Formic Acid
Formic acid is an acid found in geranium oil and yerba santa. It has been used as an astringent and counterirritant. If moderate to high levels of formic acid are ingested, death or permanent injury can occur.
References
Leung, Albert Y. 1980. Encyclopedia of Common Natural Ingredient used in Food, Drugs and Cosmetics. John Wiley and Sons, N Y. 409 pp.
Toxnet, N.L.M. Toxicology Data Network. National Library of Medicine. Bethesda, Maryland.
Windholz, Martha, Susan Budavani, et. al. 1976. The Merck Index. Merck and Co., Inc. New Jersey.