Widely used plant that’s a member of the mint family. May be listed by its Latin names Lavandula angustifolia or Lavandula officinalis. Primarily a fragrance ingredient, although it may have antibacterial properties. In-vitro research indicates that components of lavender, specifically linalool and linalyl acetate, can be cytotoxic, which means that topical application of as little a concentration as 0.25% causes cell death.  This study was conducted on endothelial cells, which are cells that line blood pathways in the body and play a critical role in the inflammatory process of skin.
As linalool and linalyl acetate are both rapidly absorbed by skin and can be detected within blood cells in less than 20 minutes, endothelial cells are an ideal choice for such a test.  The results of this research also demonstrated that lavender has a damaging effect on fibroblasts, which are cells that produce collagen.
The fragrance constituents in lavender oil, linalool and linalyl acetate, oxidize when exposed to air, and in this process their potential for causing an allergic reaction is increased.  If you’re wondering why lavender oil doesn’t appear to be problematic for you, it’s because research has demonstrated that you don’t always need to see it or feel it happening for your skin to suffer damage. 
- Prashar A, Locke I, Evans C. Cytotoxicity of lavender oil and its major components to human skin cells. Cell Prolif. 2004;37(3):221-9.
- Jager W, Buchbauer G, Jirovetz L, Fritzer M. Percutaneous absorption of lavender oil from a massage oil. J Cosmet Sci.. 1992;43(1):49-57.
- Hagvall L, Sköld M, Bråred-Christensson J, Börje A, Karlberg A. Lavender oil lacks natural protection against autoxidation, forming strong contact allergens on air exposure. Contact Dermatitis. 2008;59(3):143-50.
- Basketter D, Darlenski R, Fluhr J. Skin irritation and sensitization: mechanisms and new approaches for risk assessment. Skin Pharmacol Physiol. 2008.;21(4):191-202.