In the occupational setting, falling incidents are commonly referred to as slips, trips, and falls (STFs).[19] Falls are an important topic for occupational safety and health services. Any walking/working surface could be a potential fall hazard. An unprotected side or edge which is 6 feet (1.8 m) or more above a lower level should be protected from falling by the use of a guard rail system, safety net system, or personal fall arrest system.[20] These hazardous exposures exist in many forms, and can be as seemingly innocuous as changing a light bulb from a step ladder to something as high-risk as installing bolts on high steel at 200 feet (61 m) in the air.