not only will it still give you a rash after months, but botonists
have gotten a rash from specimen of poison ivy years old.

The chemical (urushiol oil) doesn't deteriorate with time, vaporizes
with heat (so its in the smoke when burning).

It also binds very quickly to the proteins in your skin, and once
bound to the proteins doesn't wash off. According to my dermatologist
friend, you only have a few minutes after exposure to wash it off.

Exposure is usually culmulative, which means if you don't get a rash
at first, you will get one eventually, and each time it will be worse.

You do have to crush or bruise the plant to get the oil out onto your
skin, so lightly brushing up agains the leaves often doesn't result in
a rash.

tim