There are many advantages of LEDs in energy and environmental conservation, but their short life in many outdoor applications prompt a necessity to have a detailed understanding of their degradations to prolong their lifetime, which can also conserve LED material and even expand their applications. Using ab initio density functional theory formulation, we identify the detail paths of the LED degradation in outdoor applications. We discovered that the main stressors are humidity and the light that is emitted from the LED chip itself. This is rather ironical. A mathematical model is developed based on the ab initio study, and excellent experimental agreements are found. With this model, we can predict the situations where no and slow degradations can be achieved, and these are verified experimentally here. We can also predict quantitatively the time to a specific degradation severity. Quality index of the housing material for LEDs can also be determined.