What Is Poison Oak?

Poison oak carries the same kind of oil as poison ivy. A USDA Forest Service guide says reactions, treatments and preventive measures are the same for poison ivy, poison oak and poison sumac, and the National Park Service says most people break out in an itchy red rash within hours to a few days after contact with the urushiol oil in poison oak, which is potent even in small doses.

There are two main kinds in the US. The National Park Service says Pacific poison oak is native from British Columbia down through California, growing below about 5,000 feet in mixed evergreen forests, woodlands, chaparral and riparian areas, in both sun and shade. The Forest Service's Fire Effects Information System says Atlantic poison oak occurs from New Jersey to Florida, west to eastern Texas and north to southeastern Kansas, and that unlike eastern poison ivy it is not a climber and does not produce aerial roots.

The National Park Service says poison oak leaves have more rounded and irregular lobes than poison ivy leaves, which have pointed tips and are not distinctly lobed. CDC's NIOSH says the saying leaves of three, let it be is a helpful reminder for poison ivy and poison oak, but does not apply to poison sumac, which usually has clusters of 7 to 13 leaves.

The Park Service says the reaction tends to get worse with increasing exposure and is particularly dangerous when triggered by inhaling the smoke of burning poison oak. The Forest Service guide says the rash, itch and blisters disappear in 14 to 20 days without treatment, and that over-the-counter corticosteroids with 1 percent hydrocortisone help relieve itching.