What it is and why it's in the formula
Undecylenic Acid USP is listed at 5% on the product label — the only ingredient in the formula with a specific disclosed percentage. It's a naturally derived fatty acid that has a long history of use in over-the-counter foot and nail cosmetic products. It's important to be clear about what this product is and isn't: PureNail Pro is formulated and marketed as a cosmetic nail and skin care mist, not as a treatment for a diagnosed fungal or skin infection, and it is not intended to diagnose, treat, cure, or prevent any disease. Anyone dealing with a suspected nail or skin infection should speak with a doctor rather than relying on a cosmetic spray alone.
Where it fits in the full formula
Undecylenic Acid is one of 16 ingredients disclosed openly on the PureNail Pro label — none of the formula sits behind vague "proprietary blend" wording. You can see the complete breakdown, including which oils pair with it, on the full ingredients page.