Volatile fragrance chemical that occurs naturally in cloves, basil, and bay leaves, among other plants. Eugenol is often part of the fragrance in cosmetics products, and is known to cause sensitivity. It is best to avoid leave-on products that contain eugenol.
References for this information:
Toxicology, September 2012, pages 20-24
Molecules, June 2012, pages 6,953-6,981
Cell Proliferation, August 2006, pages 241-248