Knowledge Base
Air Quality Parameters
Understanding what each AirGradient sensor measures is essential for accurately interpreting your air quality data from the dashboard, exports, or API. CO2, PM, TVOC
Understanding what each AirGradient sensor measures is essential for accurately interpreting your air quality data from the dashboard, exports, or API. This comprehensive reference guide provides detailed definitions, measurement units, and educational resources for parameters we publish.
| Parameter Name (unit) | Description | Why it's measured this way and what the unit means | Time Resolution | References from AirGradient Blog | Other References |
|---|---|---|---|---|---|
| PM2.5 (µg/m3) raw | Raw, uncorrected, mass concentration of airborne particles with diameter <= 2.5 µm. | Mass (µg/m3) represents the total weight of inhalable particles in air. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| PM 0.3µm particle count | Count of all airborne particles with a diameter equal to or larger than 0.3 µm, measured as number concentration. | Particle count tracks the number of particles, regardless of mass. Small particles can be high in number but low in mass, so count reveals pollution events mass-based PM may miss. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| CO2 (ppm) raw | Carbon dioxide concentration in the air. | PPM (parts per million) reflects volumetric gas concentration. Standard unit for tracking CO2 buildup from human activity. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| Temperature (°C) raw | Ambient temperature near the sensor. | Temperature in °C is a direct physical measure affecting comfort and sensor behavior. For our indoor monitor, the raw temperature reflects ambient room temperature well. For our outdoor monitors, the raw temperature diverges from ambient because the sensor sits inside the insulated casing of the monitor. Therefore, we apply a temperature and humidity correction (shown as corrected values later in the sheet, refer to Temperature (°C) corrected). |
1-min (Wi-Fi), 3-min (Cellular) | AirGradient Education Webinar: Temperature and Relative Humidity Correction | |
| Humidity (%) raw | Ambient relative humidity near the sensor. | % RH indicates water vapor content relative to air saturation. Affects sensor function and air quality. For our outdoor monitors, the raw temperature diverges from ambient because the sensor sits inside the insulated casing of the monitor. Therefore, we apply a temperature and humidity correction (shown as corrected values later in the sheet, refer to Humidity (%) corrected). | 1-min (Wi-Fi), 3-min (Cellular) | AirGradient Education Webinar: Temperature and Relative Humidity Correction | |
| TVOC (ppb) | Total concentration of volatile organic compounds converted to ppb using Sensirion (sensor manufacturer) conversion formula. | Reported in ppb but calculated using Sensirion's conversion formula from VOC index. This is not an absolute concentration but a relative estimate based on sensor response, useful for trend comparison. | 1-min (Wi-Fi), 3-min (Cellular) | How Accurate is the Sensirion SGP41 TVOC Sensor? | Compliance of Sensirion's VOC Sensors with Building Standards Related to All Sensirion Products with a VOC Index Output |
| TVOC index | Scaled index value representing total VOC activity, based on sensor signal relative to recent baseline. 100 = baseline, higher values indicate increased TVOC concentration, lower values indicate a reduction of TVOCs. | Unitless index (1 - 500) based on deviation from 24h baseline. 100 = baseline, >100 = more VOCs present compared to the 24-hour baseline, <100 = fewer VOCs compared to the 24-hour baseline. Index is used because MOx sensors respond broadly to many VOCs, making exact ppb estimates unreliable. Better for detecting relative changes in air quality. The VOC Index adapts its gain based on environment: in clean air it becomes more sensitive to small changes, and in polluted air, it reduces sensitivity to avoid saturation, ensuring relative changes remain meaningful. | 1-min (Wi-Fi), 3-min (Cellular) | TVOC - When it's useful and when it's useless | What is Sensirion's VOC Index? |
| NOX index | Scaled index value representing NOx activity, where 1 = baseline and higher values indicate increased NOx. | Unitless index (1 - 500) based on deviation from 24h baseline. 1 = baseline, >1 = more oxidizing gases present. The NOx Index does not include gain adaptation, as NOx events are typically more consistent in composition. | 1-min (Wi-Fi), 3-min (Cellular) | What is Sensirion's NOx Index? | |
| PM1 (µg/m3) | Mass concentration of particles with diameter <= 1 µm. | Approximate mass concentration of particles smaller than 1 µm. µg/m3 provides a standard mass-based metric for fine particulate pollution. | 1-min (Wi-Fi), 3-min (Cellular) | They Add More Pollutants; We Add More Accuracy. Why We're Stepping Back from PM1 and PM10 Marketing | |
| PM10 (µg/m3) | Mass concentration of particles with diameter <= 10 µm. | Approximate mass concentration of particles smaller than 10 µm. Plantower sensors are optimized for PM2.5, making PM10 values less reliable. Larger particles such as pollen (>10 µm) are generally not detected. µg/m3 gives weight of larger respirable particles like dust per air volume. | 1-min (Wi-Fi), 3-min (Cellular) | They Add More Pollutants; We Add More Accuracy. Why We're Stepping Back from PM1 and PM10 Marketing | |
| TVOC raw log(R) | Logarithmic value of raw resistance output from the VOC sensor. | Reflects electrical resistance of the MOx sensor surface. A MOx sensor consists of a heated metal oxide that changes resistance with oxygen interaction. Reducing gases (like VOCs) lower resistance, oxidizing gases increase it. Output is broadband and non-specific. | 1-min (Wi-Fi), 3-min (Cellular) | How Accurate is the Sensirion SGP41 TVOC Sensor? | What is a metal oxide (MOX) sensor? |
| NOX raw log(R) | Logarithmic value of raw resistance output from the NOx sensor. | Value shows MOx resistance change in presence of oxidizing gases. Heated metal oxide gains resistance when exposed to NOx. Output reflects general oxidizing gas presence, not specific concentrations. | 1-min (Wi-Fi), 3-min (Cellular) | What is a metal oxide (MOX) sensor? | |
| PM2.5 (µg/m3) corrected | Calibrated PM2.5 value adjusted using the calibration formula chosen in the dashboard. | Corrections may improve accuracy by accounting for various environmental or sensor-specific factors. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| CO2 (ppm) corrected | Calibrated CO2 value adjusted using the calibration formula chosen in the dashboard. | Corrections may improve accuracy by accounting for various environmental or sensor-specific factors. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| Temperature (°C) corrected | Temperature reading adjusted using the calibration formula chosen in the dashboard. | Corrections may improve accuracy by accounting for various environmental or sensor-specific factors. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| Humidity (%) corrected | Relative humidity adjusted using the calibration formula chosen in the dashboard. | Corrections may improve accuracy by accounting for various environmental or sensor-specific factors. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| Heat Index (°C) | Calculated heat index combining corrected temperature and humidity readings into a perceived temperature. | The heat index, also known as the apparent temperature, is a measure of how hot it really feels when relative humidity is combined with the air temperature. | 1-min (Wi-Fi), 3-min (Cellular) | ||
| Location ID | The location ID automatically assigned to the location in the dashboard. | ||||
| Location Name | The location name assigned to the location in the dashboard by the user. | ||||
| Location Group | The location group assigned to the location in the dashboard. | ||||
| Location Type | Indicates whether the location is indoor or outdoor. | ||||
| Sensor ID | A combination of the monitor SN and the maker. | ||||
| Local Date/Time | Date and time information in the local timezone. | ||||
| UTC Date/Time | Date and time information in Universal Coordinated Time. | ||||
| Channel | PM readings of monitors with two PM modules are distinguished this way. "1" stands for PM module 1; "2" stands for PM module 2; and "-" provides the average of both modules. | ||||
| # of aggregated records | At lower time resolutions (for example 5 min, 1 hour, etc.), this indicates how many data points went into the average. | ||||
| Place Open | Indicates if the place was open when the data was captured or closed as configured in the dashboard. |