Rules
Safety rules
Ten rules that cover most of what goes wrong with essential oils at home. They are written to be checked before a blend is made, not after a reaction appears.
1. Dilute, and dilute by arithmetic
Undiluted essential oil on skin is a chemical burn risk, not a strong version of a diluted blend. Work out the number before you open a bottle: at 20 drops per millilitre, a 1% blend in 30 ml of carrier oil is 30 × 20 × 0.01 = 6 drops. Write it down. The commonest serious error is counting drops into an empty bottle and then topping up with carrier, which multiplies the concentration.
The ceilings used on this site are 0.5% for facial blends and children 6-15, 1% for everyday adult leave-on use, 2% for short-term localised adult use, and 3% as an absolute adult maximum for robust oils only. Phenol-rich oils such as thyme and oregano stop at 2%, and phototoxic citrus peel oils stop at 0.5% on any skin that will see sun within 12 hours.
2. Patch test every new oil, and every old one after a gap
Four steps, 48 hours: dilute to 1% in carrier oil; apply 2 drops to the inner forearm; cover with a plaster or a small square of gauze and leave it 24 hours; read the site at 24 and again at 48 hours. Stop early and wash with carrier oil — not water — if it itches or burns within the first hour.
A negative patch test lowers risk; it does not eliminate it. Sensitisation develops over repeated exposures, so an oil that passed in March can fail in September.
3. Read the contraindication columns honestly
Pregnancy, breastfeeding, epilepsy, hypertension, cardiac conditions, bleeding disorders, anticoagulant and antihypertensive medication, asthma, and liver or kidney disease each change the answer. So does a household with a cat, a dog, a bird or a reptile. If none of the columns on a monograph describes you, ask a clinician; do not interpolate.
Cats cannot glucuronidate many terpene alcohols and phenols. A diffuser running in a closed room with a cat in it is a real exposure route, and tea tree, eucalyptus, peppermint and citrus are the oils most often named in feline cases.
4. Keep phototoxic oils off sun-exposed skin
Cold-pressed bergamot, lime, lemon, grapefruit and bitter orange peel oils carry furanocoumarins that absorb UVA and dump the energy into skin tissue. The reaction appears 12 to 18 hours after application and looks like a burn with streaks and blistering, followed by pigmentation that can last months.
Distilled citrus oils are not phototoxic. If your bottle does not say which process was used, treat it as cold-pressed. Never put these oils on the face, neck, décolletage or the backs of the hands at any dilution.
5. Store for the shelf life, not for the memory
Essential oils oxidise. Linalool and limonene hydroperoxides are the most common sensitisers found in household bottles, and they build up in a partly empty bottle in months, not years. Buy small, decant nothing into plastic, keep the cap on, keep the bottle below 20°C and away from light, and write the opening date on the label.
Shelf lives used here: 12-18 months for expressed citrus peel oils, 24 months for most distilled flowers and herbs, 36 months for lavender, tea tree and most wood oils, and effectively indefinite for frankincense, myrrh, sandalwood and vetiver, which improve with age. A sharp, acidic or turpentine-like smell means the oil has turned; discard it rather than using it up.
6. Rotate rather than repeat
Continuous use of one oil is the fastest route to sensitisation, particularly with the aldehyde-rich oils — lemongrass, litsea cubeba, cinnamon — and the phenol-rich ones. Rotate between oils with different principal constituents every two to three weeks, and keep a written note of what you have used on which area.
Cross-sensitisation is real: a reaction to geraniol raises the odds of reacting to citronellol and linalool, so "switching" between rose, geranium and palmarosa is not a rotation in the sense meant here.
7. Never ingest, and never let a child near the bottle
Essential oils are not food. Unless a qualified clinician has specifically directed oral use, swallowing essential oil is poisoning: a teaspoon of eucalyptus or wintergreen is a medical emergency. Keep bottles in a locked or high cupboard, never in a handbag or a bathroom cabinet, and never in a container a child could mistake for food or a dropper bottle of medicine.
If oil is swallowed, do not induce vomiting. Contact your local poisons service or emergency number and have the bottle to hand. If oil splashes into an eye, flush with cool water for 15 minutes and seek medical attention.
8. Diffuse with the door open
A diffuser is not a room treatment, it is a low-dose exposure for everyone in the room, including people who did not choose it. Run it 15 to 30 minutes at a time with ventilation, never continuously, never in a closed bedroom overnight, and never in a room with an infant, a pregnant person, an asthmatic or an animal that cannot leave.
If anyone develops a headache, a cough, a sore throat or irritated eyes, switch it off and ventilate the room.
9. Do not blend around medications or conditions you have not declared
Topical absorption is low but not zero, and several constituents interact with drugs and conditions in documented ways. Peppermint and eucalyptus near the face of an infant or a young child can trigger laryngospasm. Rosemary and camphor-rich oils are traditionally avoided in epilepsy. Fennel, anise and other methyl-chavicol or anethole-rich oils are avoided in pregnancy and in oestrogen-sensitive conditions.
This site prints the caution rows because the information belongs to the reader, not because it can substitute for a pharmacist's review of your prescription list.
10. Treat every page here as information, not as treatment
Nothing on this site diagnoses, treats, cures or prevents disease. If you have a symptom that persists, the correct next step is a clinician, not a stronger blend. If you have already reacted — redness, swelling, blistering, itching that starts hours after contact — wash with carrier oil, stop using the oil, and seek medical advice if it covers more than a small area or is worsening after 24 hours.
There is no oil that is safe for everyone, and no dilution that makes a hazardous oil suitable for a population it is contraindicated in.
Pocket list
- Dilution convention used throughout: 20 drops per millilitre.
- 1% in 30 ml carrier = 6 drops. 2% in 10 ml = 4 drops. 3% in 30 ml = 18 drops.
- Patch test at 1% on the inner forearm, read at 24 and 48 hours.
- Wash a reacting area with carrier oil, not water, then stop using the oil.
- In an emergency: local poisons service or emergency number first, this site never.