Irritation and sensitivity are possible with the use of almost any essential oil, says Jimm Harrison, author of Milady’s Aromatherapy: Therapeutic Use of Essential Oils for Esthetics. This is one reason most aromatherapy experts recommend against using undiluted oils on the skin. Still, certain oils are more likely to cause irritation, including thyme, oregano, cinnamon, and clove. They are safest under a 10% dilution, says Harrison, who also cautions users about the potential irritant properties of citrus oils, needle tree oils, eucalyptus, tea tree oil, and rosemary.

“If irritation occurs when an oil is applied, or spills, on the skin, remove it by applying a fair amount of vegetable oil, such as olive or safflower, and wipe it from the skin,” says Harrison. “Water should not be used alone to wipe off an irritant oil. Water will push the essential oils deeper, and more aggressively, into the skin.” For that reason using essential oil in a foot bath or applying it to wet skin increases the chances of irritation.