You may fall and get hurt can be too the weight of the backpack
When you exercise, your body produces something called endorphins (it's sort of like morphine), and it makes your body feel less pain and make you feel more happy. In this case, the endorphins combat your stress and make you feel better mentally.
The long-term effects of damage to your lungs (possibly lung cancer) and the possibility of giving those around you second-hand smoke.
