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Some umbellifers, including Persian hogweed and giant hogweed, are phototoxic. Phototoxic reactions are caused by substances which damage the skin if they are exposed to natural or artificial sunlight when in contact with the skin.
In most cases, phototoxic reactions occur as a result of skin contact with phototoxic plants followed by exposure to ultraviolet rays from natural or artificial sunlight.
In order to cause a phototoxic reaction upon skin contact, most plant saps must remain in contact with the skin for 30-120 minutes. Umbellifers known to cause this type of skin reaction include Persian hogweed and giant hogweed (Heracleum sp.).
The main symptom is a skin rash.
The rash usually appears 1-3 days after exposure, and is limited to the areas that have been in contact with the plant and then exposed to sunlight.
The reaction has the appearance of severe sunburn, with reddening of the skin, blisters and severe stinging. The skin area may suffer prolonged hyperpigmentation (darkening of the skin) for up to several months after the phototoxic reaction.
The exposed area may also be hypersensitive to UV light for a long period of time (months to years) after the reaction.
You can find out more about selected umbellifers here: Identifying poisonous umbellifers in Norway.