The resin canals contain a poisonous oil (urushiol) to which many people are allergic on contact, some strongly so and others only mildly. The itching (and blistering) reaction is delayed, usually 1-2 days after exposure but sometimes in especially sensitive persons only a few hours, and in others sometimes a week or more. The oil is not volatile, and one cannot get an allergic reaction from merely being near a plant. The plant must be bruised to break the resin canals, but the oil can then be carried on shoes or clothing, on an animal such as a cat or dog, as droplets in smoke from burning plants, or on the surface of water (as may happen with poison sumac in a bog or fen). Only an extremely minute amount of oil is required to produce a reaction in a sensitive person.

The chief reason for the usual advice, upon exposure, to wash as quickly as possible with soap or preferably alcohol (in which the oil dissolves) is to remove excess oil beyond what has, almost immediately, reacted with proteins in the skin. It is this excess oil which can be transferred by clothing or scratching fingers. The fluid in the blisters of an infection does not spread the rash.

1. Plant a large shrub or occasionally a small tree with pinnately compound leaves of (5-) 7-11 (-13) leaflets, usually in wet, ± open habitats.

T. vernix

1. Plant a climbing vine or low shrub with trifoliolate leaves, in dry, mesic, or floodplain habitats.

2. Plant a climbing vine or scrambling vine-like shrub (if sprawling on the ground, the stems superficial, deep-seated rhizomes not present), aerial roots present on climbing stems; petioles ± densely pubescent.

T. radicans

2. Plant a non-climbing, low, erect shrub from creeping rhizomes, aerial roots never formed; petioles essentially glabrous, except sometimes when very young.

T. rydbergii