What dose of any substance is harmful instead of harmless? Is this a philosophical question or a practical one? If it's a practical one, we don't know, because if there are any effects they're too weak to infer with certainty. Unlike for example those of benzene in your sunscreen or acne products, or flame retardants in your furniture.

For PFOA, based on animal trials, somewhere above 10–70 ng/kg body weight-day.