Notholithocarpus densiflorus, or tanoak, is an evergreen oak that grows in the Western US. Reaching heights of up to 40 meters, the tree is taller than most oaks1 despite its slender stature, typically growing to no more than 18 inches across. Named for the high amounts of tannins in it's bark, the tree was particularly valuable to the leather industry before synthetic tannins were developed. However it may be most notable for the "dust" it produces: a mixture of pollen and wooly fibers that grow on young leaves notorious in logging circles for it's effects on the respiratory system (especially when combined with high altitudes and summer temperatures north of 100°F).2 Today, trail crews in southern Oregon (tanoak's most prominent location) encounter it often.
"Some people have a pretty bad reaction to tanoak dust, where they actually find it hard to breathe, in a way kind of like an anaphylactic reaction—I've given people Benadryl for it before. It starts to itch at your throat, and then pretty soon it gets worse, and worse, until you're just hacking globs of dust and saliva conglomerate. I think it's worse than poison oak, and I've had poison oak really bad. Poison oak you can do a lot to prevent—although at a point there's not avoiding it—and you can do a lot to ease the symptoms afterwards. But the dust is just everywhere. There is no way around it."
—Gabe Howe, Siskiyou Mountain Club Executive Director