Are there any carcinogenic essential oils?
There are carcinogenic essential oil constituents. These include asarone, estragole (methyl chavicol), safrole and methyleugenol. Both estragole and methyleugenol are found in Basil leaves and they are quite concentrated in pesto. However, the WHO has declared pesto to be safe for human consumption because research has shown that anticarcinogens in Basil leaves counteract the toxic effects of the carcinogens. Unfortunately these particular anticarcinogens are not found in Basil essential oils, but there are others that are found, depending on the type of Basil oil.
In some cases we can be fairly certain that an essential oil containing one of these carcinogens is also carcinogenic, but in other cases, this is not so. It depends on three factors: 1) the percentage of the constituent; 2) the potency of that constituent, and 3) how much, if any, of anti-carcinogenic constituents are also in that essential oil. Rose oil, Elemi oil and Holy Basil oil all contain very small amounts of carcinogens, but the oils are not generally considered to be carcinogenic. On the other hand, Calamus oil, Sassafras oil and Huon Pine oil are considered to be carcinogenic, and they are not much used in aromatherapy.
Some widely-used essential oils should only be used in restricted amounts because of their content of carcinogens. For example, the estragole chemotype of Basil oil contains 75-87% of estragole and should be used at a maximum topical concentration of 0.1%. But the linalool chemotype of Basil has very little estragole and has lots of anticarcinogenic linalool and eugenol, so needs no restriction. More information on this can be found in Essential Oil Safety.