Among the general population, it is estimated that roughly 70% are sensitive to urushiol. It is challenging to estimate the incidence rates of poison ivy, oak, and sumac exposure among agricultural workers in the United States because so many farms are exempt from mandatory reporting to the Bureau of Labor Statistics (37,44).
Generally, poison ivy can be found as crawling vines east of the Rocky Mountains near bodies of water in the east and Midwest, and more spread-out from water in the south. Somewhat similarly, poison sumac can be found in boggy areas. Poison oak is generally found west of the Rocky Mountains as a vine, shrub, or small tree. None of these three plants grow well above 4000 feet of elevation, and are not found in desert areas. They are native to the Americas and very rarely found in Europe. Poison oak is restricted to North America, while poison ivy and poison sumac can be found in North and South America, and poison ivy is also found in East Asia. Within the Americas, the three plants are seldom found in Texas, Arizona, central Mexico, northern Canada, or Alaska and are unknown in the Hawaiian islands. The plants themselves vary greatly in appearance depending upon geographic region (34,37,38).
Ragweed, the most important allergenic plant in the Ambrosia genus and Compositae family, is found naturally in North America but also in Australia, Europe (occasionally), and India. Feverfew or carrotweed (Parthe-nium hysterophorus) is native to the southern United States but is called the "scourge of India" as it caused an epidemic of AACD in India. Liverwort (Frulania) thrives in humid climates, including the Pacific Northwest, tropics, and subtropics (34,38,41,43).
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