Essential oils are concentrated material. A single 0.05 ml drop can carry dozens of distinct compounds, some at concentrations high enough to irritate, to sensitise the skin, or to act on the central nervous system at high exposures. Safety is not abstinence. Safety is choosing the right oil, the right dilution, the right person and the right situation.
This page sets out the core principles: dilution by application type, age-related guidance, oils to avoid in certain conditions, drug interactions, photosensitivity, and oxidation risk. Each section carries specific warnings. For any individual oil, consult its dedicated page for the full guidance.
This page is reference material and does not replace medical advice. In suspected oral poisoning, particularly involving children, contact a poison centre or hospital immediately.
Essential oil concentration on skin by product type and user. Exceeding the threshold may cause irritation. Calculated as percent of essential oil in total product volume.
Convention: 1 ml ≈ 20 drops
Undiluted essential oils on skin can cause irritation, redness, burning or rash. Repeated exposure can lead to permanent sensitisation, after which the same oil triggers an allergic reaction at every contact, even at low concentrations.
Dilution is mandatory for nearly all dermal use. The carrier is usually a vegetable oil (jojoba, sweet almond, grapeseed), a cream, a gel or another lipid-based base. Water does not dilute an essential oil. It only disperses it temporarily into droplets that can still attach to skin.
A small set of exceptions permits neat use: spot application on minor burns, insect bites, herpes lesions, mouth ulcers, pressure sores, in minimal amounts and under experienced supervision.
Appropriate concentration depends on the area of skin involved, the sensitivity of that area and how often the product is used. Whole-body massage covers more skin and requires a lower percentage than a spot application. Face and mucous membranes require lower levels than limbs.
Bath water is a special case. Essential oils do not dissolve in water; if dropped directly into a bath, they float as droplets that attach to the skin on entry. They must be combined with full-fat milk, a carrier oil, honey or an emulsifier before being added to water.
Ambient diffusion is a much lower-risk route than skin contact. A few drops in a diffuser or a bowl of hot water in a reasonably ventilated room poses almost no risk. Prolonged direct inhalation from a bottle (more than 30 minutes) can cause headache, dizziness and nausea.
| Application | Recommended concentration | Maximum |
|---|---|---|
| Whole-body massage | 1.5 – 2.5 % | 5 % |
| Facial use | 0.5 – 1 % | 2 % |
| Spot application | 3 – 5 % | 10 % |
| Bath (dispersed) | 5 – 10 drops | 15 drops |
| Diffusion | 3 – 5 drops per 100 ml water | 10 drops |
Working rule: 1 ml of essential oil ≈ 20 – 40 drops depending on dropper. Average is roughly 1 ml ≈ 30 drops. A 1 % dilution corresponds to about 3 drops of essential oil in 10 ml of carrier.
Infants and small children have thinner, more permeable skin than adults. Skin of premature newborns is roughly 2.5 times more permeable than adult skin; before 30 weeks of gestation it is 100 to 1,000 times more permeable. Liver and kidney enzyme systems remain immature for the first months, slowing the elimination of any compound that enters the bloodstream.
Oils rich in 1,8-cineole or menthol can depress respiration in young children and must not be applied to or near the face. Eucalyptus globulus and Peppermint are not used in children under six, while Eucalyptus radiata is gentler and acceptable in diluted form for older children. Sweet birch and Wintergreen carry very high methyl salicylate levels and are excluded entirely for children.
Older skin is drier and thinner, more permeable in certain areas. Cytochrome P450 metabolism often slows with age. Starting concentrations should be lower, particularly for people taking multiple medications.
| Age | Recommended | Maximum |
|---|---|---|
| Premature | 0 % | |
| 0 – 3 months | 0.1 % | 0.2 % |
| 3 – 24 months | 0.25 % | 0.5 % |
| 2 – 6 years | 1 % | 2 % |
| 6 – 15 years | 1.5 % | 3 % |
| Over 15 | 2.5 % | 5 % |
Avoid applying essential oils to or near the face of children under five. Do not place oils in a child's nose. Do not expose children to concentrated essential oil vapours. Never give essential oils orally to children.
Most essential oil constituents cross the placenta when applied dermally or inhaled. The amount reaching the foetus from ordinary topical use is small, but for certain classes (phenylpropanoid ethers such as methyleugenol, safrole, estragole; ketones such as thujone, pulegone, camphor; salicylates) the safety margin is narrow and these are excluded throughout pregnancy.
See the Pregnancy page for the full list. Oils excluded entirely in pregnancy include Anise, Anise (star), Fennel (sweet), Fennel (bitter), Sage (Dalmatian), Hyssop (pinocamphone CT), Wormwood, Parsley seed.
Through breast milk, exposure is usually below 1 % of the maternal dose, low enough not to be a concern for most diluted topical use. Oral dosing should be avoided during lactation unless medically indicated.
Asthma and airway hyper-reactivity. Strong odours can trigger attacks in susceptible people. Avoid direct inhalation of concentrated oils. Massage can be used at no more than 1 %, preferring gentle oils such as Lavender.
Epilepsy. Several monoterpene ketones stimulate the central nervous system and can trigger seizures at high doses. Oral use of Sage (Dalmatian), Hyssop (pinocamphone CT), Camphor, Wormwood, Mugwort is contraindicated. Febrile children are more prone to seizures than adults; caution with the oils above applies even without a diagnosis.
Bleeding disorders and concurrent aspirin, warfarin or heparin. Oils rich in methyl salicylate (Wintergreen, Sweet birch) are aspirin-like and raise bleeding risk. Avoid oral use one week before surgery or childbirth.
GERD and peptic ulcer. Menthol-rich oils can relax the lower oesophageal sphincter and worsen reflux. Peppermint and cornmint should be avoided orally in GERD.
Kidney or liver disease. Clearance of concentrated compounds is reduced. Oral use only under a clinician trained in essential oils.
| Condition | Oils to avoid or restrict |
|---|---|
| Epilepsy | Sage (Dalmatian), Hyssop (pinocamphone CT), Camphor, Wormwood, Mugwort |
| Asthma | Cinnamon bark, Clove bud, Oregano, Thyme (thymol/carvacrol CT) |
| Children under 6 | Eucalyptus globulus, Peppermint, Wintergreen, Sweet birch, Cinnamon bark |
| Anticoagulants | Wintergreen, Sweet birch |
| GERD, peptic ulcer | Peppermint, cornmint, Wintergreen, Sweet birch (oral) |
| Estrogen-dependent cancer | Anise, Anise (star), Fennel (sweet), Fennel (bitter) |
A handful of essential oils carry acute toxicity or carcinogenicity high enough that they should not be sold to end users. They belong to controlled industrial flavour and fragrance use, not personal use, not aromatherapy.
This group includes Mustard, Horseradish, Sassafras, Boldo, Snakeroot, wormseed, unrectified bitter almond, unrectified cade. They contain allyl isothiocyanate, safrole or hydrocyanic acid at levels capable of serious harm.
A second group carries acute toxicity risk if ingested by children and must be sold with child-resistant closures: Cinnamon bark, Clove bud, Eucalyptus globulus, Peppermint, Wintergreen, Sage (Dalmatian), Tea tree, Wormwood, Oregano, pennyroyal.
Several essential oils carry dermal maxima due to irritation, sensitisation or phototoxicity risk. Above the threshold an immediate reaction (redness, burning) is possible, or repeated exposure may induce lasting sensitisation.
The limits below apply to leave-on products. For rinse-off products (body wash, shampoo) the limits can be more generous, but each formulation requires its own assessment.
| Oil | Maximum dermal level | Reason |
|---|---|---|
| Bergamot (expressed) | 0.4 % | Phototoxic / Phototoxicity |
| Lime (expressed) | 0.7 % | Phototoxic / Phototoxicity |
| Lemon (expressed) | 2.0 % | Phototoxic / Phototoxicity |
| Cinnamon bark | 0.07 % | Irritant and sensitizer |
| Clove bud | 0.5 % | Irritant |
| Oregano | 1.1 % | Irritant |
| Lemongrass | 0.7 % | Sensitizer |
| May chang | 0.8 % | Sensitizer |
Sensitisation is a delayed immune response. First exposure is usually silent. After repeated exposure over 10 to 15 days, the immune system learns the substance as an allergen. From that point, every subsequent exposure triggers a fast reaction, sometimes within hours, with redness, itching, dermatitis, occasionally spreading.
Unlike irritation, sensitisation persists. Once sensitised to a constituent, a person may react to it for life, even at very low concentrations, even when that constituent appears in a different oil. People with a history of atopic dermatitis, perfume allergy or damaged skin are at higher risk.
Common sensitising constituents include Cinnamaldehyde, Citral, Eugenol, Isoeugenol, sesquiterpene lactones such as alantolactone, methyl salicylate, and the oxidation products of Limonene and α-Pinene. Oils rich in these constituents (clove, cinnamon, lemongrass, may chang, oakmoss absolute) should be kept below the dermal limits.
An aged or poorly stored oil develops sensitisation risks that the fresh oil did not have. When Limonene, α-Pinene or d-3-carene meet oxygen over time, they form oxidation products such as hydroperoxides that are markedly more sensitising than the parent molecule.
Citrus oils, pine, tea tree and monoterpene-rich oils generally are the most vulnerable. After 12 months at room temperature in an opened bottle, the monoterpene content of a lemon oil can fall from 97 % to under 31 %, and what disappears is converted to oxidation products capable of causing skin reactions.
Prevention lies in proper storage and rapid turnover. See Storing essential oils for shelf life by class and signs of oxidation.
Several essential oil constituents induce or inhibit hepatic cytochrome P450 enzymes, altering the metabolism of oral drugs. The effect is significant for oral dosing, modest for diluted topical use and generally negligible for ambient diffusion.
Drug groups to note include anticoagulants (warfarin), opioid analgesics, SSRI and tricyclic antidepressants, statins, benzodiazepines, oestrogen contraceptives, certain antidiabetic drugs. Anyone on multiple medications should consult a clinician before oral essential oil use.
Do not apply essential oils to skin areas carrying a transdermal drug patch. The oils can substantially increase drug absorption, leading to local overdose.
Oral use carries the highest risk and should be performed only under a clinician authorised by local law to prescribe internal essential oils. High doses irritate the gastrointestinal tract and can damage liver, kidney or central nervous system.
Mucous membranes (eyes, nose, mouth, vagina, rectum) are far more sensitive than skin. Never apply undiluted essential oils to mucous membranes. Several oils are excluded from mucous membrane use even when diluted: Cinnamon bark, oakmoss absolute, treemoss absolute, massoia.
Dropping undiluted oils into the ear can damage the eardrum. To treat the outer ear canal, apply diluted oil to a cotton wad and rest it at the ear opening; do not push it deep.
Eye splash: flush the eye with water for at least 15 minutes. Remove contact lenses after the first 5 minutes and continue rinsing. Hold the lids apart for a thorough rinse. Seek medical advice if irritation persists.
Skin reaction: wash the area gently with unperfumed soap and water for at least 10 minutes. Expose the skin to air but avoid direct sunlight. A warm oatmeal bath can soothe larger areas. Barrier cream or a mild corticosteroid is the usual medical response.
Accidental ingestion: do not induce vomiting. If the person is conscious and not convulsing, rinse the mouth with water. Contact a poison centre or hospital immediately. Children who ingest any amount must be taken to medical care.
Excessive inhalation: move the person to fresh air. Support breathing if needed and call emergency services.
When seeking medical help, bring the bottle so the species and composition can be identified.
Safe essential oil use rests on a few principles. Dilute correctly for the application. Reduce concentrations for children, the elderly and pregnant women. Avoid restricted oils in specific conditions. Respect the dermal limits for oils carrying irritation, sensitisation or phototoxicity risk.
Store properly to prevent oxidation. Read the label to verify botanical name, chemotype and plant part. Reserve oral use for medical supervision. When in doubt, choose a lower concentration and consult the individual oil page for full guidance.