you're heating the plastic and destabilizing its structure.  imagine for a moment we're talking about handling chocolate. if the chocolate gets hot enough to destabilize it gets fucking everywhere. if it stays cold it does not transfer any substantial amount. the analogy breaks down here because plastic has many different components in it which leach out at different rates. perhaps you've seen old plastic which has lost its vigor (brittle, faded, flaking, etc.) this occurs when the long list of chemicals added to the polymer are dwindling.
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