Irritation
Irritation is a local allergic response of the skin to any substance that the body finds stressful. All essential oils have the potential to be irritants although a response is far more likely in a person who has a history of allergy reactions or someone who is already suffering from an allergic reaction such as hay fever.

Irritation is also possible as the result of a reaction between some skin products and an essential oil applied to the skin. Most modern moisturising skin preparations, shower gels, etc leave chemical residues in the epidermis and these may result in an irritation in combination with essential oils. All essential oils high in terpene content have increased risk of causing irritation as terpenes themselves are skin irritants.

Guidelines for safe use of Essential Oils
Keep within the recommended safe concentration for an individual especially if covering a large body area with henna.

1. Always keep a bottle of vegetable or olive oil handy to remove a design that is causing discomfort. At a pinch, full fat milk will do, but not water. Water will be ineffective in removing an essential oil as the essential oil is soluble in oil, so it will preferentially remain on the skin after washing with water. Water may even exacerbate a problem by spreading the essential oil over a greater area.
2. To prevent deterioration and chemical breakdown of oils keep them in tightly sealed dark bottles, away from heat, light, children and pets.
3. Topical application of photo-sensitising oil is not recommended, but any application of such should be kept away from sunlight or any other source of UV light for 3-4 days.
4. Never use essential oils on the face or near the eyes.
5. People with allergies should have a skin test before application.
6. Special caution should be used with children, epileptics, people with high blood pressure or diabetes and throughout pregnancy.
7. Know the properties of the oils you are using.