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The only trait that affects toxicity is Responsive (+2 Magic Affinity, +20% toxicity change). This trait works exactly as stated, your slave will gain toxicity 20% faster and lose it 20% faster.

I strongly suspect that you are using a potion on your slaves but not considering it to be a potion because of the name of the item. Aphrodisiac, Hair Dye, and Beauty Mixture are all potions that increase toxicity.

If you want to track down the source of your toxicity, then the simplest way to do it would be to change the code to report increases in toxicity as debug text. Though it wouldn't tell you the source, you would know when it happened. 

Get the debug mod if on Windows x64(https://itch.io/t/1137280/debugmod-v1), otherwise you'll need to open ".../Strive/logs/DEBUG_TRACE.txt" in the Strive user data to monitor the debug text. Use a decent text editor(like Sublime Text 3 or Notepad++) to change this function in ".../files/scripts/person/person.gd" from:

func tox_set(value):
    var difference = value - stats.tox_cur
    stats.tox_cur = clamp(stats.tox_cur + difference*stats.tox_mod, stats.tox_min, stats.tox_max)

to this: (make sure to change the leading spaces for each line to 1 or 2 tabs or it will break, forum only allows spaces)

func tox_set(value):
    var difference = value - stats.tox_cur
    stats.tox_cur = clamp(stats.tox_cur + difference*stats.tox_mod, stats.tox_min, stats.tox_max)
    if difference > 0:
        print("Toxicity Increased " + str(difference))
        globals.traceFile("Toxicity Increased " + str(difference))