Almost anything can be harmful at a high enough dose — and almost anything can be harmless at a low one.
Examples
Water, vitamin A, iron and caffeine are essential or harmless at normal intakes and harmful at extreme ones.
Units matter
A substance at parts per billion is a very different situation from grams per day.
Route matters
Eating, breathing in and skin contact lead to very different exposures.
Evidence & sources
Toxicity depends on the dose: the same substance can be harmless at a low dose and harmful at a high one. Evidence: established. Known: A core principle of toxicology. Unknown: Some people are more sensitive than others. Source: US National Library of Medicine, ToxTutor: Dose and Dose Response (Current revision).