I have a deck built preceding 2003, when arsenic was discontinued from use on wood, so it's probable that the wood on my deck contains arsenic. Now, prelude aside..
It has not rained in a week. I dropped a comb on the deck, picked it back up, and continued combing my hair. Is it possible that any arsenic clung to the comb, and then got into my scalp? I washed my hair afterwards, to be safe.

