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Yes—you can build a Wi-Fi air-quality monitor that appears in Home Assistant with an ESP32, ESPHome firmware and the sensor hardware suited to the pollutants you actually need. The simplest route uses the ESPHome-supported SM300D2 module, while a modular design gives you better control over CO₂ measurement, particulate sensing and calibration. The important limitation is interpretation: a PM-based number from ESPHome is an estimate, not the complete U.S. EPA Air Quality Index, and the SM300D2’s CO₂ field is calculated eCO₂ rather than a direct CO₂ measurement.
What the finished system does
The ESP32 provides Wi-Fi and runs ESPHome. You describe the hardware and entities in an ESPHome YAML file, compile and flash the firmware, then add the device to Home Assistant through its native ESPHome API. Each configured reading becomes a Home Assistant sensor entity that can be displayed on a dashboard or used in an automation.
Home Assistant’s Air quality integration can combine pollutant entities and provide pollutant-aware conditions and triggers. It supports PM2.5, PM10, CO₂, carbon monoxide, volatile organic compounds (VOCs), ozone, nitrogen dioxide, and gas or smoke detector data. You can therefore use the raw readings directly, or build ventilation and warning automations around thresholds that make sense for your home.
Choose the sensor architecture
All-in-one: SM300D2
ESPHome has an official SM300D2 platform. With one UART-connected module, the configuration can expose seven measurement groups:
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- CO₂ (reported by the module as ppm)
- Formaldehyde (µg/m³)
- TVOC
- PM2.5
- PM10
- Temperature
- Relative humidity
The SM300D2 project publishes an operating and accuracy table claiming CO₂ of 400–5,000 ppm with ±50 ppm accuracy and PM2.5 of 5–100 µg/m³ with ±10% accuracy. Those are the project’s module claims, recorded in an ESP-Air project snapshot accessed in 2026; they are not a universal validation of every seller’s board revision.
Modular sensors
A modular build uses separate ESPHome components for the measurements that matter most. The component catalog includes direct CO₂ platforms such as SCD30, SCD4X and SenseAir, particulate platforms such as SPS30, PMSA003, PMSX003, SDS011 and HM3301, and separate VOC, NOx, temperature and humidity sensors.
This approach adds wiring and configuration, but lets you choose a direct CO₂ instrument, a particulate sensor with better documentation, or a narrower and less expensive combination. It is also easier to replace one sensor when its calibration drifts or when you later need another pollutant.
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| Question | SM300D2 all-in-one | Modular design |
|---|---|---|
| Pollutant coverage | CO₂/eCO₂, formaldehyde, TVOC, PM2.5, PM10, temperature and humidity in one module | Select only the pollutant platforms required |
| CO₂ type | eCO₂ inferred from TVOC, not directly measured | Can use a direct CO₂ sensor such as SCD30, SCD4X or SenseAir |
| Wiring and setup | One UART sensor and one ESPHome platform | More buses, pins, power checks and YAML components |
| Calibration evidence | Use the particular module revision’s documentation | Evaluate each sensor’s calibration and accuracy documentation separately |
Understand the eCO₂ limitation before automating
The SM300D2 project explicitly states that its CO₂ sensor is actually an “eCO2 (equivalent carbon dioxide) sensor”: the value is calculated or approximated from the TVOC reading rather than measured as CO₂. Treat that number as an indicative air-quality signal. Do not present it as measured CO₂ or use it as the sole basis for precise ventilation control.
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If an automation must respond to actual carbon-dioxide concentration—for example, controlling outdoor-air ventilation in an occupied room—use a modular build with a direct CO₂ sensor and follow that sensor’s calibration and placement guidance.
Assemble the hardware safely
- Select an ESP32 development board. Confirm that it has the USB connection and GPIOs needed for your chosen interface.
- Select the sensor architecture. Use the SM300D2 for a compact seven-group monitor, or combine dedicated platforms when direct CO₂ or specific particulate performance is more important.
- Wire the sensor interface. The SM300D2 ESPHome example uses UART. Connect power, ground, transmit and receive lines according to the module’s datasheet and the pin assignment you choose on the ESP32.
- Verify electrical compatibility. Check the sensor’s supply voltage and logic levels against the exact ESP32 board and module revision. The available documentation does not provide one wiring diagram that is valid for every ESP32 board revision.
- Keep the air inlet unobstructed. Mount the sensor where room air can reach it, away from direct exhausts, condensation and sources of heat that could bias temperature or humidity.
Configure ESPHome
Create a device configuration in ESPHome and give it a unique name. Add the platform for your sensor, the UART pins (for the SM300D2), and a deliberately chosen update interval. Name entities clearly—such as PM2.5, PM10, eCO₂, TVOC, temperature and humidity—so they are understandable after Home Assistant discovers them.
For a modular build, add one ESPHome component per sensor family and assign the correct bus or UART address. Keep units explicit in the entity names or metadata, especially for particulate readings in µg/m³ and gas readings in ppm.
Compile the configuration and flash it to the ESP32 over USB for the initial installation. After the device joins Wi-Fi, subsequent firmware updates can normally use ESPHome’s network update path. Watch the compile and device logs for UART errors, unavailable entities or repeated resets before mounting the hardware permanently.
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Add the device to Home Assistant
- Open Home Assistant and go to Settings → Devices & services.
- Use the ESPHome integration to discover or add the ESP32 device.
- Complete adoption through the native ESPHome API connection.
- Open the device page and confirm that every expected pollutant, environmental and diagnostic entity is present.
- Add the entities to a dashboard, then create thresholds or schedules only after observing their normal readings for several days.
Use the Air quality integration when you want pollutant-aware triggers and conditions rather than a dashboard of unrelated numeric sensors. Keep the raw PM2.5, PM10 and gas entities available even if you also create a combined air-quality indicator.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the ESPHome AQI value means
ESPHome’s aqi sensor accepts PM2.5 and PM10 readings and can apply AQI or CAQI calculations. Its documentation warns that this is a PM-only estimate, not the complete U.S. EPA AQI.
The EPA AQI accounts for ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide. Its particle method uses 24-hour averages or NowCast-style hourly estimates. A fast-changing PM reading from a room sensor does not automatically satisfy those averaging rules, and it omits the other pollutants unless you measure them separately.
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Common failure modes
No sensor entities appear
Recheck the selected ESPHome platform, UART transmit/receive orientation, power and ground connections, and the configured GPIO numbers. Confirm that the module’s voltage and logic levels are compatible before reconnecting it.
Values are implausible or stuck
Inspect the device logs for communication errors, allow the sensor to warm up as specified by its documentation, and check that the inlet is not blocked. Compare the reading with a known-good instrument before changing automations.
CO₂ automation behaves poorly
Determine whether the entity is eCO₂ inferred from TVOC. If it is, replace it with a direct CO₂ platform for decisions that require measured concentration, then revisit thresholds and placement.
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The AQI number disagrees with an official report
Check whether your entity uses PM only, which pollutants are missing, and whether it uses instantaneous, hourly or 24-hour data. Those methodological differences can produce legitimately different values.
Quick Recap
A practical build plan
- Start with the ESP32 and SM300D2 if you want the fewest parts and broad room-monitoring coverage.
- Expose and name all seven measurement groups, but mark the CO₂ entity as eCO₂ in your dashboard.
- Observe raw readings before creating alerts; establish normal ranges for the room and time of day.
- Add PM-based calculations only with a label that identifies the method and averaging assumptions.
- Move to a modular design with a direct CO₂ sensor or dedicated particulate sensor when the measurement’s accuracy, calibration behavior or automation consequence justifies extra wiring.
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