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Contact with this annoying oil produces a rash in three out of four people. The rash can begin within a few hours after contact, or it can start three to five days later. The rash starts with itchiness and swelling, followed by a reddish inflammation of tiny pimples. Blisters then form and then couple in a chain-like reaction. This fluid then hardens to a yellowish crust. Left untreated, the rash (a typical histamine response) will last three to five weeks.