According to this guide, confined spaces include boilers, cupolas, degreasers, furnaces, pipelines, pits, pumping stations, reaction or process vessels, septic tanks, sewage digesters, sewers, silos, storage tanks, ship's holds, utility vaults, and vats. A confined space has limited openings for entry and exit, unfavorable natural ventilation, and is not designed for continuous worker occupancy. Hazardous atmospheres that may be present include oxygen deficient atmospheres, flammable atmospheres, or toxic atmospheres. Testing of the atmosphere in the space may be accomplished by using properly calibrated instruments to determine what gases are present at the top, middle, and bottom of the space. If an unfavorable atmosphere exists, ventilation must be performed before entry is made. If ventilation is not possible, adequate personal respiratory protective equipment must be used. Only air supplying respirators should be used in confined spaces where there is not enough oxygen. Over 50 percent of the workers who die in confined spaces are trying to rescue other workers. Only workers trained in and properly outfitted for rescue work should make such an attempt. When evaluating possible hazards in a confined area consideration should also be given to possible temperature extremes, engulfment hazards, noise, the presence of slick and wet surfaces, and falling objects. A checklist for the evaluation of confined spaces for safe entry is included.