Frankincense

Frankincense, also known as olibanum, is the aromatic oleo-gum resin tapped from trees of the Boswellia genus. Harvesters score the bark and collect the milky sap as it hardens into amber "tears". Around 90% of the world's supply comes from the Horn of Africa, with further production in Oman's Dhofar region and in India; the species behind the names you see on a cosmetic label are usually Boswellia carterii (also spelled carteri), Boswellia sacra, Boswellia serrata or Boswellia papyrifera.

On an ingredients list, frankincense turns up in two quite different forms, and the difference matters more than most articles admit. Boswellia Carterii Oil or Boswellia Sacra Oil is the steam-distilled essential oil — a volatile, intensely fragrant fraction built largely from monoterpenes such as alpha-pinene and limonene. Olibanum, Frankincense Extract and similar names refer to the resin itself, or an extract of it, which retains the heavier triterpenoid compounds known as boswellic acids.

Frankincense has been burned as incense and used in traditional preparations for thousands of years, and it remains a fixture of perfumery — that resinous, faintly citrus-balsamic smell is why it appears in so many "ritual" and "spa" formulas. Its formally recognised cosmetic functions are more modest than the marketing suggests: ingredient databases list Boswellia Carterii Oil as masking and tonic, and olibanum as masking and skin conditioning. Expect to find it low down the ingredients list of facial oils, balms, serums, cleansers and body products.

Two rather different things are going on, depending on which form of frankincense a formula actually contains.

• The distilled oil. Terpenes are small and volatile, which is why the oil smells the way it does and why it is classified as masking. In laboratory testing, terpene-rich frankincense fractions show measurable free-radical scavenging — how much of that carries over into a diluted finished product sitting on skin is far less clear.
• The resin. Boswellic acids are large and non-volatile, so they stay behind in resin and solvent extracts; a product listing only the distilled oil is unlikely to deliver them in any quantity.
• What happens once a bottle is opened. Monoterpenes react with oxygen over time, breaking down into new compounds that the immune system recognises more readily than the fresh oil does. That is the chemistry behind the usual advice about air, light and heat: an oil left open for months is a different mixture from the one that was bottled, and a more sensitising one.

Preclinical work has applied frankincense oil to UVB-exposed rodent skin and reported shifts in antioxidant enzyme levels and collagen-related markers — an animal model, not a human trial, and no guide to what happens on a human face. Beyond the chemistry, the oil contributes a little emollient slip and, above all, aroma, which is a genuine part of how a product feels to use.

Frankincense is a fragrant botanical, not an active — and fragrance is the most common reason people react to a facial oil. If your skin is at all reactive, it is an easy ingredient to skip.

• Patch test first. Dab a little on the inner forearm for a few days running before putting it near your face — especially with facial oils, where it can sit high in the formula.
• Use it late in the routine. It belongs in the oil or balm step, after your water-based serums, where it helps seal in what you have already applied.
• Never apply it neat. If you have a bottled essential oil rather than a finished product, dilute it in a carrier such as jojoba or squalane; around 1% is a commonly cited guideline for facial use.
• Store it dark, cool and closed. Monoterpenes oxidise on contact with air, and oxidised terpenes are more likely to provoke a reaction — so don't nurse an opened facial oil for years.
• Give it a night off from strong actives. On evenings you use a retinoid, a high-strength acid or benzoyl peroxide, skip the fragrant oil; if your skin does react, you will know what caused it.
• Let tolerance set the frequency. Some skins are fine with it regularly; if yours is reactive, keep it occasional or leave it out.