17. August 2021
Plant + light = rash

Many plants contain substances that, when activated by the UV light of the summer sun, can cause severe skin irritation. The best-known example is giant hogweed, but other acanthus species have a similar effect.

“Many people have already experienced these phototoxic reactions in their own gardens”, says Prof. Dr. Torsten Zuberbier, ECARF-chairman of the board. Typical symptoms include redness, itching, blistering and, in some cases, pain at the contact sites. The skin reactions can become very unpleasant, similar to a severe burn. However, they are not an allergy, nor are people with allergies more affected than others.


The tricky thing is that when you come into contact with these substances, you don’t notice anything at first. Phototoxic reactions only occur after a few hours, often with a delay of 24 to 48 hours. “It is particularly difficult in these cases to trace the reaction back to the plant that triggered it”, says Zuberbier.


Here’s what happens:

The triggers are usually furanocoumarins. They belong to the defence arsenal that protects plants against infection with bacteria or fungi. When furanocumarins are exposed to UVB or UVA radiation, they form active oxygen radicals. These in turn destroy the outer wall of the skin cells when they come into contact with them.

The skin produces an inflammatory reaction to the cell damage. Redness, blistering, swelling and itching occur – the symptoms of photodermatitis. Skin pigmentation can also increase.


What should you do?

Even when the sky is overcast, UV radiation penetrates through the clouds, and skin irritation can therefore occur. This is why phototoxic plants should be handed with care. “As a rule, everyone should wear gloves when gardening and avoid direct skin contact with these plants”, advises Zuberbier. If contact occurs, the skin should be cleansed immediately with soap and water. “Incidentally, this should also be done after a picnic in the garden”, cautions Zuberbier. Some types of fruit, including citrus fruits and in particular certain orange varieties, also contain phototoxic ingredients. You should therefore be sure to wash your hands after squeezing them.

If a skin reaction occurs, the first step should be to cool the area. If the pain is severe, you should consult your doctor or health care professional. Wound treatment and antibiotic treatment may be necessary in severe cases.


Which plants are phototoxic?

There are a number of plants that have a phototoxic effect. A subset of furanocoumarins, called psoralens, are mainly found in Umbellifers and lemon plants.

In vegetables and spices, the proportion of phototoxic substances is usually so low that they do not pose any risk during harvesting or consumption.


Plant namePhototoxic partFlowering/harvesting season
Bishop’s weedFruitsJune – September
Burning bush
(Dictamnus albus)
Leaves with bristles, stems, seed capsulesJune – July
Garden angelica
(Angelica archangelica)
Fresh plant juiceJune – August
(Conium maculatum)
Plant juiceJune – August
Queen Anne’s lace
(Ammi majus)
Seeds in particularJune – September
(Peucedanum ostruthium)
Plant juiceJune – August
Giant hogweed
(Heracleum mantegazzianum)
All parts of the plant,
especially plant juice
June – August
Common rue
(Ruta graveolens)
Leaf surfaceJune – July
(Heracleum sphondylium)
All parts of the plantJune – August
Herbs and vegetables
(Pimpinella anisum)
Plant juiceSeptember/October
(Anethum graveolens)
Plant juice
Fig (Ficus Carica)Milky plant juiceFrom June, main harvest September
(coriandrum sativum)
Plant juiceSpring – Fall
(apium graveolens)
Roots in particular
Only phototoxic if the roots are infected with the fungus Sclerotina sclerotiorum (the root has a pink-red colour)
From August onward
(Foeniculum vulgare)
Bulbs in particularFrom June
(Daucus carota subsp. sativus)
Roots and leaves (plant juice)From 7 weeks after sowing
(Levisticum officinale)
Plant juiceBefore flowering
(Pastinaca sativa)
All parts of the plantFrom September onward
(Petroselinum crispum)
Plant juiceSpring – Fall
Cow parsley
(Anthriscus sylvestris) 
All parts of the plantMay – July


Can photoallergic reactions also occur?

Yes, but they are quite rare. The symptoms are similar to those of a phototoxic reaction, but the immune system is involved. This is caused by an allergy to substances in the plants. Sunlight causes changes in the chemical architecture of plant substances. This produces what is called a photoantigen, which is an allergy-triggering substance. It causes a type IV hypersensitivity reaction.

These reactions are diagnosed by a dermatologist using a photo patch test.



Zuberbier Torsten. ECARF expert meeting in July 2021. Torsten Zuberbier is the head of the European Centre for Allergy Research Foundation (ECARF).


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