Cinnamon essential oil is one of the most studied essential oils and has been used in hundreds of different research projects over the years. It is extracted from cinnamon trees, most commonly cinnamomum verum. Cinnamon EO can be purchased as Cinnamon Leaf EO (CL) or Cinnamon Bark EO (CB). Both essential oils are extracted from the same tree, but one is extracted from the leaves, while the other is extracted from the bark. The leaves are processed directly with steam or CO2 in a distillation process, while the bark is peeled off the tree, put through a wood chipper, and then distilled with steam or carbon dioxide.
The use of cinnamon essential oil can cause adverse reactions, especially for those who have sensitive, irritated, or dry skin, and those with medical skin conditions. Cinnamon essential oils contain three chemicals that have been documented to cause significant side effects which include cinnamaldehyde, eugenol, and cinnamic acid. Cinnamon leaf EO has a higher concentration of eugenol, and cinnamon bark EO has a higher level of cinnamaldehyde and is known to be more irritating. Both oils also contain trace amounts of more than forty other chemical compounds.
Complications from these three compounds, and possibly other trace compounds found in the oils, can include irritation (irritant contact dermatitis), contact urticaria (immediate hypersensitivity), allergic contact dermatitis (delayed hypersensitivity), and photosensitivity/phototoxicity. These types of reactions are examples of why cinnamon essential oils, in particular, cinnamon bark EO, are not recommended for higher concentration use or are contraindicated for use in skincare products, including wash-off products such as soap.
Some suppliers and resources state that both cinnamon leaf and cinnamon bark essential oils should not be used in any skin product, even soap, while others provide a recommended usage rate that is typically very low, with ranges from 0.1%-0.7% of the total product concentration. Out of the 23 suppliers that I viewed for this article, 16 stated that cinnamon essential oil was not for skin use, while 3 stated a maximum usage rate of 0.1%, and the others listed concentration usage rates up to 0.7%. Wholesale Soap Supplies recommends no more than 0.5% in soap for their cinnamon leaf essential oil.
In addition to possible skin irritation, both cinnamon bark and cinnamon leaf essential oil contain eugenol, a chemical compound that can accelerate trace. Eugenol, which has an acidic proton, immediately reacts with the lye and creates a surfactant that can accelerate trace and increase the reaction rate. In cold process soap making, this can be an undesirable effect and can lead to thickening of the recipe, seizing, and other complications. If your soap is slow to trace, or you are in need of an accelerant for your hot process or liquid soap, those who have read The Ultimate Guide to Soap books know that cinnamon essential oils can help speed this reaction and eliminate the need for extended mixing with an immersion blender. Using a trace accelerant like a small amount of cinnamon leaf essential oil (several drops will often do the trick) is perfect for recipes that are slow to trace like castile soap, or accelerating the reaction rate in HTHP or HTLS. It should be noted that clove essential oil has a higher concentration of eugenol and we typically recommend it as the first selection because of this.
Although the cinnamaldehyde found in cinnamon essential oils makes a beautifully sweet and spicy aroma and essential oils are considered a natural fragrance option, I personally don't recommend their use in soap making in at any concentration more than 0.1-0.2%. Cinnamon leaf is less of an irritant than cinnamon bark, so there may be a little leeway with higher concentrations. If you wish to use cinnamon essential oils, consider using it as part of a fragrance oil blend, as it makes a wonderful middle note.
Cinnamon fragrance oils, rather than essential oils, can be purchased from fragrance oil and cosmetic supply companies. These can be a much safer and less irritating option to obtain a cinnamon aroma. It should be noted that some cinnamon fragrance oils may still cause irritation if used in higher concentrations from small amounts of cinnamaldehyde or other irritants, so be sure to adhere to supplier and IFRA usage recommendations.
When formulating soap and other cosmetics, always be sure to keep your personal preferences and skin needs in mind, in addition to those who may be receiving or purchasing your products. If you do like to include a higher concentration of cinnamon EO above 0.3%, you might consider notice on your label that includes something along the lines of "cinnamon oil is a potential irritant." By including a notice, it can allow customers to make an informed decision about their purchase and products used.
If you would like more information about the adverse reactions of using cinnamon essential oil on the skin, I would recommend researching cinnamaldehyde, eugenol, and cinnamic acid and how they react with and affect skin. (This article from NCBI and this article written by Robert Tisserand are great places to start)
Student Question: "Do you use cinnamon EO in your personal soap and skincare products?" No. I personally do not use cinnamon essential oils in my formulations due to the fact that I have very sensitive skin. Products that contain higher concentrations have left my skin feeling dry, and have caused redness, burning, and itching, even with wash-off products like soap.