Complete tutorial : https://help.sinric.pro/pages/tutorials/air-quality-sensors/mq135.html

AirQualitySensor.h

#ifndef _AIRQUALITYSENSOR_H_
#define _AIRQUALITYSENSOR_H_

#include <SinricProDevice.h>
#include <Capabilities/ModeController.h>
#include <Capabilities/RangeController.h>
#include <Capabilities/PushNotification.h>

class AirQualitySensor 
: public SinricProDevice
, public ModeController<AirQualitySensor>
, public RangeController<AirQualitySensor>
, public PushNotification<AirQualitySensor> {
  friend class ModeController<AirQualitySensor>;
  friend class RangeController<AirQualitySensor>;
  friend class PushNotification<AirQualitySensor>;
public:
  AirQualitySensor(const String &deviceId) : SinricProDevice(deviceId, "AirQualitySensor") {};
};

#endif

sinricpro-air-quality-sensor.ino

// Uncomment the following line to enable serial debug output
//#define ENABLE_DEBUG

#ifdef ENABLE_DEBUG
  #define DEBUG_ESP_PORT Serial
  #define NODEBUG_WEBSOCKETS
  #define NDEBUG
#endif

#include <Arduino.h>
#ifdef ESP8266
  #include <ESP8266WiFi.h>
#endif
#ifdef ESP32
  #include <WiFi.h>
#endif

#include <TroykaMQ.h> // https://github.com/amperka/TroykaMQ
#include <SinricPro.h>
#include "AirQualitySensor.h"

#define WIFI_SSID         ""   // Your WiFI SSID name 
#define WIFI_PASS         ""   // Your WiFi Password.

#define APP_KEY           ""   // Should look like "de0bxxxx-1x3x-4x3x-ax2x-5dabxxxxxxxx" (Get it from Portal -> Secrets)
#define APP_SECRET        ""   // Should look like "5f36xxxx-x3x7-4x3x-xexe-e86724a9xxxx-4c4axxxx-3x3x-x5xe-x9x3-333d65xxxxxx" (Get it from Portal -> Secrets)
#define DEVICE_ID         ""   // Should look like "5dc1564130xxxxxxxxxxxxxx" (Get it from Portal -> Devices)

#define BAUD_RATE         115200              // Change baudrate to your need (used for serial monitor)
#define EVENT_WAIT_TIME   60000               // send event every 60 seconds


#define BAUD_RATE         115200
#define EVENT_WAIT_TIME   60000               // send event every 60 seconds

#if defined(ESP8266)
  #define SENSOR_PIN        A0
#elif defined(ESP32) 
  #define SENSOR_PIN        34
#endif

MQ135 mq135(SENSOR_PIN);

const int AQI_THRESHOLD_SEVERE = 700;
const int AQI_THRESHOLD_VERY_POOR = 600;
const int AQI_THRESHOLD_POOR = 500;
const int AQI_THRESHOLD_MODERATE = 400;

int ppm = 0;
int lastPPM = 0;
String lastAirQualityLevelStr = "";

AirQualitySensor &airQualitySensor = SinricPro[DEVICE_ID];
 
// ModeController
void updateMode(String mode) {
  airQualitySensor.sendModeEvent("modeInstance1", mode, "PHYSICAL_INTERACTION");
}

// RangeController
void updateRangeValue(int value) {
  airQualitySensor.sendRangeValueEvent("rangeInstance1", value);
}

// PushNotificationController
void sendPushNotification(String notification) {
  airQualitySensor.sendPushNotification(notification);
}

/********* 
 * Setup *
 *********/

void setupAirQualitySensor() {
   // before calibrating the sensor, warm it up for 60 seconds calibrate the sensor in clean air
   mq135.calibrate();
   // if you know the resistance of the sensor in clean air
   // you can specify it manually, eg: 160 mq135.calibrate(160);   
   Serial.print("Ro = ");
   Serial.println(mq135.getRo());
}

void setupSinricPro() {
  SinricPro.onConnected([]{ Serial.printf("[SinricPro]: Connected\r\n"); });
  SinricPro.onDisconnected([]{ Serial.printf("[SinricPro]: Disconnected\r\n"); });
  SinricPro.begin(APP_KEY, APP_SECRET);
};

void setupWiFi() {
  #if defined(ESP8266)
    WiFi.setSleepMode(WIFI_NONE_SLEEP); 
    WiFi.setAutoReconnect(true);
  #elif defined(ESP32)
    WiFi.setSleep(false); 
    WiFi.setAutoReconnect(true);
  #endif

  WiFi.begin(SSID, PASS);
  Serial.printf("[WiFi]: Connecting to %s", SSID);
  while (WiFi.status() != WL_CONNECTED) {
    Serial.printf(".");
    delay(250);
  }
  Serial.printf("connected\r\n");
}

void setup() {
  Serial.begin(BAUD_RATE);
  setupAirQualitySensor();
  setupWiFi();
  setupSinricPro();
}

/********
 * Loop *
 ********/

 void handleAirQualitySensor() {
  if (SinricPro.isConnected() == false) {
    return; 
  }

  static unsigned long last_millis;
  unsigned long        current_millis = millis();
  if (last_millis && current_millis - last_millis < EVENT_WAIT_TIME) return;
  last_millis = current_millis;
    
  ppm = mq135.readCO2();

  Serial.printf("Present ppm: %d\r\n", ppm); 
  
  if (ppm == lastPPM) {
    Serial.printf("PPM did not changed. do nothing...!\r\n");
    return; 
  }

  /* determine air quality level */
  String airQualityLevelStr = "";

  if (ppm > AQI_THRESHOLD_SEVERE) {
    airQualityLevelStr = "SEVERE";
   } else if (ppm > AQI_THRESHOLD_VERY_POOR) {
    airQualityLevelStr = "VERY POOR";
   } else if (ppm > AQI_THRESHOLD_POOR) {
    airQualityLevelStr = "POOR";
   } else if (ppm > AQI_THRESHOLD_MODERATE) {
    airQualityLevelStr = "MODERATE";
   } else {
    airQualityLevelStr = "GOOD";
   }

  /* Update air quality level label if changed */
  if(lastAirQualityLevelStr != airQualityLevelStr) {
    lastAirQualityLevelStr = airQualityLevelStr;
    updateMode(lastAirQualityLevelStr);
  }
  
  delay(500);

  /* update PPM label */
  updateRangeValue(ppm);

  /* send a push notification when air quality level is bad! */
  if (ppm > AQI_THRESHOLD_SEVERE) {
    sendPushNotification("Air quality is SEVERE!");
  } else if (ppm > AQI_THRESHOLD_VERY_POOR) {
    sendPushNotification("Air quality is VERY POOR!");
  } else if (ppm > AQI_THRESHOLD_POOR) {
    sendPushNotification("Air quality is VERY POOR!");
  }

  lastPPM = ppm;  // save to compare next time.
}

void loop() {
  handleAirQualitySensor();
  SinricPro.handle();
}