Vinegar’s active ingredient, acetic acid, can be produced naturally through bacterial fermentation, as in apple cider vinegar, or industrially, via chemical reactions from such source materials as methanol. But concentrations strong enough to be effective against anything but the youngest, most tender weeds, vinegar is no longer a food product but an herbicide called horticultural vinegar and not so friendly.
In concentrations this strong, vinegar becomes hazardous and can cause environmental damage. Vinegar is a contact or "burndown" herbicide, killing what it touches within hours or days. The worst part is that it may looks like it’s working, but weeds will then resprout from the roots, particularly perennial species.
That partial success worries Jeff Gillman, author of The Truth About Organic Gardening, because it often incites a gardener to continue using vinegar, even if it's not the best for his or her garden. The gardener — seeing results but not entirely satisfied — often trades up to higher concentrations, replacing household vinegar (5% acetic acid) with a horticultural product (typically 20%).
Despite the signal word danger on most such labels, gardeners may instead just see vinegar and be careless. Sobering details: In concentrations over 11%, acetic acid can burn skin and cause eye damage, and concentrations of 20% and above are corrosive to tin, aluminum, iron, and concrete and can even cause blindness. Such herbicides should be applied while wearing goggles and protective clothing.
https://www.goodhousekeeping.com/home/gardening/a20705796/vinegar-weed-killer-danger/
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