   //Serial.println(Rs);
   //Serial.println(R0);
   //Serial.println(ratio);
-  Serial.println(concentration);
-  return concentration;
+  if(concentration>0)
+  {
+    Serial.println(concentration);
+    return concentration;
+  }
+  else
+  {
+    Serial.println("0");
+    return 0;
+  }
 }
 
 float readNO2()

  
  Serial.println("---------------------");
  
  // MAX 4466 amp
  //Serial.println(analogRead(A0));  // read sound levels
  //Serial.println();


  // TSL2561 light sensor

  //Serial.println(Rs);
  //Serial.println(R0);
  //Serial.println(ratio);
  if(concentration>0)
  {
    Serial.println(concentration);
    return concentration;
  }
  else
  {
    Serial.println("0");
    return 0;
  }
}
