#define MQ135_DO 4

void setup() {
  Serial.begin(9600);
  pinMode(MQ135_DO, INPUT);
  Serial.println("MQ135 Warming Up....");
  delay(200);
}

void loop() {
  int airState = digitalRead(MQ135_DO)  ;
  if(airState == 0){
    Serial.println("Poor Air Quality Detected ");
  }else{
    Serial.println("Air Quality Normal ");
  }
  delay(1000);
}