Oxidation occurs when an oil has been exposed to light and/or heat, or simply over time; it is a reaction of oil molecules with oxygen molecules. This can cause skin to become seriously irritated or sensitized, so is definitely something we need to recognize when using essential oils.
How to Know if Oil is Oxidized
Generally, oils higher in monoterpenes, phenols, and oxides have a faster oxidation rate than those with a higher percentage of other chemical families. How do we know when an oil is no longer safe to use because of oxidation? First of all, the quality over time does NOT depend on the supplier, provided the oil was not sitting around in a warehouse or on a shelf too long before you received it. When purchasing oils, look for a distillation date, which will help. You will also notice a loss or change of odor, and the oil may become viscous or cloudy. If you are familiar with a particular oil and use it a lot, you will just know something is “off”. In addition to skin issues, oxidized oils can also cause respiratory issues and will lose their therapeutic benefits.
Oils which are Prone to Oxidation
Some common oils which are most prone to oxidation are: Bergamot (Citrus bergamia), Grapefruit (Citrus paradisi), Lime (Citrus aurantifolia), Lemon (Çitrus limon), Mandarin (Citrus reticulata var mandarin), Manuka (Leptospermum scoparium ct east cape), Melissa (Melissa officinialis), Neroli (Citrus aurantium var amara), Orange (Citrus sinensis), Rhododendron (Rhododendron anthopogon), Tangerine (Citrus reticulata), and Yuzu (Citrus junos).
It’s Oxidized – Now What?
What to do with an oil which has oxidized? You can often use it to make cleaning products (just wear gloves in this instance to avoid possible irritation), or sadly, just toss it. Always store your oils in a cool dark place, and if purchasing a large bottle of essential oil, always decant into smaller bottles to prolong shelf life.