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) in order to prevent or reduce 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 for aircraft design or for other structures which experience a large number of stress cycles over their lifetimes. Consideration is also given to durability of materials against possible deterioration which may impair performance over the design lifetime.