For individual use, essential oils, absolutes and CO2s are most often sold in 5 ml, 10 ml and 15 ml (1/2 ounce) sizes. For more expensive oils, it is common to find them available in sizes starting at 1 or 2 ml.
Although essential oils do not become rancid, they do oxidize, deteriorate and lose their beneficial therapeutic properties over time. Oils such as the citrus oils will oxidize and begin to lose their aroma and therapeutic properties in as little as six months. Not all essential oils diminish in aromatic quality as time passes. The aroma of essential oils such as patchouli and sandalwood mature with age, however, Robert Tisserand explains that all essential oils oxidize and are subject to losing their therapeutic value in time. All essential oils benefit from proper storage and handling.
Glass, Plastic or Aluminium?
To avoid deterioration and protect the aromatic and therapeutic properties of your essential oils, store them in amber or cobalt blue bottles. Dark glass such as amber or cobalt helps to keep out deteriorating sunlight. Be leery of purchasing any oils sold in clear glass bottles. Clear glass bottles are not harmful to essential oils, but clear glass does not protect the oils from damaging sunlight. Beer deteriorates quickly and is often bottled in amber glass for a similar reason. Avoid purchasing pure essential oils sold in plastic bottles as the essential oil will eat at the plastic, and the essential oil will become ruined over a short period of time. Some vendors sell oils in lined aluminum bottles. It has been said that aluminum bottles are acceptable if the interior of the bottles are lined.
Avoid purchasing essential oils that are stored in bottles that have a rubber dropper incorporated into its screw-top cap. Droppers with rubber bulbs should not be kept with the essential oil bottle as the highly concentrated oil can turn the rubber bulb into gum and ruin the essential oil. Most essential oils are sold in bottles that contain an "orifice reducer." An orifice reducer is a small, usually clear insert inside the bottle opening that acts as a dropper. Unlike rubber droppers, orifice reducers will not harm essential oils. You simply tip the bottle to dispense the oil drop by drop.
Store Your Essential Oils in a Cool Location or the Refrigerator
Prominent aromatherapy expert Robert Tisserand advocates storing essential oils in a refrigerated environment, whenever possible. If that is not possible, store them in as cool a location as possible. Avoid keeping essential oils in a location that is prone to temperature changes.