Toxicity is the degree to which a substance is able to damage an exposed organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell (cytotoxicity) or an organ (organotoxicity).

Toxicity can be acute, subchronic, or chronic:

•Acute toxicity involves harmful effects in an organism through a single or short-term exposure.

•Subchronic toxicity is the ability of a toxic substance to cause effects for more than one year but less than the lifetime of the exposed organism.

•Chronic toxicity is the ability of a substance or mixture of substances to cause harmful effects over an extended period, usually upon repeated or continuous exposure, sometimes lasting for the entire life of the exposed organism.

The great majority of essential oils are non-toxic and perfectly safe when used sensibly; that is to say, in the small quantities and low dilutions described in this database. There are, though, some which are highly toxic, even in small amounts, and others which may give rise to toxicity if used over a long period of time. There are also several groups of people who are far more vulnerable to possible damage from the use of essential oils than most of us. They are: babies and young children, pregnant women, people suffering from epilepsy, and the elderly; and special caution should be taken when using essential oils for anybody in one of these categories.

The most toxic essential oils have never been commonly used in therapy, and are virtually impossible to obtain. However there is a group of oils which could be considered as 'borderline', in that they present certain risks, but are quite easy to get hold of, and it is perhaps here that the greatest care is needed. Some of these are described in books, particularly those originating in France, without any cautions regarding their use, but we need to bear in mind that in France the majority of aromatherapists are doctors, who have undertaken a training in the use of essential oils after their general medical training. They may, on occasion, use with great caution, and with a complete understanding of their possible effects of the body, some oils which could be lethal if misused. In much the same way, both homoeopaths and allopathic doctors use minute amounts of toxic plants to produce a cure, but this does not mean that the lay person, or an aromatherapist without a medical background (and, in the USA and Great Britain that means almost all of them) should attempt to do the same.

There are also some oils whose possible toxic effects have not been fully understood until recently, through new research and through clinical observation, and these too may be described as though they are completely safe in a number of reference works.

Among the oils which are potentially dangerous, but which you may be able to buy without difficulty, are Camphor, Mug-wort (often sold under its French name of Armoise), Pennyroyal, Sassafras, Thuja, Wintergreen and Wormwood. All these oils present risk of poisoning, and it is best to leave them strictly alone.

Sage is an oil which has been commonly held to be quite safe, but observations have suggested otherwise, and therapists are now producing evidence which confirms this. Clary Sage is a safer alternative unless you are trained, experienced and very sure of what you are doing. There is also some suggestion that Thyme is potentially hazardous, and you should certainly use this oil in very low concentrations only, and avoid it completely if treating children.

The essential oil derived from Bitter Almonds is very dangerous indeed, as it contains cyanide. This should not be confused with the almond oil used as a carrier for massage, which is obtained from the sweet almond, and is perfectly safe. The almond essence sold for food flavouring has either had the cyanide removed by a chemical process, or is more commonly a synthetic product.

Aniseed, again often given is French name of Anise, has serious effects if used over a long period of time. It can damage the nervous system, and the circulation, and can be addictive in the same manner as narcotic drugs. It is wise, in any case, to avoid using any essential oil repeatedly over a long period of time, as residues may build up in the body and give rise to toxic effects. Even with the most safe essential oils, the body simply stops responding to them if they are used continually, so it is more sensible to vary the oils used, or take a little 'holiday' from your favourite oils from time to time.

The most toxic oils frequently have the effect of damaging the kidneys and/or the liver and prolonged use of the dubious oils can do the same. This is because these organs have the job within our bodies of filtering out many dangerous substances, so high concentrations of poisons eventually accumulate in them.

It should be emphasise that in speaking of toxicity or - in plain terms, poisoning - we are not only referring to taking essential oils by mouth. This is by far the most dangerous method of use, and you will realise from many other entries in this database that we do not recommend the use of essential oils in this way. A very large number of responsible aromatherapists share this view.

However, essential oils are very effectively absorbed into the body, whether we inhale them or apply them to the skin, and they find their way into the bloodstream. It is this very fact which makes aromatherapy the effective system which it is, but it also means that toxic oils can be taken into the body by the same routes.

A complete list of toxic essential oils is given in the article for hazardous oils.