Issue in MQ135 Sensor using esp 12e?

Other Hardware Sensors
1

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