks that periodic spot checks miss.
- Carbon monoxide: Continuous monitoring with a permanently installed carbon monoxide detector. CO is acutely dangerous at elevated concentrations and requires immediate response capability.
- PM2.5: Continuous or extended monitoring. Concentrations can spike from cooking or candle use and then decline, making a single snapshot unreliable.
- VOCs: Periodic baseline testing. TVOC levels spike during building activities such as painting or flooring installation, then decline. A baseline before and after significant work provides the most useful comparison.
- Radon: Long-term alpha track sampling, 90 days minimum. No consumer real-time monitor produces a result suitable for a remediation decision.
Consumer Monitors vs. Professional Testing: Knowing When to Escalate
Consumer IAQ monitors serve a specific and legitimate function. They provide ongoing situational awareness of CO2, humidity, and PM2.5, and they can trigger ventilation decisions in real time. Their limitations become relevant when the decision at stake requires legally or medically defensible data.
What Consumer Monitors Can and Cannot Detect Reliably
A well-designed indoor air quality monitor reliably measures CO2 concentration, PM2.5 particle counts, temperature, and relative humidity within its rated accuracy range, as outlined in EPA guidance on low-cost air pollution monitors and indoor air quality. These are its appropriate uses. Consumer monitors cannot identify specific VOC compounds or report individual compound concentrations.
MOS-based VOC sensors report aggregate TVOC, not which compounds are present. Consumer monitors do not detect mold spores. Humidity readings above 60% RH indicate conditions that support mold and mildew growth, but that is a growth risk indicator, not a mold detection capability. Consumer monitors cannot produce the certified radon measurements required for real estate transactions or post-remediation verification.
When Symptoms or Elevated Readings Require Professional Assessment
When persistent symptoms, headaches, respiratory irritation, or fatigue, correlate with time spent in a specific space, and a consumer monitor shows elevated CO2 or TVOC readings, the appropriate next step is a professional IAQ survey, not continued consumer monitoring. Sick building syndrome is a recognized occupational health concept, and when occupant health is the concern, professional assessment is the standard that applies:
- Professional assessment: uses calibrated lab instruments, accredited sampling protocols, and certified analysis.
- WELL Building Standard certification: requires third-party verified air quality testing, not consumer monitor data.
- Radon-specific escalation: the EPA maintains a network of state radon offices that can direct occupants to accredited radon testing services.
Where Humidity Fits in an Indoor Air Quality Monitoring Programme
Relative humidity is not a peripheral IAQ variable. It directly affects how particulate matter behaves in air, whether mold and mildew growth conditions are met, static electricity generation in low-humidity environments, and occupant respiratory comfort. ASHRAE’s recommended range of 40 to 60% RH for occupied spaces is not arbitrary. It reflects the intersection of these effects at a range where all four risks are minimised.
For a broader understanding of how this variable is defined and why it matters, our guide on relative humidity covers the underlying principles in detail.
Humidity as a Core IAQ Variable, Not an Afterthought
Elevated humidity above 60% RH increases hygroscopic particle growth, causing PM2.5 particles to absorb water and grow larger. This changes how they behave in a ventilation system and affects filtration efficiency. It also directly creates the surface moisture conditions that mold requires. Low humidity below 30% RH allows particles to remain suspended longer and generates static electricity, which is a materials and equipment concern in electronics and pharmaceutical environments.
Understanding the tools used to track this variable is addressed in depth in the articles on humidity sensors and hygrometers. Monitoring RH is not optional in a complete IAQ programme. It is one of the five core variables, and the one most directly correctable through a purpose-built humidification system.
Precision Humidity Control After Monitoring Identifies a Problem
Once monitoring confirms that a space consistently runs outside the 40 to 60% RH target, a corrective system must be capable of maintaining that range reliably. For guidance on the available corrective approaches, our article on how to control humidity covers methods and technology comparisons.
For commercial and industrial facilities, humidity control isn’t resolved by adding a portable unit. The specification requires precision maintenance of the target range continuously, without wetting surfaces, equipment, or ducts, across facility types including:
Precision humidification using an equal-sized droplet grid of self-evaporating droplets addresses this requirement. Droplets produced through a compressed air and water process self-evaporate before reaching any surface, enabling humidity control up to 99% RH with plus or minus 1 to 2% precision, under proper system design. Direct exposure to the fog stream will cause wetting. This precision is what closes the gap between a monitoring program that identifies a humidity deviation and a corrective system that actually holds the target range.
For further detail on the sensor and monitoring side of this, our guides on how humidity is measured, humidity meters and monitors, and humidity control systems provide supporting depth.
Final Thoughts
Measuring indoor air quality is a matter of matching the right sensor to the right pollutant, running the appropriate sampling duration for each, and interpreting results against the standard that applies. Consumer monitors are a legitimate starting point for CO2, PM2.5, and humidity. They are not the endpoint when radon remediation, occupant health symptoms, or WELL certification is the concern.
Building type informs where to start. Pollutant characteristics determine how long to measure. Established thresholds from the EPA, WHO, and ASHRAE provide the numerical benchmarks that turn a raw reading into a decision. Understanding how to control humidity once monitoring identifies a deviation is the logical next step for any facility operating outside the 40 to 60% RH range.
Facilities that have identified a humidity control gap through their monitoring programme and need a precision system designed for continuous industrial operation can speak with a Smart Fog engineer about humidification requirements for their specific environment.
FAQ
What is a good indoor air quality reading for CO2, PM2.5, and humidity?
For CO2, readings below 1,000 ppm indicate adequate ventilation for a typical occupied space. ASHRAE 62.2 uses 1,100 ppm as a ventilation design target. For PM2.5, the WHO 2021 guideline sets a 24-hour mean of 15 µg/m³ as a health-protective threshold. The EPA NAAQS 24-hour standard is 35 µg/m³. For relative humidity, ASHRAE recommends maintaining 40 to 60% RH in occupied indoor spaces to minimise mold risk, particulate suspension, and static electricity.
What tools are used to measure indoor air quality at home?
Indoor air quality is measured using dedicated monitors and test kits matched to each pollutant type. A multi-sensor indoor air quality monitor typically covers CO2, PM2.5, temperature, and relative humidity using NDIR, laser particle counter, and capacitive sensors. Radon gas requires a separate alpha track detector deployed for 90 days or more. VOCs are measurable with consumer monitors using MOS sensors, though these cannot identify specific compounds. A carbon monoxide detector is a separate permanently installed device and is required per NFPA 720.
How do I know if my indoor air quality is poor without a monitor?
Without a monitor, there is no reliable way to know pollutant concentrations. Some indicators suggest a problem worth investigating: persistent headaches, fatigue, or respiratory irritation that improve when leaving the building; visible condensation or mold and mildew growth; strong or persistent odours from building materials; inadequate ventilation in tightly sealed spaces. These are signals to acquire a monitor or commission professional testing, not conclusions about air quality on their own.
What are the most important indoor air pollutants to test for first?
The starting priority depends on building type. In pre-1980 construction, radon gas is the highest-stakes unknown because it is odourless, invisible, and associated with lung cancer at sustained exposure above 4 pCi/L. In any building, a CO2 baseline and a humidity reading provide immediate information about ventilation adequacy and mold risk conditions. VOC baseline testing is warranted after renovation, new construction, or installation of new flooring or finishes. A permanently installed carbon monoxide detector is required in any occupied space regardless of building age.
How often should you check or recalibrate an indoor air quality sensor?
Calibration requirements vary by sensor type. Electrochemical sensors in CO and NO2 monitors typically degrade after two to three years and should be replaced at the manufacturer’s stated lifespan, not only when the device stops functioning. NDIR CO2 sensors have longer operational lifespans but benefit from periodic calibration against a known reference. Capacitive humidity sensors are generally stable but should be verified against a calibrated reference instrument annually in critical applications. Any device that shows readings inconsistent with expected conditions should be recalibrated or replaced before being used as the basis for a ventilation or remediation decision.
Does the Air Quality Index apply to indoor air quality measurement?
No. The air quality index is a public communication tool the EPA uses to report on five outdoor pollutants tracked at federal ambient monitoring stations. It uses a 0 to 500 scale calibrated to outdoor concentrations. Indoor environments have different pollutant sources, different concentration ranges, and different health thresholds. Using the AQI as an indoor benchmark is technically misleading. Indoor IAQ measurement uses pollutant-specific thresholds from standards including ASHRAE 62.2, the WHO 2021 Global Air Quality Guidelines, EPA NAAQS, and the WELL Building Standard.
When does indoor air quality testing require a professional rather than a consumer device?
Professional testing is required in four situations: when radon readings from a consumer device exceed 4 pCi/L and a remediation decision or real estate transaction is involved; when occupant health symptoms suggest sick building syndrome and a source investigation is needed; when VOC source identification is required (consumer MOS sensors cannot identify specific compounds); and when WELL Building Standard certification requires third-party verified air quality data. Consumer monitors provide directional awareness. They do not produce the certified, legally defensible measurements that professional accredited testing delivers.
How long does radon testing take to produce a reliable result?
A minimum 90-day long-term test using an alpha track detector is required to produce a result that can support a remediation decision, per EPA protocol. Radon concentrations fluctuate with seasonal pressure differences, weather, and soil moisture. A 48-hour short-term test is considered preliminary and can produce readings that do not reflect average annual exposure. The EPA’s action level of 4 pCi/L is based on long-term average exposure, which is why the long-term sampling period is the correct methodology for any decision involving remediation or real estate disclosure.
