That's not a meaningful standard for any health intervention. If I'd apply everything to my body that wasn't proven to hurt I'd spend a hundred bucks every morning and two hours in the bathroom. If "it doesn't hurt" was sufficient basis for a recommendation our doctors would tell us to swallow homeopathic medicine every morning.
It seems pretty obvious that anything you apply has to have at least some measurable impact, otherwise you're basically in the same category as the supplement industry.
https://www.consumerreports.org/toxic-chemicals-substances/d...
It's demonstrable that something like a bean skin, lodged in your teeth, will erode the teeth touching it.