A potent contact allergen of Phacelia (Hydrophyllaceae)

The major contact allergen of Phacelia crenulata (Hydrophyllaceae) has been identified as geranylhydroquinone. A maximization test of geranylhydroquinone showed this to be a potent sensitizer comparable in degree to poison oak/ivy urushiol. Comparative patch testing on humans with urushiol established that the phacelia allergen does not cross-react with poison oak or ivy.