Applications:

Indoor Air Quality: The MQ-4 sensor is widely used to detect methane leaks in households, industries, and commercial spaces. It’s particularly useful in preventing potential explosions due to gas leaks.

Outdoor Air Quality: Deployed in outdoor environments to track pollution levels and understand the overall air quality.

Environmental Research: Employed in research projects to gather data on various gases and their concentrations.

Personal Projects: Popular among makers and DIY enthusiasts for building air quality monitoring devices.

Considerations:

Warm-Up Time: The sensor might require some time to stabilize before providing accurate readings.

Cross-Sensitivity: The MQ-135 sensor can exhibit cross-sensitivity to various gases, which might affect its accuracy in mixed gas environments.

Environmental Factors: Factors like temperature, humidity, and the presence of other gases can influence its performance. When integrating the MQ-135 Air Quality/Gas Detector Sensor Module into a project, proper calibration and accounting for potential cross-sensitivity are crucial for obtaining accurate and meaningful air quality readings.

#include <LiquidCrystal.h>
int sensorValue;
const int rs = 12, en = 11, d4 = 5, d5 = 4, d6 = 3, d7 = 2;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);
void setup(){  lcd.begin(16, 2);
Serial.begin(9600);                            // sets the serial port to 9600
 } //https://kitsguru.myshopify.com/products/mq135-mq-135-air-quality-sensor-hazardous-gas-detection-module
void loop(){sensorValue = analogRead(0);       // read analog input pin 0
Serial.print("AirQua=");
Serial.print(sensorValue, DEC);               // prints the value read
Serial.println(" PPM");
lcd.setCursor(0,0);
lcd.print("ArQ=");
lcd.print(sensorValue,DEC);
lcd.print(" PPM");
lcd.println("       "); 
lcd.print("  ");
delay(100);                                   // wait 100ms for next reading
} //Credits : https://create.arduino.cc/projecthub/m_karim02/arduino-and-mq-135-gas-sensor-with-arduino-code-a8c1c6