Some essential oils carry a photodynamic agent that release free radicals. These free radicals are very reactive to UV light, and when applied to the skin, it will create a skin burn. The skin burns because it has become photosensitised, which is why we labelled them as photosensitising oils.

The oils themselves do not directly cause the phototoxicity, but an effect of the interaction between UV light and the skin areas that oil has applied.

Photosensitizing takes place when they are obtained through cold-pressing, not when obtained through steam distillation.

These essential oils are often packaged in a darker shade bottling to prevent UV penetration, therefore extending their shelf life.