Urushiol is the causative agent in poison oak and poison ivy dermatitis. Urushiol concentration in poison oak (Toxicodendron diversilobum) leaves was quantified weekly between mid-August and late November for several individual genotypes of plants that had been grown in either sun, moderate shade, or deep shade. These individual plants were grown in the same garden to reduce variation caused by environmental factors. Plants cloned from a coastal genotype had higher levels of urushiol than those from an inland site, suggesting that people who live in coastal areas may be at a greater risk for contracting poison oak dermatitis than people who live farther inland. The concentration of urushiol remained relatively constant in the inland clones until early November, at which point it increased by three to six times. This increase in urushiol may be responsible for the greater number of cases that some dermatologists have noticed in the autumn. The level of urushiol in leaf litter was very low, and so it can be considered nonallergenic.
