The few data points for some of the larger tephra falls, and the problems of determining original fall thicknesses from prehistoric deposits, leaves open the possibility of secondary thickness maxima in these layers. Alternatively, it may reflect varying methods used by different workers to determine original fall thickness. Close to an erupting vent, the main tephra hazards to man-made structures include high temperatures, burial, and impact of falling fragments. Large blocks thrown on ballistic trajectories from an erupting vent can damage structures and kill or injure unprotected people. Hot tephra may set fire to forests and flammable structures, but this is not likely to be a hazard beyond a distance of 15-20 km.
Structural damage can also result from the weight of tephra, especially if it is wet.
Also, tephra is much more cohesive when wet than when dry, and can adhere to steeper surfaces and is much more difficult to remove. Drifting of tephra by winds can locally increase accumulations and loads on sloping structures far above that resulting from unmodified fall thicknesses. At distances of tens to hundreds of kilometers, the chief hazards from tephra falls are the effects of ash on machinery and electrical equipment and on human and animal respiratory systems. These effects could impair the ability of personnel to perform even routine tasks in areas affected by tephra fall. A wide range of compositions and volumes of tephra have been erupted during the past 15,000 years from Cascade volcanoes.