A hogweed rash, technically known as phytophotodermatitis, is a severe skin reaction caused by contact with the plant’s sap followed by exposure to ultraviolet (UV) light. The clear, watery sap contains phototoxic chemicals called furocoumarins. When activated by UV radiation, these chemicals cause a chemical burn on the skin, damaging cells and preventing the skin from protecting itself from the sun. The resulting injury is comparable to a second-degree burn, characterized by painful inflammation and blistering. The duration of a hogweed rash involves an initial acute phase of blistering and a much longer phase of residual skin discoloration.
Essential Immediate Care
The most important step after suspected contact with hogweed sap is to prevent the phototoxic reaction from starting or worsening. Immediately wash the affected area thoroughly with mild soap and cool water to remove the furanocoumarins before they can be activated. Ensure you remove the sap without rubbing it into the skin, which can spread the chemicals.
After washing, the exposed skin must be completely shielded from sunlight for at least 48 hours, as the chemicals remain active. Protection should include wearing long sleeves and pants, or covering the area with a bandage or dark cloth, even indoors. If sap has entered the eyes, rinse them with clear water for 10 to 15 minutes, and seek medical attention immediately due to the risk of vision damage.
Medical attention is required if blistering is extensive, the reaction is severe, or if a child has been exposed. Seek professional care if the lesions contain pus, cover a large area, or if you experience severe pain. Topical steroid creams may be prescribed to reduce inflammation, while severe burns might require specialized care.
The Acute Phase Timeline
The active, painful phase of the hogweed rash follows a predictable timeline, with symptoms developing after a delay following sun exposure. Initial irritation and redness can begin within 15 minutes of sap contact, but the full inflammatory response takes longer to manifest. The first clear symptoms, such as burning pain and redness, usually appear 24 hours after the skin is exposed to both the sap and UV light.
Blistering and swelling intensify, reaching peak severity between 48 and 72 hours after the initial exposure. These blisters are painful and can be large, resembling a second-degree burn. The severity of this acute phase relates directly to the amount of sap and the duration of subsequent sun exposure.
If kept clean and protected, the fluid-filled blisters will begin to flatten and resolve within one to three weeks. The pain and active inflammation will subside as the body repairs the chemical burn. However, the resolution of the blisters marks the end of the acute injury, not the complete healing of the skin.
Duration of Residual Skin Changes
Once the acute blistering has healed, the most persistent symptom is post-inflammatory hyperpigmentation. This condition appears as a darkening of the affected skin area, often turning purple, dark brown, or black. This discoloration is caused by the inflammatory injury triggering skin cells to produce an excess of melanin.
The duration of this hyperpigmentation is highly variable and lasts significantly longer than the initial rash. While mild cases may fade within several weeks, most marks persist for several months. In severe cases, especially without strict sun avoidance, the discoloration can last for a year or more.
The affected skin remains highly sensitive to UV light for a prolonged period, meaning sun exposure can easily deepen the hyperpigmentation. It is recommended to use a sunscreen with an SPF of 30 or higher and to keep the area covered for at least six months following the injury. If the initial blistering was deep or severe, the damage may result in permanent scarring.