What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
Nearly every sector shed jobs including healthcare, typically a source of strength, which was hit by strikes last month.
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The US Environmental Protection Agency’s Response Management Program (RMP) requires more than 12,500 high-risk facilities to develop protocols to prevent catastrophes, or limit fallout, and was largely designed to protect workers, first responders, and fence-line communities.