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A green lime, a knife, a midday outdoors. Lime peel holds molecules that do not evaporate and have no smell, yet react with UV light when they sit on skin. This piece reads citrus phototoxicity through the figures in the library: which species, which method, what percentage.

A brown mark shaped like a drop

Dermatology has a very particular kind of dark mark: brown streaks shaped like drops, running down the side of the neck, behind the ears, across the chest, exactly where perfume was dabbed or sprayed. Its name, berloque dermatitis, comes from the French breloque, a pendant, because the marks often look like a hanging charm.

According to the library's phototoxicity page, the pattern was first described in 1916. The earliest cases involved women who wore perfume containing bergamot oil and then exposed their necks to the sun. Hours to days later, dark patches appeared exactly where it had been applied, and could last weeks to months.

What matters is that the mark needs no allergy. Phototoxicity is a direct reaction between a substance on the skin and UV light, mainly UVA. The immune system does not have to recognise the substance beforehand, so it can happen on first exposure. The distinction is important: someone who has never reacted to bergamot can still have a phototoxic reaction if the concentration and the sunlight are sufficient.

Furocoumarins in the peel

The culprits are a group of molecules called furocoumarins: a furan ring fused to a coumarin framework, forming a flat, multi ringed structure. They occur in the peel of many citrus species and in the roots of some plants of the carrot family. They are heavy, non volatile in pure form, and contribute almost nothing to the smell.

The flat shape is what makes them hazardous. The library describes the mechanism: furocoumarins slip between the base pairs of DNA in skin cells. When UVA supplies the energy, the molecule forms a covalent bond with a pyrimidine base, and linear furocoumarins can also bind to a base on the opposite strand. The result is DNA damage and skin inflammation.

The signs appear late. According to the library data, the reaction peaks around 36 to 72 hours after UV exposure: redness, swelling, sometimes blistering, then an area of hyperpigmentation. The delay means many people never connect the mark on their skin with the bottle they used two or three days earlier.

Not all furocoumarins are equally potent. The library's table ranks psoralen strongest, bergapten strong, oxypeucedanin and imperatorin moderate, while bergamottin, though the most abundant in bergamot, is rated negligible. Isopimpinellin and bergaptol are not phototoxic.

Bergapten and UV light

In citrus oils, bergapten is the most closely watched molecule. Its chemical name is 5-methoxypsoralen, formula C12H8O4, molecular weight about 216. The library's constituent page notes that it has no significant odour value: a substance that can be present in a bottle without the nose noticing at all.

Bergapten content varies markedly by source. According to the library, expressed lime contains about 0.17 to 0.33 percent, expressed bergamot 0.11 to 0.33 percent, bitter orange 0.035 to 0.073 percent, expressed lemon from nearly zero to 0.035 percent, and mandarin only traces. Bergamot FCF, the processed version, keeps between 0 and 0.0091 percent.

Timing matters too. The library notes that skin sensitivity to furocoumarins peaks about one to two hours after application, when the molecules have reached the dermis, then declines. When a limit has been exceeded, the guidance is to avoid sun and UV for about 12 to 18 hours. Heavy clothing blocks most UV; thin fabric gives only the equivalent of SPF 5 to 15; sunscreen reduces the risk but does not remove it.

Darker skin has a higher threshold but is not immune. According to the library data, about 15 percent bergamot is needed to cause a reaction on dark brown skin, against about 2.4 percent on light skin. The figure explains why safety limits sit far below the reaction threshold: they have to hold for every skin.

Which species, which method

Furocoumarins are heavy and non volatile, so they do not travel with steam in conventional distillation. From this simple physical property comes the most important rule of this piece: from the same fruit, the expressed oil can be phototoxic and the distilled oil is not. Expression bursts the oil sacs in the peel and collects everything inside, the non volatile part included; distillation carries only what can evaporate.

The library's expressed lime page gives a limit of 0.7 percent for leave-on products on sun exposed skin, along with eleven recorded cases of photodermatitis. The distilled lime page, same species Citrus x aurantifolia, reads not phototoxic, though some Mexican distilled samples have been reported with traces of citropten and bergapten. Distilled lime also smells different: sharper, without the freshness of the expressed oil.

Species decides as well. The library lists several citrus oils that are not phototoxic even when expressed, because their natural furocoumarin content is very low: mandarin, sweet orange, tangerine, yuzu, satsuma and lemon leaf. The word citrus on a label is therefore not enough. Two more lines need reading: the Latin name of the species, and the method of extraction.

One side note is worth recording: citrus oils oxidise quickly. The library's storage page notes that as a citrus oil ages, the chemistry around the furocoumarins changes and can raise the overall reaction to UV. An old expressed bottle is not safer than a fresh one.

Limits for leave-on products

Phototoxicity limits apply only to products that stay on the skin: body oils, lotions, perfumes. Rinse off products such as soap, shampoo and shower gel are recorded by the library as generally unrestricted, because the oil does not remain on skin long enough. This is why expressed bergamot can appear in a bar of soap without the calculation a body oil requires.

For leave-on products, the library's table of maximum levels, following IFRA guidance, reads: expressed bergamot 0.4 percent, expressed lime 0.7 percent, expressed bitter orange 1.25 percent, expressed lemon 2.0 percent, expressed grapefruit 4.0 percent. The figure is the percentage of essential oil in the finished product, not in the blend of oils.

At the molecular level, European rules based on the SCCNFP opinion cap bergapten at 15 ppm in leave-on cosmetics and 1 ppm in sunscreen and bronzing products. The library's bergapten page gives the equivalent IFRA level, 0.0015 percent total furocoumarins in the finished product. The two figures agree: 0.4 percent bergamot multiplied by the highest bergapten content of 0.33 percent gives about 13 ppm, just under the 15 ppm line.

When several phototoxic oils are used together, the risk is treated as additive in proportion. The library gives an example: if bergamot and cumin, each limited to 0.4 percent, are used in equal parts in the same product, each must drop to 0.2 percent. A formula with three expressed peels has to be calculated as one block, not as three separate sums.

FCF and distillation

The three letters FCF, furocoumarin free, on a bottle of bergamot are the trace of an extra processing step. According to the library's bergamot FCF page, the expressed oil is fractionally distilled or filtered by adsorption to remove bergapten and other furocoumarins. A very small amount can still remain at ppm level.

The dermal limit changes accordingly. The library records that bergamot FCF can be used up to 16.6 percent on skin while still meeting the SCCNFP bergapten threshold. The arithmetic shows where the figure comes from: 16.6 percent multiplied by the highest residual bergapten content of 0.0091 percent gives about 15 ppm. The limit does not disappear; it simply moves further out.

The cost of FCF is in the smell. The library describes the FCF version as lighter than traditional expressed bergamot, because part of its character is lost in processing. For lime, the equivalent choice is the distilled oil, which also loses the freshness of the expressed one. A formulator chooses between the full scent at a very low dose and a thinner scent at a higher one.

If a skin reaction appears after using a product containing citrus oils and then going into the sun, the library's safety page advises washing the area gently, avoiding sunlight, and seeking advice from a health professional if the reaction persists. The piece on cold pressing citrus peel follows the peel from oil sac to bottle.

The molecule that leaves the brown mark has no smell.

It follows the method of extraction, not the name of the fruit.

Two lines on a label, the Latin name and the method, say more than the word citrus.