A) 500.  Only 1 nanogram (billionth of a gram) needed to cause rash.
 
 
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  B) Specimens of urushiol several centuries old have found to cause
  dermatitis in sensitive people.  However, 1 to 5 years is normal for
  urushiol oil to stay active on any surface including dead plants.
 
 
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  C) Rubbing the rashes won't spread poison ivy to other parts of your body
  (or to another person). You spread the rash only if urushiol oil -- the
  sticky, resin like substance that causes the rash -- has been left on your
  hands.  The rash spreads only by spread of the invisible sap on the skin,
  not by leaking blister fluid from the rash itself. That blister fluid is a
  product of your own body, and contains no poison ivy sap. The reason the
  rash often continues to break out in new areas beyond 72 hours is that
  people unknowingly continue to contact it --- from unwashed clothes
  (especially shoes and laces), sporting or gardening equipment, pets, etc.
  Anything that comes in contact with the plant can carry active sap for
  months! The sap is so potent, you can pass it from object to object a dozen
  times just by light contact, and the last object can still cause an intense
  rash. People leave an invisible "trail" of urushiol where they sit, touch,
  sleep or bathe. They end up re-exposing themselves over a period of weeks.
  So don't worry about your oozing blisters being contagious --- just start
  washing everything you may have touched!
 

  Web sites with more information on this nasty plant:

  http://www.quickcare.org/skin/poison.html

  http://poisonivy.aesir.com/welcome.html

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