Confined spaces are areas where there is limited access into and exit from the space, such as tunnels, tanks, sewers, and silos. Working in these spaces requires special precautions to limit the hazard to the worker. Confined space fans attach to ducts to circulate air and remove fumes. Confined space entry and retrieval systems consist of a frame, lifeline, and other components for raising, lowering, and supporting workers. Permits and permit holders include checklists and authorizations needed before workers can enter a confined space. Manhole entry, retrieval, and fall prevention equipment raises and lowers workers, and prevents people from falling into open manholes.