NIOSH warns construction workers and supervisors about the many hazards present in this industry, with emphasis on trench cave ins. Construction is one of the nation's most hazardous occupations, with an annual work related fatality rate three times greater than the rate for all industry sectors. One of the preventable accidents is trench cave ins which kill approximately 70 construction workers each year. Factors contributing to trench cave ins include soil stability which is affected by the proximity to highways, large machinery, backfilled areas, and existing structures. Timber or other materials can be used to shore up trenches to ensure that the earth does not collapse on workers. Sides can be sloped to reduce the overburden. Shield or trench box systems are manufactured for protecting workers in excavations or can be designed by qualified engineers. Additional hazards facing workers in the construction field include working at heights, working with heavy machinery, manually handling materials, and working around sources of electricity such as overhead powerlines.