I’ll be the first to admit that there’s nothing more soothing and homely than a beautiful aroma. Like the smell of fresh oranges and lemons while you’re baking dessert. Or a touch of calming lavender oil sprinkled on your pillow after a long day. Pleasant aromatic plants and essential oils as a whole are thought to have a range of potential benefits from memory to mental state.

But many people use this as a reason to use essential oils directly on their skin. Some companies (*cough* MLMs) come out with all sorts of outlandish claims – and there’s a bunch of reasons not to trust them. So what really happens when we put these highly concentrated oils on our faces and bodies?

The answer is that essential oils can be dangerous and toxic – and no, not the kind of toxic you hear about when people are talking about DIY sunscreen alternatives or selling you juice ‘cleanses’. They are unsafe to use on the skin in high concentrations, and even sometimes at all.

Using certain essential oils can pose two serious problems – sensitization and phototoxicity. Today we’re getting sciencey and discussing these side effects of topical essential oil use and how to avoid them.

The first issue with many popular essential oils are allergic reactions caused by sensitization.

What causes sensitization?

Some essential oils contain allergenic compounds. These are usually monoterpenes and sesquiterpenes within the oil. In tea tree oil (the biggest sensitizing culprit and most highly praised as a skin care oil) the suspects are ascaridole, terpinolene, α-terpinene, 1,2,4-trihydroxymenthane, α-phellandrene, and limonene.

In sufficient quantities, these can trigger a sudden allergic reaction. A person might use the oil once or many times with no reaction before suddenly ‘becoming’ allergic. This sensitisation reaction is permanent – once it’s happened, you’ll have to avoid these oils completely for the rest of your life.

What does sensitization look like?

Sensitization to essential oils causes allergic contact dermatitis that manifests as an itchy rash, burning skin, redness, blistering, hives and scarring. See this page for more information and images of dermatitis caused by essential oil.

Culprit oils that cause sensitization:

The big culprit here is tea tree oil. This is the oil most commonly found to cause allergic contact dermatitis. In concentrations of 5%, it’s been found to trigger sensitization reactions in up to 3.5% of people. Co-reactivity is another issue – once you are sensitized to tea tree, you are likely to also react to other things containing similar compounds. Such co-triggers include oil of turpentine, Balsam of Peru and colophonium (pine resin).

Many essential oils other than tea tree also contain molecules capable of eliciting allergic reactions. These include:

Clove

Fennel

Basil

Clary Sage

Black pepper

Cypress

Frankincense

Jasmine absolute

Juniper berry

Rose

Spruce

Cassia

Cinnamon

Verbena

Backhousia

Inula

How can you avoid sensitization?

Essential oils are capable of triggering sensitization when used in high concentrations. The more you use them, the more you are at risk for a sensitization reaction. Essential oils are not meant to be applied neat. As most studies found reactions for tea tree oil at a concentration of 5%, it’s thought safest to avoid concentrations over 1%. Many other oils need to be brought down to concentrations as low as 0.01% to be considered safe.

If you insist on using essential oils in skin care, dilution is necessary. All essential oils should be diluted into a carrier oil (which is a basic oil containing far fewer aromatic components, like olive or coconut) appropriately before being applied to the skin. Oil does not mix with water, so diluting using water will not work.

This also brings up the necessity of being selective of products containing essential oils unless you know how much is in there. This can sometimes be tricky – for example, tea tree can go by several other names in an ingredients list:

Melaleuca alternifolia oil

Tea extract

Tea leaf, absolute

Tea oil

CAS RN: 68647-73-4

Also, studies have shown that the byproducts of oxidized oil cause worse reactions, so only use them before the ‘best before’ date.

Beyond sensitization reaction, essential oils in skin care can also be phototoxic; making them unsafe to expose to the sun.

What causes phototoxicity?

Some essential oils contain a family of compounds called Furanocoumarins. When exposed to sunlight (specifically UVA, which can penetrate clouds and glass), they react and cause an allergic response in humans called “phytophotodermatitis“. However unlike contact dermatitis you don’t need an initial exposure to be sensitized to these compounds. As a result phytophotodermatitis can happen to anyone the first time they use these oils.

What does a phototoxic reaction look like?

This isn’t a condition that is easy to miss. The response to phototoxic oils can include painful blistering, redness and weeping resembling chemical burns, swelling (edema), scabs and scarring with lasting hyperpigmentation. Imagine the worst sunburn of your life times 50. If you’re morbidly curious, Aga over at Core Conscious Living documented her terrible reaction to putting diluted bergamot essential oil on her face before a hike. Ouch.

Culprit oils that are phototoxic:

Many citrus peel oils are culprits for phototoxicity. The main popular oils to avoid are:

Bergamot

Bitter orange

Grapefruit

Lemon

Lime

Other less common oils include:

Angelica Root Essential Oil

Cumin

Fig Leaf Absolute

Mandarin Leaf

Opopanax

Rue

Tagetes

For a full list of potentially phototoxic oils, see AromaWeb’s page.

How can you prevent phototoxic reactions?

You should never ever apply these oils to the skin during daylight hours, in any concentration. Experts suggest you do not expose your skin to the sun for 12-18 hours after any application of these oils. Since UVA is present all day, even through clouds and indoors, the only way to be completely safe is to avoid any skin care containing these oils at all wherever possible.

The surefire way to stay safe and prevent sensitization or phototoxic reactions? Don’t use essential oils on your skin! They can seem like harmless fun but you have to remember that they’re potent ingredients and you need to treat them as such. You shouldn’t throw them around at random because a MLM representative told you they’d cure all your ills.

I’m not saying all essential oils are evil. But just because they’re “natural” sure as heck doesn’t mean they’re harmless. The same goes for other ‘natural’ skin remedies and products like soap. Doing your research will prevent dangerous mistakes and also help expand your knowledge to end the cycle of misinformation. To be safe, I generally try to keep all essential oils out of my routine as much as possible. They might smell great, but there are other ingredients that work better and more safely.