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document updated 8 years ago, on Jun 20, 2015

Poison ivy / poison oak / sumac has a pretty intense chemical that requires a very little amount to cause problems — the average [accidental] dose is 100 nanograms. It contaminates surfaces invisibly, and will remain potent for 5 - 50 years. And it grows EVERYWHERE.

So it could be used for evil purposes.

To harvest and purify urushiol:

collect the sap [2] from a plant in the Toxicodendron family
dissolve the sap in acetone [2] [3] [4]
pour off the acetone, and discard the insoluble part (referred to in various places as "acetone powder")
evaporate the acetone at room temperature (You do NOT want to heat this, because the vapors are nasty. If you have nice lab equipment, you can evaporate more quickly under a vacuum.)
what's left behind is urushiols

Urushiol rashes can get pretty severe [1] [2] [3] [4] so protect yourself when handling the stuff, and be aware of the harm you can cause.

Note that it can take 24 - 48 hours from exposure until a rash develops, so this isn't a fast-acting weapon.

If you were REALLY evil, you could burn or otherwise vaporize urushiol, which will give someone a rash inside their lungs. However, this is known to be fatal. So yeah, don't do that.