Our aim is to design a sensor that can detect acid gas and formaldehyde gas in air. As iPhone is the most popular smart phone with powerful hardware basis as well as software support, the sensor will be integrated on an iPhone so that it can be portable and user friendly. During the sampling period the sensors will detect the gases and the functions of flashlight and taking photos in iPhone will be used to acquire information from the sensors. Then the concentration of the gases will be analyzed by the app we create in the iPhone and it will also provide useful suggestions such as how to protect yourself or whether leave that place or not. Also user can share their result online or see the results from other people.