Phototoxicity - essential oils, the sun, and safety

Phototoxicity has long been a well-known topic and has been studied extensively. You can get phototoxic skin reactions from certain essential oils, perfumes, medications, sunscreens, or even the fluids of fresh plants when they come into contact with your skin or are used internally while you’re exposed to UV radiation. Phototoxicity appears as a skin reaction that resembles a burn. It’s very avoidable if you remember a few basic principles, for example when using essential oils. Increasing awareness about phototoxicity is one of the key ways to completely prevent symptoms.

In this article, we’ll explain what phototoxicity is and how it shows up on the skin. We’ll also go through phototoxic products and review how to avoid a phototoxic reaction. You don’t need to avoid the sun entirely; it’s enough to know what you can and can’t safely use in the sun.

Why are plants sometimes phototoxic?

One way volatile molecules protect a plant is by being harmful to creatures that might eat the plant. In that case – hopefully – the herbivore would learn to avoid plants that smell or taste a certain way. And one of the clever ways plants have evolved to teach herbivores a lesson – whether insects, insect larvae, birds, or mammals – is through phototoxicity.

For example, methoxsalen, found in limes, is particularly toxic to certain butterflies and moths. The dose is usually not lethal, but it can cause damage to various internal organs or, in the case of mammals, phototoxicity on lighter skin areas like ears, lips, eyelids, and in cows, the udder.

These compounds need sunlight for their toxic effects to show. Unlike very small creatures, large mammals like cows and humans do not suffer from internal toxicity because the sun doesn't penetrate their internal organs. The previously mentioned methoxsalen is a furanocoumarin (FC). 

Some furanocoumarins are used as medicines combined with UVA light exposure in the treatment area. These substances prevent cells from dividing, so they can be used as a treatment for conditions like psoriasis. They also make the skin's melanocytes produce melanin, which darkens the skin, so they've been used cosmetically for vitiligo as well as non-medically to enhance tanning. However, tanning and medical treatments carry an increased risk for the treated area because several furanocoumarins damage cellular DNA when combined with UV light. As a result, they are classified as carcinogens and mutagens.

When does a phototoxic reaction occur in humans?

In humans, a phototoxic reaction is similar to a severe sunburn, because the phototoxic chemical exaggerates the skin's reaction to UV light. The two main sources of UV light are the sun and tanning beds, and it’s not a good idea to use phototoxic substances when exposed to either. Common sources of phototoxic chemicals include:

• Fresh lemons and limes  
• Phototoxic essential oils  
• Phototoxic plants or foods  
• Phototoxic medications and drugs  

As a phenomenon, phototoxicity has been well understood since the 1940s, but still, a lack of awareness can be seen in the occurrence of skin reactions in people.

Since the late 1970s, the fragrance industry has taken steps to protect consumers by ensuring that perfumes don’t contain phototoxic substances in amounts that pose a risk. Instead, the risk has returned in a new form as aromatherapy has grown in popularity.

It’s important to understand that phototoxicity isn’t linked to skin sensitivity; phototoxic ingredients react to UV light the same way on all skin types.

What does a phototoxic reaction look like?

A visible reaction often looks like a burn, with pigmented skin around the edges. In mild cases, there’s often a slight stinging, or just pigmentation without any visible stinging. In more severe phototoxic reactions, blisters resembling burn wounds can form on the skin. After the injury heals, uneven pigmentation often lingers for weeks or months, but it eventually fades.

Keep the timing in mind  

Phototoxic reactions are sneaky in that at first there's usually no pain or inflammation – you don’t know it's happening while it’s happening. The burns develop hours later and get worse over the next 1–3 days. When a phototoxic substance is applied to the skin at a sufficient concentration, exposure to UVA radiation at any time in the next 12 hours, and possibly even longer, can trigger a phototoxic reaction.

Phototoxic Chemicals

Almost all phototoxic chemicals found in plants belong to a class known as furocoumarins (FCs). There are quite a few FCs, and their phototoxicity ranges from negligible to strong. FCs occur in essential oils only in very small percentages, usually less than 1%, but even 0.1% can be phototoxic. FCs are mainly found in two plant families: Apiaceae and Rutaceae, although some species in the Asteraceae, Moraceae, and Fabaceae families also contain them.

If you're going to be in the sun, it's recommended to completely avoid essential oils that are exposed to light, because even very low concentrations could potentially pose a risk. Another way to avoid light exposure is to use oils from which the FCs have been removed, typically through distillation. A common example of this is bergapten-free bergamot oil, or Bergamot FCF (furocoumarin-free).

How does phototoxicity actually happen on the skin?

Whether it's from internal use or local external application, when a phototoxic chemical reaches skin cells, it enters the cell and moves to the nucleus. Under the influence of UVA radiation, the phototoxic molecule becomes chemically reactive and starts releasing so-called free radicals and reactive oxygen species into its surroundings. These, in turn, begin to damage cell membranes, alter protein structures, and the resulting DNA damage triggers inflammatory reactions and even cell death.

At the same time, inflammation occurs, and in many cases, melanocytes – the cells that produce the skin pigment melanin – multiply quickly, causing pigmentation.

The mechanism is similar to what happens in burns when damaged cells release signaling molecules. This is followed by redness, warmth, and pain on the skin.

Phototoxicity in Essential Oils

Citrus oils can be either distilled or pressed. Furanocoumarins (FCs) are relatively heavy, non-volatile molecules, so they don’t transfer into the essential oil during the distillation process. Since distilled citrus oils practically contain no FCs, they aren’t photosensitive. Pressing, on the other hand, is a mechanical process where the peel of the whole fruit is scraped, allowing the essential oil to be collected. Then the juice is extracted from the fruit.

Some essential oils contain very small amounts of furanocoumarins (FCs), not enough to make them photosensitive. These include cumin, ginger, mandarin fruit (pressed), parsley herb, patchouli, mandarin (pressed), and yuzu (pressed), as well as most distilled citrus oils.

Bergamot essential oil is famously highly phototoxic because it contains relatively high amounts of FCs.

Phototoxicity from sunscreens

Surprisingly, some ingredients used in sunscreens have been linked to causing photosensitivity, including:

• Benzophenones
• Cinnamates
• Salicylates
• PABA
• Octocrylene

The safest and most effective sunscreens are those based on physical properties rather than chemical ones.

This is the best way to avoid a phototoxic reaction  

• Don’t use any phototoxic essential oils on your skin if you’re going out in the sun or to a tanning bed. Remember to ALWAYS dilute all essential oils carefully when applying them to the skin.  
• The risk is low if the essential oils are in a rinse-off product, like shampoo or body wash.  
• Avoid the sun and tanning beds if you’re using phototoxic medications. Be aware of the effects and risks of the medicines and skincare products you’re using.  
• If you’re using a phototoxic product, make sure that area of skin is not exposed to UV rays for at least 12 hours, either by covering it completely or staying indoors.  
• One good way to care for your skin after sun exposure is to spray lavender hydrosol or apply a moisturizer with a drop of lavender essential oil.  
• However, if you do get a phototoxic reaction, treat it at home like a burn. In some cases, it might also be a good idea to show the injury to a doctor.