In very tall buildings, many occupants would be physically incapable of negotiating the stair shafts from the upper floors to the ground level, particularly under emergency conditions. Although smoke spread is a factor that must be considered for low buildings, it takes on much more serious significance for high-rise buildings because escape routes may become untenable before the occupants can be evacuated. This Digest discusses the causes and the characteristics of smoke movement in buildings. Characteristics of Smoke Two important features of smoke are that it contains toxic products of combustion and reduces visibility. The two factors are generally related.
Dense smoke is usually highly toxic and reduces visibility substantially.
Oxygen and carbon dioxide may vary about 10 per cent from normal amounts without substantially affecting the physical properties of the atmosphere. The concentration of particulate smoke, even under conditions of very low visibility, is not sufficient to alter the characteristics of an atmosphere enough to affect its movement. It can be assumed, therefore, that smoke will probably follow the over-all air movement within a building. Although a fire may be confined within a fire resistive compartment, smoke can readily spread to adjacent areas through such air leakage openings in the enclosure of the compartment as cracks, openings around pipes, ducts and doors. These leakage openings will permit a substantial smoke flow even with quite small pressure differentials.