Phototoxicity is a skin reaction that occurs when a light-sensitive substance is present on the skin and is then exposed to UV (mainly UVA) radiation. The immune system is not involved, and the reaction can occur on first exposure. Symptoms include redness, pain, blistering and post-inflammatory hyperpigmentation that can persist for weeks to months.
The principal offenders in essential oils are furocoumarins (FCs). These molecules intercalate between DNA bases, and on activation by UVA they form covalent bonds with pyrimidines, causing DNA damage and severe skin inflammation.
FCs are heavy, non-volatile molecules in pure form. They are present in cold-pressed (expressed) citrus peel oils and some Apiaceae root oils, but largely absent from the same plants when processed by distillation.
Five-step diagram: from furocoumarins on the skin, through UVA activation, to DNA damage and berloque dermatitis. The final two pathways run in parallel — DNA adducts pose a long-term cancer risk; hyperpigmentation produces lasting skin darkening.
FCs are flat, polycyclic molecules. The basic structure is a furan ring fused to a coumarin ring system. When the furan ring is at the coumarin 6,7-bond, the result is a linear FC (psoralen). When at the 7,8-bond, the result is an angular FC (angelicin).
Both structures can intercalate between DNA base pairs to form an unstable complex. When UVA supplies activation energy, the FC forms a covalent bond with one pyrimidine base, then with a second base on the opposing DNA strand. This biadduct cross-links the two strands of DNA.
Psoralens form both monoadducts and biadducts. Angelicins form monoadducts only. For this reason psoralens are considered more strongly phototoxic, though angelicins can still produce moderate phototoxicity.
Clinical consequences are delayed, peaking 36-72 hours after UV exposure. The acute reaction includes erythema, swelling and blistering. Post-inflammatory hyperpigmentation follows and can persist for weeks to months. This pattern was first described in 1916 and is called berloque dermatitis.
The name comes from the French "breloque" meaning trinket or pendant, because the hyperpigmented marks often appear as droplet shapes on the neck and chest, tracing where perfume was dabbed.
Early clinical reports involved women who had applied perfumes containing Bergamot oil and then exposed their necks to sunlight. Within hours to days, dark patches appeared at the application sites.
Severe cases include second-degree burns after applying undiluted Bergamot oil and entering a UV tanning bed. Blistering burns developed over 48 hours and required hospitalization.
IFRA recommends maximum dermal use levels for each phototoxic oil. Above these concentrations, treated skin should not be exposed to UV light for 12-18 hours after application. These limits apply to leave-on products. Rinse-off products (soap, shampoo, bath preparations) are generally exempt.
| Essential oil | Maximum dermal level | Key FC |
|---|---|---|
| Fig leaf absolute | Do not use on skin | FC mixture |
| Verbena (lemon) oil | Do not use on skin | Mixed |
| Taget oil | 0.01 % | α-Terthienyl (thiophene) |
| Rue oil | 0.15 % | Bergapten, Methoxsalen |
| Mandarin leaf oil | 0.17 % | Dimethyl anthranilate |
| Bergamot oil | 0.4 % | Bergapten |
| Cumin oil | 0.4 % | Bergapten + FCs |
| Lime (expressed) | 0.7 % | Bergapten + Oxypeucedanin |
| Angelica root oil | 0.8 % | Bergapten + Imperatorin |
| Orange (bitter, expressed) | 1.25 % | Bergapten |
| Lemon (expressed) | 2.0 % | Bergapten + Oxypeucedanin |
| Laurel leaf absolute | 2.0 % | Mixed |
| Grapefruit (expressed) | 4.0 % | Bergapten + Bergamottin |
When multiple phototoxic oils are combined, risk is treated as additive in proportion. If both Bergamot and Cumin (each limited to 0.4 %) are used in the same product in equal parts, each must drop to 0.2 %.
FCs are heavy and non-volatile, so they do not carry over with steam in conventional distillation. The consequences:
Lemon expressed contains Bergapten and is phototoxic. Lemon distilled has no Bergapten and is not phototoxic.
Lime expressed is phototoxic. Lime distilled is not phototoxic.
Bergamot cold-pressed is phototoxic. Bergamot FCF (furanocoumarin-free), produced by fractional distillation to remove Bergapten, is not significantly phototoxic.
Some citrus oils are not phototoxic even when expressed: Mandarin, sweet orange, tangelo, tangerine, yuzu, satsuma, lemon leaf. The reason is naturally low FC content.
Not all FCs are equally phototoxic. Some FCs are not phototoxic at all.
| Constituent | Type | Phototoxic potency |
|---|---|---|
| Psoralen | Linear (psoralen) | Strongest |
| Methoxsalen (5-methoxypsoralen) | Linear | Strong |
| Bergapten (5-methoxypsoralen) | Linear | Strong |
| Angelicin (isopsoralen) | Angular (angelicin) | Moderate |
| Imperatorin | Linear | Moderate |
| Citropten | Lactone (coumarin) | Moderate |
| Oxypeucedanin | Linear | Moderate |
| Bergamottin | Linear | Negligible |
| Isopimpinellin | Linear | Not phototoxic |
| Bergaptol | Linear | Not phototoxic |
| 5-Methoxy-7-geranoxycoumarin | Lactone | Unknown |
Skin sensitivity to FCs peaks around 1-2 hours after application, the time FCs reach the dermis. It declines thereafter as the FCs diffuse deeper and clear.
At concentrations within IFRA limits, no phototoxic reaction occurs even with UV exposure within a few hours. Above the limits, skin should avoid UV for 12-18 hours.
With undiluted essential oils, phototoxic responses can persist for 18 hours or longer. At 0.5 % Methoxsalen, reactions continue to 36 hours and resolve by 48 hours.
Heavy clothing blocks most UV. Lightweight fabrics give only SPF 5-15. Sunscreen reduces but does not eliminate the risk.
FCs are also present in several foods: parsnip, celery, grapefruit juice and angelica root. High dietary intake can raise total circulating FC load.
PUVA therapy (Psoralen + UVA) uses high oral doses of Methoxsalen with UVA to treat skin conditions such as vitiligo and psoriasis. It is controlled medical use and is not equivalent to essential oil exposure.
With phototoxic oils, oral ingestion substantially raises systemic FC and therefore the risk of cutaneous phototoxicity on subsequent UV exposure. Oral use of expressed citrus oils is not recommended where subsequent sun exposure is likely.
If a product contains phototoxic oils above IFRA limits or undiluted oil is used, avoid UV exposure for 12-18 hours.
For citrus character in leave-on daytime products, choose FCF or distilled variants: Bergamot FCF, Lemon distilled, Lime distilled, or Mandarin and sweet orange.
Darker skin tolerates FCs better than light skin but is not immune. About 15 % Bergamot is required to elicit a reaction on dark brown or black skin, compared to 2.4 % on light skin.
Rinse-off products such as soap, shampoo and bath preparations are not a phototoxic risk because the oils do not remain on skin. However, sauna use with vaporized oils can deposit FCs on skin.