Issue in MQ135 Sensor using esp 12e?

Hi all, My project is about mq135 sensor to calculate air quality in domestic field.Can any one help me how to calculate sensor value in ppm to matches a standard air quality index....Here is my code

#include "MQ135.h"
#define ANALOGPIN A0    //  Define Analog PIN on Arduino Board
#define RZERO 206.85    //  Define RZERO Calibration Value
int I_high=100;
int I_low=0;
int B_high=100;
int B_low=0;
MQ135 gasSensor = MQ135(ANALOGPIN);

void setup()
{
  Serial.begin(9600);
  float rzero = gasSensor.getRZero();
  delay(3000);
  Serial.print("MQ135 RZERO Calibration Value : ");
  Serial.println(rzero);
}

void loop() {
  float ppm = gasSensor.getPPM();
  delay(1000);
  digitalWrite(13,HIGH);
  Serial.print("CO2 ppm value : ");
  Serial.println(ppm);
  float rs = gasSensor.getResistance();
  delay(3000);
  Serial.print("MQ135 Correction Factor : ");
  Serial.println(rs);

  float air_quality=((100-0)/(ppm-0))*(ppm-0)+0;
  Serial.println(air_quality);
}

Please help me...Thanks in advance