What is sensitization?
Sensitization is a reaction to a substance which had not previously caused an adverse reaction. This can present as redness, itchiness, burning, welts, etc. on the skin. It can happen on the first use, but it more likely after multiple usages. A great example is that a gardener may not develop a rash the first time he contacts poison ivy (highly sensitizing), but the second time he contacts it, his skin reacts with significant rash.
How does sensitization occur?
If you use any substance for too long, at too high of a dose, or too often, you may become sensitized to that substance. In aromatherapy, this usually happens with topical applications. For example, if a mild essential oil, such as lavender or tea tree, is used undiluted or even at a high dilution (say 50%) every day for several months, the risk of sensitization becomes high. If it is used rarely, undiluted, the risk of sensitization is low. If it is used highly diluted (less than 5%) on a daily basis, the risk is low. Once a person has become sensitized to a substance, like an essential oil, that sensitization is a life-long problem, requiring avoidance of that substance.
Which Essential Oils cause sensitization?
Some essential oils, such as cinnamon bark and oregano are considered to be "highly sensitizing", that is, it will cause sensitization in the majority of people unless used with proper dilution and respect. Some are "moderately sensitizing", others are considered "non-sensitizing". Lavender and tea tree fall into this last category. However, that does not mean they should be used daily, undiluted. It means that with proper dilution, it is unlikely to cause sensitization.
How to prevent sensitization?
First of all, always dilute. For "non-sensitizing" essential oils, diluting at 10% is reasonable. This means 10 drops of essential oil in 1 teaspoon of a carrier oil (such a coconut, avocado, grapeseed, or other fixed oil). For highly sensitizing essential oils, some are recommended at less than 0.5%, which would translate to 1 drop of essential oil in 1 cup of carrier oil (as a rule of thumb). One popular way to dilute essential oils for topical use is in a roller bottle. In our lavender example, a 10% dilution in a 10mL roller bottle would mean 20 drops of lavender essential oil, then filling the rest of the bottle with a carrier oil.
Roller bottles are an inexpensive and convenient method of preventing sensitization. Is sensitization still possible, yes, but has the risk been lessened, yes.
