Indoor air quality (IAQ) is influenced not only by CO₂ and humidity, but also by volatile organic compounds (VOCs) such as cooking fumes, cleaning chemicals, paints, perfumes, and off-gassing from furniture.
The SGP40 is a high-quality VOC sensor from Sensirion that provides a reliable VOC Index for Home Assistant using an ESP32.

This guide covers both ESPHome and MQTT integration methods.

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1. What Is the SGP40 VOC Sensor?

The SGP40 measures volatile organic compounds, producing a VOC Index:

• 0–100 → Excellent air
• 100–200 → Normal
• 200–400 → Polluted air
• 400+ → Very poor air (cooking/chemicals)

It does not measure specific gases; instead it outputs a calibrated score for overall VOC presence.

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2. Hardware Required

• ESP32 DevKit
• SGP40 sensor board
• Jumper wires
• (Optional) BME280/SHT45 for humidity compensation
• USB cable

The SGP40 uses I2C, like most environmental sensors.

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3. Wiring Diagram (I2C)

ESP32 → SGP40
3.3V → VIN
GND  → GND
GPIO21 → SDA
GPIO22 → SCL

METHOD 1 — ESPHome Integration

ESPHome supports SGP40 with optional humidity compensation for better accuracy.

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4. ESPHome YAML – SGP40 Only

esphome:
  name: esp32-sgp40
  platform: ESP32
  board: esp32dev

wifi:
  ssid: "YOUR_WIFI"
  password: "YOUR_PASSWORD"

logger:
api:
ota:

i2c:
  sda: 21
  scl: 22
  scan: true

sensor:
  - platform: sgp40
    name: "VOC Index"

This creates:

• sensor.voc_index

Auto-discovered by Home Assistant.

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5. ESPHome YAML – SGP40 + Humidity Compensation (Recommended)

VOC readings improve greatly if humidity is also supplied to the SGP40 algorithm.

Example with SHT45:

sensor:
  - platform: sht4x
    temperature:
      name: "Temperature"
      id: temp_air
    humidity:
      name: "Humidity"
      id: hum_air
    update_interval: 10s

  - platform: sgp40
    name: "VOC Index"
    compensation:
      temperature: temp_air
      humidity: hum_air

Now ESPHome uses real temperature & humidity to correct VOC output.

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METHOD 2 — MQTT Integration

This matches the same Home Assistant structure used in BME280, SHT45, and other MQTT articles.

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6. Home Assistant configuration.yaml — MQTT VOC Sensor

Add under your existing:

mqtt:
  sensor:

Correct MQTT YAML Block

mqtt:
  sensor:
    - name: "VOC Index"
      state_topic: "home/air/voc"
      value_template: "{{ value_json.voc }}"
      unit_of_measurement: "Index"

This creates one sensor reading the VOC score.

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7. ESP32 Arduino Code for MQTT (SGP40 VOC Index)

#include <WiFi.h>
#include <PubSubClient.h>
#include <SensirionI2CSgp40.h>

#define WIFI_SSID "YOUR_WIFI"
#define WIFI_PASS "YOUR_PASSWORD"
#define MQTT_SERVER "192.168.0.10"

WiFiClient espClient;
PubSubClient client(espClient);

SensirionI2CSgp40 sgp40;
uint16_t voc_index;

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);

  sgp40.begin(Wire);

  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) delay(500);

  client.setServer(MQTT_SERVER, 1883);
}

void loop() {
  if (!client.connected()) {
    while (!client.connected()) client.connect("ESP32_VOC");
  }

  voc_index = sgp40.measureRawSignal(0, 0); // no compensation

  String json = "{\"voc\":" + String(voc_index) + "}";
  client.publish("home/air/voc", json.c_str());

  client.loop();
  delay(3000);
}

Adding temperature & humidity compensation

Use readings from any sensor (BME280, SHT45, DS18B20).

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8. Home Assistant Dashboard Example

type: gauge
entity: sensor.voc_index
min: 0
max: 500
severity:
  green: 0
  yellow: 150
  red: 300

9. Useful VOC Automations

Alert when kitchen fumes spike

automation:
  - alias: "VOC High Alert"
    trigger:
      - platform: numeric_state
        entity_id: sensor.voc_index
        above: 250
    action:
      - service: notify.mobile_app
        data:
          message: "Air quality is poor — open a window."

Automatically turn on ventilation

  - alias: "Auto Ventilate on VOC Rise"
    trigger:
      - platform: numeric_state
        entity_id: sensor.voc_index
        above: 200
    action:
      - service: fan.set_percentage
        target:
          entity_id: fan.cabinet_fan
        data:
          percentage: 100

Restore low fan speed after cleaning VOC drops

  - alias: "Restore Fan After Ventilation"
    trigger:
      - platform: numeric_state
        entity_id: sensor.voc_index
        below: 120
    action:
      - service: fan.set_percentage
        target:
          entity_id: fan.cabinet_fan
        data:
          percentage: 30

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10. Troubleshooting

VOC Index always high

• Sensor needs 24h burn-in for calibration
• Airflow must reach sensor → avoid sealed boxes

VOC Index drops too slowly

• High humidity slows VOC clearing
• Use humidity compensation for better accuracy

Sensor not detected

• Swap SDA/SCL wiring
• Enable scan: true in ESPHome I2C config
• Cheap breakout boards sometimes mislabeled pins

Readings fluctuate rapidly

• Update interval too short → increase to 5–10 seconds

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11. SGP40 vs Other VOC Sensors

Sensor	VOC Accuracy	Notes
SGP40	★★★★★	Best accuracy, recommended
CCS811	★★★☆☆	Needs baseline calibration
MQ Gas Sensors	★☆☆☆☆	Very inaccurate, high power
SGP30	★★★★☆	Older but still good

SGP40 is the current recommended choice for Home Assistant.

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