Structural engineers ensure that buildings and bridges are built to be strong enough and stable enough to resist all appropriate structural loads (e.g., gravity, wind, snow, rain, seismic (earthquake), earth pressure, temperature, and traffic) to prevent or reduce the loss of life or injury. They also design structures to be stiff enough to not deflect or vibrate beyond acceptable limits. Human comfort is an issue that is regularly considered in the limits. Fatigue is also an important consideration for bridges and aircraft design or for other structures that experience many stress cycles over their lifetimes. Consideration is also given to the durability of materials against possible deterioration which may impair performance over the design lifetime.