All essential and fragrance oils are created of complicated blends of various chemical compound, things like, linalool, benzyl salicylate, geraniol, coumarin, and eugenol.
You may have seen a number of these chemical compounds listed at the end of ingredient lists. They are known as sensitizers by the IFRA (International Fragrance Association) and needed to be listed in the ingredient lists if they have concentrations above 001% in leave-on products and 0.01% in wash-off products. Each individual sensitizer also has a maximum allowable limit.
If you’re allergic to a particular compound, you will need to make certain to avoid it! And, sometimes, a formula is sensitive to a particular chemical compound. To determine if a certain chemical compound is present in the fragrance or essential oil, look at the SDS (safety data sheet) or GCMS (Gas Chromatography Mass Spectrometry Analysis). The essential oil company should have these documents and allot of times have them listed on their website. Essential oil composition varies from crop to crop, so while you can get a general idea of the composition of the oil, if you need exact levels to concentrate sensitizer levels you will want to be purchase your essential oils from somewhere that can give you the precise breakdown of exactly what you are using.