The following guidelines are based on the essential oil chemistry and proper storage.
Essential oils should be stored in a cool, dark place to maximize therapeutic shelf life. Essential oils exposed to extreme heat, light or oxygen on a regular basis will oxidize quickly and are more likely to cause skin irritation and other issues. Oils past their shelf life should only be used for cleaning or discarded.
These guidelines listed are based on years of Pompeii Organics GC/MS analysis experience in addition to research by Robert Tisserand as stated in his book Essential Oil Safety, 2nd Edition.