Knowing the cause of a skin rash is the first step to treating it. Sometimes, however, telling the difference between hives and the rash caused by contact with poison ivy is difficult. Both appear as red discolorations on the skin, and both may be itchy or painful. Fortunately, there are differences between the two, mainly in their relative causes, appearance and duration.
Difference in Causes
The rash caused by poison ivy is a result of coming into contact with the irritating oil urushiol, which liberally covers the plant's surface. This coating acts as a deterrent to browsers and parasites seeking to harm the plant, but it is usually only spread through direct contact with the plant itself. People who have not handled poison ivy prior to the appearance of symptoms are unlikely to have come into contact with urushiol, which suggests other causes for the rash.
Hives, on the other hand, do not have a single cause. Hives are the result of an immune response to an allergen. Often, they are the result of eating certain foods or coming into contact with an allergen through other means.
Differences in Appearance
Hives appear on the skin as blotchy red spots. They may be raised or flat, and they range in size from very small, pinprick-sized spots to large, irregular-shaped masses. They may even grow into each other and merge into areas of generalized redness and itching or pain. The rash caused by poison ivy may be either mild or severe, depending on the amount of urushiol picked up and the sufferer's sensitivity to it, but it also turns the skin red.
Unlike hives, however, a poison ivy rash is often confined to a straight line, which corresponds to the path of the plant's leaves across the victim's skin. Though poison ivy rashes begin with a hive-like appearance, over time they grow and form blisters that do not usually spread beyond their immediate area.
Differences in Duration
Hives and poison ivy rashes also differ in the speed with which they strike and how long they last. Hives tend to come and go quickly, with symptoms appearing within minutes of exposure to the allergen and quickly reaching their maximum extent. Hives can last for any length of time, from seconds to hours.
Rashes caused by poison ivy are not perfectly uniform in their onset or presentation, as a great deal depends on the severity of the exposure and the sensitivity of the person with the rash, but the red welts come on more slowly than hives and persist longer. Poison ivy's characteristic blisters begin to appear sometime between 12 and 72 hours after the exposure, and symptoms of the rash can last as long as two weeks.