Hot answers tagged pressure-treated
I'd remove the wooden post, do a full repair of the broken step, and install a bolt-on steel or aluminum post with a vinyl cover on the step below. Of course, this requires a rebuild of your rail assembly to extend it, but it should be full-length anyway.
Offgassing has never really been a concern with treated lumber. Direct skin contact and ingestion were concerns with CCA treatment, and to a lesser degree with the products that have replaced it. As long as you're not spending a lot of time in contact with wet wood, and as long as your pets and family members aren't gnawing on it, I wouldn't be concerned.
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