Citrus oil whose volatile components (chiefly substances known as furocoumarins) are sensitizing to skin. Topical application of grapefruit oil may cause sensitizing reactions from its chief fragrance chemical, limonene.
References for this information:
Food and Chemical Toxicology, March 2012, pages 756-760
Journal of Agricultural and Food Chemistry, December 2005, pages 9,790-9,794