Tea tree oil is steam-distilled from the leaves of Melaleuca alternifolia, a tree native to the north coast of New South Wales. It is one of the few essential oils with a substantial clinical evidence base, and also one of the more common causes of contact allergy in cosmetics.
The active constituents
Tea tree oil contains over a hundred compounds. The most significant is terpinen-4-ol, which accounts for most of the antimicrobial activity and typically makes up 30 to 40 percent of the oil.
An international standard, ISO 4730, sets minimum terpinen-4-ol content and maximum 1,8-cineole content. Cineole is more irritating and less active, so a good quality oil is high in the former and low in the latter.
The evidence
Tea tree oil has documented antibacterial, antifungal and anti-inflammatory activity. It has been studied for acne, with some trials reporting comparable outcomes to benzoyl peroxide at 5 percent, more slowly but with less dryness and irritation.
That is a stronger evidence base than most botanicals in skincare carry, and it is why tea tree oil appears in serious formulations rather than only in natural-positioned ones.
The oxidation problem
This is the part that matters most and is least often explained.
Tea tree oil oxidises on exposure to air, light and heat. Oxidation degrades terpinen-4-ol and generates peroxides and other products that are substantially more sensitising than the fresh oil.
Most reported allergic reactions to tea tree oil involve oxidised material. Which means age, storage and packaging affect not only how well it works but how likely it is to cause a reaction.
Contact allergy
Tea tree oil is a recognised contact allergen and appears on standard patch test series in dermatology. Sensitisation rates in the general population are low but not negligible, and higher among people with existing eczema or damaged skin.
Never use it undiluted
Neat tea tree oil applied directly to skin is a common cause of irritant and allergic reactions. Cosmetic products contain it at low concentrations for that reason.
Oral toxicity
Tea tree oil is toxic if swallowed, with documented cases of confusion and loss of coordination following ingestion. Keep away from children and never take it internally.
Pregnancy
Topical tea tree oil at cosmetic concentrations is not generally flagged as a concern, but essential oil safety data in pregnancy is limited across the board. Worth confirming with your doctor.
