The poison they say he was killed with is polonium 210. However, the half life (how fast it decays) is only about 5 - 6 months. Arafat died in 2004. That means its impossible to show he died from polinium 210 poisoning, but that won't stop people from saying he was killed by it.
from the article: French, Swiss investigators to visit West Bank for Arafat death probe