diff --git a/MQ135.cpp b/MQ135.cpp
index 676ed31..db2a390 100755
--- a/MQ135.cpp
+++ b/MQ135.cpp
@@ -11,6 +11,8 @@ the datasheet but the information there seems to be wrong.
 @section  HISTORY
 
 v1.0 - First release
+v1.1 - Add rzero param in constructor,
+       correcting atmospheric CO2 level for 2019 year
 */
 /**************************************************************************/
 
@@ -20,20 +22,20 @@ v1.0 - First release
 /*!
 @brief  Default constructor
 
-@param[in] pin  The analog input pin for the readout of the sensor
+@param[in] pin    The analog input pin for the readout of the sensor
+@param[in] rzero  Calibration resistance at atmospheric CO2 level
 */
 /**************************************************************************/
-
-MQ135::MQ135(uint8_t pin) {
+MQ135::MQ135(uint8_t pin, float rzero) {
+  _rzero = rzero;
   _pin = pin;
 }
 
-
 /**************************************************************************/
 /*!
 @brief  Get the correction factor to correct for temperature and humidity
 
-@param[in] t  The ambient air temperature
+@param[in] t  The ambient air temperature in Celsius
 @param[in] h  The relative humidity
 
 @return The calculated correction factor
@@ -60,7 +62,7 @@ float MQ135::getResistance() {
 @brief  Get the resistance of the sensor, ie. the measurement value corrected
         for temp/hum
 
-@param[in] t  The ambient air temperature
+@param[in] t  The ambient air temperature in Celsius
 @param[in] h  The relative humidity
 
 @return The corrected sensor resistance kOhm
@@ -78,7 +80,7 @@ float MQ135::getCorrectedResistance(float t, float h) {
 */
 /**************************************************************************/
 float MQ135::getPPM() {
-  return PARA * pow((getResistance()/RZERO), -PARB);
+  return PARA * pow((getResistance()/_rzero), -PARB);
 }
 
 /**************************************************************************/
@@ -86,14 +88,14 @@ float MQ135::getPPM() {
 @brief  Get the ppm of CO2 sensed (assuming only CO2 in the air), corrected
         for temp/hum
 
-@param[in] t  The ambient air temperature
+@param[in] t  The ambient air temperature in Celsius
 @param[in] h  The relative humidity
 
 @return The ppm of CO2 in the air
 */
 /**************************************************************************/
 float MQ135::getCorrectedPPM(float t, float h) {
-  return PARA * pow((getCorrectedResistance(t, h)/RZERO), -PARB);
+  return PARA * pow((getCorrectedResistance(t, h)/_rzero), -PARB);
 }
 
 /**************************************************************************/
@@ -112,7 +114,7 @@ float MQ135::getRZero() {
 @brief  Get the corrected resistance RZero of the sensor for calibration
         purposes
 
-@param[in] t  The ambient air temperature
+@param[in] t  The ambient air temperature in Celsius
 @param[in] h  The relative humidity
 
 @return The corrected sensor resistance RZero in kOhm
diff --git a/MQ135.h b/MQ135.h
index 22cefec..f1322e3 100755
--- a/MQ135.h
+++ b/MQ135.h
@@ -11,6 +11,8 @@ the datasheet but the information there seems to be wrong.
 @section  HISTORY
 
 v1.0 - First release
+v1.1 - Add rzero param in constructor,
+       correcting atmospheric CO2 level for 2019 year
 */
 /**************************************************************************/
 #ifndef MQ135_H
@@ -23,8 +25,7 @@ v1.0 - First release
 
 /// The load resistance on the board
 #define RLOAD 10.0
-/// Calibration resistance at atmospheric CO2 level
-#define RZERO 76.63
+
 /// Parameters for calculating ppm of CO2 from sensor resistance
 #define PARA 116.6020682
 #define PARB 2.769034857
@@ -36,14 +37,16 @@ v1.0 - First release
 #define CORD 0.0018
 
 /// Atmospheric CO2 level for calibration purposes
-#define ATMOCO2 397.13
+/// On august 2019 from https://www.co2.earth
+#define ATMOCO2 409.95
 
 class MQ135 {
  private:
   uint8_t _pin;
+  float _rzero;
 
  public:
-  MQ135(uint8_t pin);
+  MQ135(uint8_t pin, float rzero=76.63);
   float getCorrectionFactor(float t, float h);
   float getResistance();
   float getCorrectedResistance(float t, float h);