Rhubarb is a perennial plant grown for its edible stalks, but its large, fan-shaped leaves are toxic. While the leaves are poisonous if consumed, the risk of simply touching them is minimal for most individuals. The primary danger comes from accidental ingestion, which can lead to systemic toxicity. Since the skin acts as a robust barrier against the toxic compounds, gardeners and home cooks can generally handle the leaves safely, provided they avoid eating them.
The Source of Toxicity: Oxalic Acid
The toxicity of the rhubarb leaf is mainly attributed to its high concentration of oxalic acid and its salt forms, known as oxalates. These compounds are naturally present in many plants, including spinach and broccoli, but the leaves of the rhubarb plant contain them in much higher concentrations. Oxalates are corrosive and act as a nephrotoxin, meaning they specifically target the kidneys.
When ingested, oxalic acid binds to calcium ions within the body, forming insoluble calcium oxalate crystals. This process removes circulating calcium from the bloodstream, which can disrupt various bodily functions. The microscopic, needle-shaped calcium oxalate crystals can cause irritation in the mouth and throat, and their accumulation in the kidneys can lead to the formation of kidney stones and, in severe cases, acute kidney failure. The concentration of oxalates in rhubarb leaves is estimated to be around 0.5 to 1.0% of the leaf’s weight, which is significantly higher than the levels found in the safe-to-eat rhubarb stalks.
Differentiating Dermal Contact from Ingestion Risk
The central distinction in handling rhubarb leaves is the difference between contact and consumption. The skin is a highly effective barrier that prevents the toxic oxalates from being absorbed into the bloodstream in significant amounts. This means that touching the leaves, even when harvesting or trimming them, does not typically lead to systemic poisoning.
The risk from dermal contact is localized and usually mild, presenting as a minor skin irritation or contact dermatitis in sensitive individuals. This reaction is more likely to occur if the skin is broken or if a person is exposed to the leaf sap for an extended period. A gardener with sensitive skin might experience slight redness or itching after handling a large batch of leaves without gloves. The symptoms of ingestion, which include burning in the mouth and throat, severe gastrointestinal distress, and potential kidney damage, are far more serious.
The immediate and severe systemic effects of oxalate toxicity only occur when the compound is absorbed through the digestive tract. The high concentration of oxalic acid in the leaves requires a substantial quantity to be eaten to cause severe illness in an adult, though smaller amounts can cause nausea and vomiting. For small children or pets, the threshold for serious toxicity is much lower, which underscores why ingestion is the primary concern.
Immediate Care and Safety Precautions
For dermal exposure, the care required is simple and focused on cleaning the affected area. If you touch rhubarb leaves, especially if they are damaged and the sap is released, wash your skin thoroughly with soap and water. Monitor the area for signs of irritation, such as persistent redness or itching, which would indicate mild contact dermatitis.
If accidental ingestion occurs, immediate medical attention is necessary, as this is a medical emergency. Do not attempt to induce vomiting unless instructed to do so by a healthcare professional or poison control. Contact the national toll-free Poison Help hotline at 1-800-222-1222 for expert guidance. Gardeners should always remove and properly discard the leaves immediately after harvesting the stalks, keeping them away from children and pets who might ingest them.