"Toxins" is such a broad category you can say just about anything about it. I have trouble taking any health advisory seriously when they only talk about nonspecific "toxins" instead of naming the compounds.
This is a situation where the word "toxins" is fine, and they're being broad on purpose because it's just a claim of existence.
> I have trouble taking any health advisory seriously
Please keep in mind you're not talking to the person that made a health advisory. You're talking to someone who is saying why it's possible for water to be unsafe to shower in.