A sprig of purple holy basil has a square stem, opposite leaves, and stalks and veins tinged a deep violet. Distilling the leaves gives an oil in which eugenol makes up 31.9 to 50.4 percent, the same molecule behind the smell of clove buds.
Purple leaves, square stem
Cut across, a holy basil stem has four edges. This is the shared mark of the mint family, Lamiaceae, the family of mint, lavender, rosemary and basil. Leaves grow in opposite pairs, their surface covered in fine hairs, and in the purple form the colour runs along the stalk and veins before spreading into the young blade. Crush one leaf between two fingers and the scent rises at once: spicy, warm, faintly sweet, closer to a jar of spice than to a bed of kitchen herbs.
The botanical name of purple holy basil is Ocimum tenuiflorum. Older books and many labels still carry the synonym Ocimum sanctum. In Hindi the plant is called tulsi, and in Hinduism it is a sacred plant. Its range stretches from India into Southeast Asia, Vietnam included, where it grows in home gardens and is cultivated for distillation.
The scent of the leaf is not spread evenly across its surface. The oil is held in tiny glandular hairs on the epidermis, and only when those cells break, from crushing, from heat or from steam, do the aroma molecules escape into the air. A standing holy basil plant therefore smells of almost nothing, while a handful of freshly crushed leaves can fill a corner of a room.
Commercial holy basil oil is steam distilled from the leaves. The result is a pale yellow liquid that carries nearly all of the warm spice of the fresh leaf, only many times more concentrated. To understand where that smell comes from, one has to read the composition table, and before that, the genus Ocimum.
The genus Ocimum and its chemotypes
The genus Ocimum contains many species that look much alike, and within each species there are lines that give entirely different oils. Two plants of the same shape and the same botanical name can carry two different main molecules. The essential oil trade calls such lines chemotypes, abbreviated CT, and names them after the dominant constituent.
European basil, Ocimum basilicum, is the clearest example. The linalool basil grown in Europe, North America and North Africa gives an oil with 53.7 to 58.3 percent linalool, floral and coolly herbal. The same species in its estragole line, grown in the Comoros, Réunion and Madagascar, gives an oil that is 73.4 to 87.4 percent estragole, smelling of anise and liquorice. Two bottles both labelled Ocimum basilicum may not resemble each other at all.
Purple holy basil stands on a third branch. Commercial oil of Ocimum tenuiflorum today is mainly the eugenol chemotype. The composition table in the library lists eugenol at 31.9 to 50.4 percent, 1,8-cineole at 12.6 to 16.5 percent, estragole at 9.7 to 12.9 percent, with β-bisabolene and β-caryophyllene in smaller shares. Each figure is a piece of the smell: eugenol gives the clove spice, 1,8-cineole a cool edge, estragole a faint trace of anise behind.
Another species familiar in Vietnam is white holy basil, Ocimum gratissimum. Its eugenol chemotype gives an oil with around 62.9 percent eugenol, higher still than purple holy basil. The English trade name clocimum is short for a clove scented Ocimum. In this genus the name on the label is only a starting point; the figures on the certificate of analysis are what tell what a bottle actually holds.
Eugenol: the smell of clove in a leaf
Eugenol is an aromatic phenol, formula C₁₀H₁₂O₂, molecular weight 164.20 g/mol. Its structure is a benzene ring carrying three groups: a hydroxyl, a methoxy and a three carbon allyl chain. The hydroxyl group gives eugenol its weak acidity, and it is also what sets it apart from close relatives such as methyleugenol.
The library describes the smell of eugenol in three layers: spicy, warm, and a slightly medicinal edge. It is a smell familiar to anyone who has opened a bag of dried cloves. On a blotter eugenol evaporates more slowly than the light terpenes, so in holy basil oil the cool layer of 1,8-cineole fades first, leaving the warm spice to linger longer on the paper.
Eugenol does not belong to a single plant. It appears in a great many materials: clove bud, leaf and stem, cinnamon leaf, pimento, West Indian bay, betel leaf. In very small amounts it also turns up in jasmine, rose and carnation, where it lends a little spice to the floral part. Each material carries its own share, and that share decides whether eugenol stands at the centre of the smell or only at its edge.
In holy basil, eugenol does not stand alone as it does in clove. It shares the space with 1,8-cineole, estragole and the sesquiterpenes. Holy basil therefore recalls clove without matching it: there is an added green leafiness, a cool note and a slight roundness at the base.
Holy basil beside sweet basil and clove
Three blotters set side by side show the distance between these materials more clearly than any table. The first is dipped in clove bud oil, the second in purple holy basil, the third in linalool basil. On the first the smell is almost a single block: eugenol makes up 73.5 to 96.9 percent, the rest is β-caryophyllene and eugenyl acetate.
The linalool basil blotter sits at the other end. Linalool is more than half, eugenol only 9.4 to 15.2 percent. The smell opens with flowers and cool grass, and the clove spice shows only as a thin outline behind. This is the basil of European cookbooks, where it is called sweet basil.
Purple holy basil sits between the two. It is spicier than sweet basil, greener than clove, and carries a cool note of 1,8-cineole that both others lack. Laid in order, the three blotters let the nose follow eugenol rising from left to right, like a scale made of leaves and buds.
The basil of the Vietnamese kitchen, the purple stemmed leaf served with phở, is another story again, and mostly carries the anise note of estragole. The Vietnamese name húng quế can therefore point to several different plants. When comparing oils, the most reliable guides remain the botanical name, the chemotype and the GC MS report that comes with the bottle.
Methyleugenol and the IFRA limits
A single methyl group added at the hydroxyl position turns eugenol into a different molecule: methyleugenol, formula C₁₁H₁₄O₂. Its smell still belongs to the clove region, with warm wood and a little vanilla sweetness added. Yet the safety profiles of the two molecules are far apart.
The United States National Toxicology Program, NTP, classes methyleugenol as reasonably anticipated to be a human carcinogen, based on sufficient evidence from experimental animals. The molecule is also on California's Proposition 65 list. Together with estragole and safrole, it belongs to the phenylpropenoids most tightly controlled in the fragrance trade.
IFRA therefore sets very low limits for methyleugenol in finished products. According to the library data, the limit for fine fragrance is 0.02 percent, and for body lotion 0.0004 percent. The figure does not apply to holy basil oil as a whole, but to the total methyleugenol contributed by every material in a formula.
Eugenol chemotype holy basil oil usually contains methyleugenol at 0.2 to 0.3 percent. That level is small, but it is not zero. The literature also records a methyleugenol chemotype of holy basil, with more than 70 percent methyleugenol, under commercial development. Both lines can carry the name Ocimum tenuiflorum, and only the certificate of analysis tells them apart.
For standard holy basil oil, the Lê Mai library lists a maximum dermal level of 1.0 percent. That figure comes not from methyleugenol but from eugenol: it is based on the highest eugenol content of 50.4 percent and a limit of 0.5 percent for eugenol. The oil is also listed with a moderate risk of skin sensitization.
Eugenol on the cosmetic label
At the end of the ingredient list on many products sold in Europe there is a row of names standing after the word Parfum: linalool, limonene, citral, geraniol, and often eugenol. These names are not separately added ingredients. They are molecules already inside the essential oils or fragrance, named out for the label.
The reason lies in EU Cosmetics Regulation 1223/2009. Eugenol belongs to the list of 26 fragrance substances with allergenic potential that must be declared above a threshold: 0.001 percent in leave on products and 0.01 percent in rinse off products. A body oil holding holy basil oil at a very low level can still cross that threshold, since eugenol makes up nearly half of the oil.
EU Regulation 2023/1545 extends this list by 56 entries beyond the former 26, with thresholds unchanged. New products placed on the European market must comply from 31 July 2026. Being named does not mean a molecule is banned. It allows someone who already knows a sensitivity to a specific substance to read the label and avoid it.
For eugenol, the skin data show a measured picture. A patch test at 8 percent on 25 volunteers produced no sensitization. Among groups of dermatitis patients tested, reaction rates ranged from 0.4 to 2.0 percent depending on test concentration. The rate is low, but enough for a molecule to be named on a label.
The piece on the names at the end of the ingredient list goes further into this list. For holy basil the point is shorter: a purple leaf carries a molecule with its own name on the label, its own limit on skin, and a methylated sibling that has to be checked batch by batch.
One leaf. One molecule filling nearly half the bottle.
The smell of clove does not need the clove tree.
The figures on the analysis say more than the name on the label.