#define MQ135_DO 8

void setup() {
  Serial.begin(9600);

  pinMode(MQ135_DO, INPUT);

  Serial.println("MQ-135 warming up");
  delay(20000);
}

void loop() {
  int airState = digitalRead(MQ135_DO);

  if (airState == LOW) {  
    Serial.println("⚠️ POOR AIR QUALITY DETECTED!");
  } else {
    Serial.println("AIR Quality Normal");
    
  }

  delay(1000);
}