McKay: The ingredients listed are organic molecules used in hair-are products, cosmetics, cleaning products, candles, and perfumes as fragrance additives. They are all naturally occurring components of various essential oils found in the plant kingdom. Synthetic versions of these are also readily available. Linalool, citronellol, and geraniol belong to a class of organic compounds known as terpenoids, which are very popular as flavor and fragrance additives. Eugenol is an allylbenzene molecule, and limonene is a terpene.
Linalool is a naturally occurring substance distilled from essential oils found in various flowers, herbs, citrus fruits, and spice plants (such as coriander and rosewood). Linalool possesses a light floral fragrance with a hint of citrus and is used in many products. It also is an intermediate in the synthesis of vitamin E (tocopherol), and can also be chemically modified to yield geraniol, citral, geranial, and citronellol. This makes linalool very valuable, not only for its inherent pleasant fragrance but also for its ability to provide the perfumer with a variety of fragrances. Citral (also known as lemonal) actually exists as two isomers, geranial (citral A) and neral (citral B). Citronellol is derived from rose and geranium plants. Geraniol, also derived from geraniums, is not only popular as a fragrance additive, but also has been found by researchers at the University of Florida to possess significant insect-repellant properties.
Eugenol is primarily extracted from clove oil, but may also be found in nutmeg, cinnamon, and bay leaf. This slightly water soluble molecule is responsible for the distinctive smell of cloves. It may also be chemically modified to form vanillin, which has a vanilla scent. Limonene is distilled from the rinds of citrus plants and has a distinct scent of oranges.
The art of perfumery is one that involves the use of many combinations of different molecules in order to achieve a subtle layering effect of the different scents, with the end result being the desired overall fragrance. For this reason, it is common to see many of these included in a list of ingredients, but more common to simply see “fragrance” as the ingredient. Products which divulge the presence of these specific substances, rather than simply the ubiquitous “fragrance,” may be aimed at a specific market that the company believes would respond favorably to them. One example of this would be some of the product lines marketed as herbal, natural, or organic.
Q: What is hydroxyisohexyl 3-cyclohexene carboxaldehyde?
McKay: While this question came from the same community member, it was particularly relevant to this month’s column because this is also one of the many fragrance ingredients used in cosmetics and toiletries.
Hydroxyisohexyl 3-cyclohexene carboxaldehyde is the INCI name (commercial name is Lyral), and it is known for its mild floral and slightly woody fragrance. At the current levels used in most products, this ingredient has been found to be a sensitizing agent and a subsequent cause of allergic dermatitis in almost all of those patients who were sensitized. For this reason, it has been recommended that this substance be used at much lower concentration in formulations, or that it be phased out entirely as an additive in products designed for skin contact.
References  Johansen, J.D.; Frosch, P.J.; et al, Contact Dermatitis, 2003, 48: 310-316, “Hydroxyisohexyl 3-cyclohexene carboxaldehyde – known as Lyral: quantitative aspects and risk assessment of an important fragrance allergen”