Why is it that some people can lay in poison ivy for hours and nothing happens, yet others have severe reactions with the slightest exposure?

To find out, we’ll need to take a closer look at what causes the reaction.

The Cause

Poison ivy (as well as poison sumac and poison oak) contain an oil called Urushiol. This oil is what causes the allergic reaction associated with poison ivy. Urushiol has a dermatitis-producing principle, pentadecylacatechol. This chemical does not evaporate and will dry quickly on objects it comes into contact with.

This chemical can remain potent for a year or more. It is important to wash any objects, especially clothing after contact with poison ivy. The resin will remain active on these articles and can cause a rash months, or even years, later.

The word ‘urushiol’ is derived from ‘urushi’, The Japanese name for lacquer. The Japanese lacquer tree contains small amounts of urushiol in its sap, and those who are sensitive to the oil can even react when they come into contact with lacquered furniture from Japan.

So why are some people more sensitive than others?

Roughly 85% of all humans will acquire or have an allergy to poison ivy. The remaining 15% are completely immune, able to roll around in it without having a reaction. There is no guarantee however, that a person who is resistant won’t become sensitive later on down the road. This is because people aren’t born with allergies; they develop them after exposure. Repeated exposures can make one even more sensitive to the irritant or allergen.

It is very uncommon for those under the age of five to get the itchy rash and blisters associated with a poison ivy reaction. Sensitivity for many can also fade with age, and quite a few adults who suffered severe reactions as children will find that they are no longer allergic or have a very mild reaction to poison ivy later in their life.

This means that the people who don’t seem allergic to poison ivy might lie in the 15 percent who are resistant, or they may just no longer be as sensitive as they were as a child.

Is there any way to prevent minimize exposure?

Aside from avoiding the plants entirely and using common sense, many forest service employees spray antiperspirant deodorant on thier exposed skin before they go out if they they think they might come in contact with poison ivy. The aluminum chlorohydrate may help prevent penetration of the oil through the skin.

If you do come into contact with poison ivy, the best preventive method to minimize exposure is to wash with lots of soap and water within 15 minutes of contact. After 15 minutes, the chemical that causes the reaction has already bonded to the proteins in your skin.

If a severe reaction develops, contact a dermatologist immediately, or go to an emergency room. Prescription medication may be needed to reduce itching and the swelling.

References:

Hawley’s Condensed Chemical Dictionary, 14th Edition, 2002, John Wiley & Sons, Inc.

Donald G. Barceloux (2008). Medical Toxicology of Natural Substances: Foods, Fungi, Medicinal Herbs, Plants, and Venomous Animals. John Wiley and Sons. pp. 681

Robert Alan Lewis (1998). Lewis’ dictionary of toxicology. CRC Press. pp. 901