The National Institute for Occupational Safety and Health (NIOSH) has a Web site that provides a definition for confined spaces and offers several links to its publications on the subject.
One of the publications, A Guide to Safety in Confined Spaces, addresses information on recognition of confined spaces and their hazards. It discusses specific safe work practices for testing, monitoring, and ventilating the atmosphere; isolation of energy sources (lockout-tagout); respirators; and standby/rescue. The publication also addresses general physical hazards such as temperature extremes, engulfment hazards, slick surfaces, and noise. A checklist for confined space entry is appended.