CO sensor
This sensor senses the carbon monoxide in the air. This sensor gives output in voltage which can be translated as ppm in air thus giving the pollution in air

SO2 Sensor
This sensor senses the amount of Sulfur Dioxide in air. This sensor gives output in voltage which can be translated as ppm in air thus giving the pollution in air.

Bluetooth module
It is a class-2 Bluetooth module with Serial Port Profile , which can configure as either Master or Slave. We are you using this module to transmit data from Arduino to Android.