Mount Hood has typically not produced thick, extensive deposits of tephra, but relatively modest amounts of tephra were produced during past lava-flow and lava-dome eruptions. Most tephra fallout was caused by clouds of volcanic ash rising from moving pyroclastic flows that were generated by lava-dome collapse. In Mount Hood's past, tephra was also produced by explosive, gas-driven, volcanic eruptions.
Future eruptions will produce tephra plumes reaching up to 1,000 to 15,000 meters (3,000 to 50,000 feet) or more above the volcano. A prolonged lava-dome-building eruption will produce numerousash clouds that could reach altitudes that would impact aviation; even minor ash can cause jet engines to stall. The finest particles in tephra clouds would be blown by the wind, which are directed toward the northeast, east, or southeast of Mount Hood about 70 percent of the time. Winds directed toward the Portland metropolitan area occur only a few percent of the time. Areas up to 5 km (3 mi) from a vent could also be subject to showers of large (more than 5 cm or 2 inches diameter) ballistic fragments within a few minutes of an eruption.
Tephra fallout produced by future eruptions of Mount Hood poses little acute threat to life, structures, or communities. However, the ash can affect people with pre-existing respiratory conditions, affect grazing animals, plus clog filters and increase wear on vehicle engines. Deposits of tephra can also short-circuit electric transformers and power lines, especially if the tephra is wet and thereby highly conductive, sticky, and heavy. This could affect hydroelectric power generation. Additionally, important watersheds for the Portland metro area and The Dalles are vulnerable to tephra fallout from future eruptions.