Don’t spray poison ivy cut the stem close to the ground and pour a little bit of stump killer on the cut end. It will go down to the root and kill it. Some of us are so sensitive to the spray that if we breathe it, it would kill us.
I remember a few years ago a professional football player was spraying his lawn with weed killer and it closed up his throat and he died.
Most people wouldn’t be hurt by it but some of us react in serious ways.
That method is, indeed, best for small infestations -- but I'm dealing with 65 acres of forested land. I frequently work from my tractor, spray after I've "bush-hogged" (leaving short, cut stems), stay upwind, and use a narrow stream from the pump sprayer. Ortho "Brush-B-Gon", properly diluted, seems to be doing the job...