Indoor air affects our health, comfort, and productivity more than many people realize. Modern air quality sensors combine multiple measurement technologies into a single device, giving you a clear, real-time picture of your environment. Each sensor focuses on a specific factor from CO₂ to particulate matter, to help you understand and improve your indoor space.
Let’s explore what these different sensors measure, why they’re important, and how to interpret the readings for a healthier home or workspace.
1. Ruuvi Air Quality Index (AQI)
What it measures:
A combined score representing overall air quality, calculated from weighted inputs including other sensor data. Expressed on a normalized scale from 0 to 100, where higher values mean cleaner, healthier air.
Why it matters:
Individual pollutants such as fine particles or chemical gases each tell part of the story. The Air Quality Index condenses this information into one simple indicator, making it easier to assess overall air quality at a glance. For health, productivity, and comfort, a high AQI means your environment supports cognitive performance, better sleep, fewer allergy triggers, and lower exposure to harmful gases. AQI is especially useful when quick decisions are needed, like opening a window, running an air purifier, or avoiding certain indoor activities.
How to monitor: Understanding the AQI Score
An AQI value close to 100 typically means low PM2.5 levels, and well-ventilated CO₂ levels: all signs of a healthy indoor environment. Scores below 80 may indicate localized issues (e.g., cooking, poor ventilation).
Values under 50 are a clear sign that action is needed, such as improving air exchange, pausing pollutant-generating activities, or investigating more deeply potential problem sources.
Note: Because AQI combines more than one pollutant, it is influenced by the most problematic pollutant at a given time. A high PM event, for example, can cause a sharp AQI drop even if and CO₂ levels are normal.
2. Carbon Dioxide (CO₂) Sensor
What it measures:
CO₂ concentration in parts per million (ppm).
Why it matters:
CO₂ is a natural byproduct of breathing. In poorly ventilated spaces, it can build up quickly, leading to drowsiness, headaches, and reduced cognitive performance even before you notice anything is wrong. High CO₂ is a clear sign that fresh air is needed, especially in offices, meeting rooms, and bedrooms. Monitoring CO₂ helps you maintain alertness and comfort throughout the day.
How to monitor: Understanding CO₂ Readings
CO₂ levels can fluctuate quickly based on occupancy, ventilation, and activities like cooking. Normal outdoor levels are around 400–430 ppm, while indoor levels above 1,000 ppm suggest increased ventilation is needed. Keep in mind that CO₂ sensors can be sensitive to sudden changes and may take a few minutes to stabilize after you move the device.
3. Total Volatile Organic Compounds (TVOC) Sensor
What it measures:
TVOC index (relative scale of organic gases in the air).
Why it matters:
VOCs are chemicals released from everyday products like paints, cleaning agents, and furniture. High VOC levels can cause irritation, headaches, and long-term health risks. The VOC sensor helps you identify sources of pollution and improve ventilation or product choices for a healthier indoor environment.
How to monitor: Knowing Your VOC Sources
VOC readings can spike after cleaning, painting, or bringing new furniture into a room. Some common household items emit VOCs even when not in use. For accurate trends, monitor over several days and note any changes in activities or products used.
3. Particulate Matter (PM) Sensor
What it measures:
PM2.5 indicates the mass concentration of airborne particles with a diameter of 2.5 micrometers or less, measured in micrograms per cubic meter (µg/m³).
PM1.0, PM4.0, and PM10 are usually estimated based on the primary PM2.5 measurement. This is because accurately measuring each particle size fraction individually is technically challenging, especially in compact, consumer-grade sensors. The sensor uses algorithms to estimate the concentrations of these other particle sizes from the detected PM2.5 data, providing a broader picture of indoor air quality.
Why it matters:
Particulate matter includes dust, pollen, smoke, and microscopic pollutants. Fine particles (especially PM2.5 and smaller) can penetrate deep into the lungs, aggravating allergies and respiratory conditions. Monitoring PM helps you respond to pollution from cooking, smoking, or outdoor air, and take action to reduce exposure.
How to monitor: Factors Affecting PM Readings
PM levels can change rapidly due to activities like cooking, vacuuming, or opening windows. Outdoor air quality, weather, and even nearby traffic can influence indoor PM. Place the sensor away from direct sources (like stovetops) for a more representative reading.
4. Nitrogen Oxides (NOx) Sensor
What it measures:
Index of combined nitrogen oxides (NO and NO₂).
Why it matters:
Nitrogen oxides are harmful gases primarily produced by combustion processes, such as traffic emissions, gas stoves, and some heating systems. Indoors, NOx can enter from outdoor air or be generated by appliances. Even at low concentrations, NOx can irritate the respiratory system, worsen asthma, and increase susceptibility to infections. Long-term exposure is linked to chronic respiratory diseases and cardiovascular issues.
How to monitor: Understanding NOx Sources
NOx levels indoors are influenced by outdoor air quality, proximity to busy roads, and the use of combustion appliances. The sensor may respond to rapid changes, especially after cooking or during periods of high outdoor pollution. For accurate readings, avoid placing the sensor directly above stoves or near vents.
Note: Interpreting trends over time is more informative than focusing on single events.
Limited lifespan affecting traditional electrochemical NOx sensors does not apply to some other NOx sensor technologies. Instead of measuring absolute concentrations of nitrogen oxides (NO and NO₂), sensor can provide a relative combined NOx index based on trends compared to previous measurements. This approach eliminates a major drawback of common electrochemical sensors, which typically require periodic calibration and have a limited operational life of 2–10 years due to sensor degradation.
5. Temperature Sensor
What it measures:
Ambient air temperature (°C, °F, K).
Why it matters:
Temperature affects comfort, sleep quality, and health. Too hot or too cold environments can disrupt focus, sleep, and even increase the risk of illness. Monitoring temperature helps you optimize heating, cooling, and energy use for comfort and savings.
How to monitor: Placement Matters
Temperature readings are most accurate when the sensor is placed away from direct sunlight, heaters, or cold drafts. Avoid placing it near windows or electronic devices that emit heat for the most reliable data.
Note: While the sensor itself can detect and report temperature changes very quickly, it may take up to an hour for the entire device to reach full thermal balance with its surroundings. This is a normal characteristic of the device.
6. Humidity Sensor
What it measures:
Relative humidity (%RH), absolute humidity (g/m³), dew point.
Why it matters:
Balanced humidity (ideally 30–50%) prevents mold growth, reduces allergens, and supports respiratory health. Too much humidity encourages mold and dust mites while too little dries out skin and airways. The humidity sensor helps you maintain healthy air and protect your home and belongings.
How to monitor: Humidity Fluctuations and Condensation
Humidity can vary room to room and throughout the day. Bathrooms and kitchens often have higher readings after showers or cooking. Place the sensor in the area you’re most concerned about, and monitor over time to spot patterns.
Note: The humidity sensor works by allowing air to pass through a special breathing sticker. If it were fully sealed, it couldn’t measure humidity at all. Over time, humid air can enter the enclosure, and if the temperature drops below the dew point, condensation will form on any surface colder than that dew point, either inside or outside the sensor.
Sensor Placement: Avoid placing a sensor in a spot with constant high humidity and rapid temperature drops are a possibility. Do not cover the breathing sticker with tape or other materials. While this would make the enclosure airtight and prevent new condensation from forming, it would also stop the sensor from measuring current humidity accurately. If you must seal the sensor, remember: the air trapped inside will determine the dew point, and condensation could still occur if the sensor is later exposed to much colder conditions.
Understanding these factors will help you get the most accurate and reliable humidity readings from your Ruuvi product and avoid issues related to condensation. If you have a challenging monitoring environment, check out this article. Our active Ruuvi community forum members might also be able to help with rarer cases.
6.1 Absolute Humidity
What it measures:
Absolute humidity represents the total amount of water vapor present in the air, expressed in grams per cubic meter (g/m³). This value stays constant regardless of temperature and tells you how much actual moisture is in a specific volume of air.
Why it matters:
Unlike relative humidity, which compares moisture content to what the air could hold at a given temperature, absolute humidity gives a precise measure of water vapor itself. This clarity makes it especially valuable in situations where temperature varies widely or when managing spaces where moisture control is critical. Applications like greenhouses, manufacturing, and archival storage benefit from exact humidity values to prevent spoilage or degradation. Even in everyday environments, knowing the true moisture load helps make more effective decisions about humidifiers, dehumidifiers, and ventilation.
How to monitor:
Absolute humidity is most informative when compared over time or observed alongside temperature and relative humidity readings. It helps identify conditions where air may feel dry even if relative humidity is moderate, or vice versa. Since comfort and air quality issues often relate more closely to the moisture content in grams per cubic meter than to percentage-based relative values, tracking this metric can reveal hidden trends in indoor air balance you might otherwise miss.
6.2 Dew Point
What it measures:
The dew point is the temperature at which the air becomes fully saturated with water vapor and condensation begins to form. It is expressed in degrees Celsius and depends on both the air’s temperature and its moisture content.
Why it matters:
Dew point temperature is a direct indicator of comfort and condensation risk. Higher values indicate more moisture in the air, which often leads to a muggy or oppressive indoor climate. Lower dew points reflect drier conditions. More importantly, when a surface cools to the dew point, condensation occurs. This is a key concern in avoiding mold growth, corrosion, and water damage in both residential and commercial environments. Dew point values also change more slowly than relative humidity, making them a reliable indicator for identifying ventilation issues or excessive indoor moisture sources.
How to monitor:
Monitoring dew point gives early warning of environments at risk of condensation. For example, if the temperature of a window or wall drops near or below the dew point, moisture will start to accumulate, potentially causing damage over time. This makes dew point especially useful in colder climates or in rooms with limited airflow. By tracking this value alongside absolute and relative humidity, you get a fuller picture of moisture dynamics in real time, helping protect furnishings, electronics, and indoor air quality.
7. Air Pressure Sensor
What it measures:
Atmospheric (barometric) pressure, in pascals (Pa), hectopascals (hPa), millimeters of mercury (mmHg), or inches of mercury (inHg)
Why it matters:
Air pressure affects weather patterns, comfort, and even some health conditions. Indoors, monitoring air pressure helps detect changes in weather, understand ventilation dynamics, and identify pressure differences that could affect building integrity or comfort. For people sensitive to pressure changes, such as those with migraines or joint pain, tracking air pressure can provide useful context for symptoms.
How to monitor: Placement
Air pressure sensors are generally robust and require minimal special placement, but for best accuracy, keep the sensor away from direct airflow from fans, vents, or open windows, which can cause short-term fluctuations. In multi-story buildings, pressure will be slightly lower on higher floors due to altitude.
8. Light Sensor (Luminosity)
What it measures:
Light intensity (lux).
Why it matters:
Light affects mood, alertness, and sleep cycles. Monitoring light levels helps you ensure spaces are well-lit for productivity during the day and dim enough for restful sleep at night. It can also help optimize plant growth or energy savings.
How to monitor: Light Changes Throughout the Day
Light levels naturally vary with the time of day and weather. For best results, monitor in the main activity area and consider both natural and artificial lighting. Note that sensor readings may be affected by direct sunlight or lamp proximity.
For best results, interpret light measurements as indicative trends rather than precise lux values, and always consider the potential impact of the casing when comparing data or making lighting decisions.
9. Movement Counter
What it measures:
The movement counter tracks how many physical movements or jolts have occurred since the last reset. It increases by one whenever motion is detected, based on internal accelerometer activity, regardless of direction.
Why it matters:
While not a full motion log, the movement count offers a simple way to detect if the device or its environment has been disturbed. This is useful for security monitoring, tracking tampering, or simply checking whether the setup has been bumped or relocated. Sudden increases in movement count, especially when unexpected, can indicate that it’s worth checking the surroundings or device placement.
How to monitor:
Compare readings over time or use data logging tools to observe if movement correlates with other changes in air quality, temperature, or positioning. In unstable mounting scenarios (like window shelves or portable setups), regular movement readings are expected.
10. Battery Voltage
What it measures:
Battery voltage reflects the current electrical potential of the device’s power source, measured in volts.
Why it matters:
Battery voltage provides an early warning of power degradation. Tracking this helps ensure consistent performance and prevents unexpected shutdowns. Understanding your device’s battery baseline also helps you anticipate replacement schedules.
How to monitor:
Monitor voltage readings during normal operation to establish a healthy range (typically between 2.5V and 3.0V for coin cells). Rapid drops or unusually low values may indicate aging batteries or environmental effects like cold temperatures.
Ruuvi Station app battery indicator
- If temperature is less than -20 °C, low battery threshold = 2 V
- If temperature is less than 0 °C, low battery threshold = 2.3 V
- Other temperatures, low battery threshold = 2.5 V
11. Acceleration (X, Y, Z)
What it measures:
The acceleration sensors detect movement along three axes: X (left/right), Y (forward/backward), and Z (up/down). Each axis is measured in units of gravitational acceleration (g), with 1g equal to Earth’s gravity.
Why it matters:
These values form the basis of motion detection and orientation sensing. Changes in one or more axes can indicate if the device has shifted, tilted, or been moved. In fixed installations, stable acceleration readings help confirm whether the sensor is positioned securely. In mobile or experimental setups, these readings can also be used to track device orientation in real time.
How to monitor:
Acceleration readings near zero indicate that the sensor is lying level or experiencing no movement in that direction. Sudden spikes, drifts, or consistently high values suggest motion or tilting and may require checking device stability or orientation.
12. Signal Strength (RSSI)
What it measures:
Signal strength, reported as RSSI (Received Signal Strength Indicator), shows how strong the Bluetooth signal is at the receiver, measured in negative decibels (dBm).
Why it matters:
RSSI provides insight into Bluetooth connection quality. Stronger signals (closer to 0 dBm) mean more reliable data transmission, while lower values (further from 0, such as -90 dBm or worse) indicate weak or unstable connections. This can be affected by walls, interference, or distance between the sensor and receiver.
How to monitor:
Use this metric to optimize placement of your sensor and receiver for strong, uninterrupted communication. Regularly fluctuating RSSI might suggest interference, low battery output, or overly long range. For stable monitoring or streaming use cases, a consistent RSSI in the -40 dBm to -90 dBm range is generally ideal.
13. Measurement Sequence Number
What it measures:
This is a running counter that increments each time a new data packet is generated by the sensor. It reflects how many measurements have been transmitted since the device was last reset.
Why it matters:
While it doesn’t directly track environmental data, the sequence number helps validate continuous operation and identify any gaps or dropped packets in long-term logging. It’s especially valuable for developers or integrators working with time-series data or packet-based analytics.
How to monitor:
Expect this number to increase steadily with time if the device is operating normally. A jump backward, reset to zero, or unexpected plateau may suggest a reboot, voltage drop, or transmission issue. Use this value to troubleshoot data continuity or monitor device uptime in automated setups.
Each of these metrics extends the capability for diagnostics, analytics, and automation. While not directly related to air quality, they provide valuable metadata, help validate sensor performance, and contribute to system integrity and insight over time.
Why Monitor All These Factors?
Each measurement gives you actionable insight into a different aspect of your indoor environment. By monitoring them together, you can:
- Detect and respond to hidden problems before they affect your health or comfort.
- Optimize ventilation and heating for well-being and energy savings.
- Create spaces that support productivity, relaxation, and better sleep.
- Protect your home, belongings, and loved ones from invisible risks.
With accurate and useful data it’s easy to make smarter choices, set alerts, and track trends over time helping you to take control of your environment, one measurement at a time.