Seaweed, a term encompassing thousands of species of marine macroalgae, is common in the ocean and a growing part of the global diet. While often celebrated for its nutritional profile, contact with or consumption of certain types can lead to adverse skin reactions. These incidents are generally uncommon but manifest as a rash through two distinct pathways: direct external contact with toxic species or systemic internal reactions following ingestion.
Skin Reactions from Direct Contact
Rashes from direct physical contact are typically a form of irritant contact dermatitis, not a true allergic response. The most documented cause is exposure to the cyanobacterium Lyngbya majuscula, often called stinging seaweed. This fine, hair-like organism grows worldwide in tropical and subtropical waters, producing powerful dermatoxins.
The rash, commonly called seaweed dermatitis, occurs when fragments of Lyngbya become trapped against the skin, most frequently under swimwear. Toxins such as lyngbyatoxin A and debromoaplysiatoxin are then rubbed into the skin by the friction of clothing or a wetsuit. Symptoms begin minutes to hours after leaving the water, presenting as a red, burning, and intensely itchy rash that often mirrors the pattern of the bathing suit.
The skin lesions can range from simple redness to the formation of small blisters and pustules. The rash is localized to the exposed areas, differentiating it from systemic reactions that affect the whole body. While other marine organisms trapped in washed-up masses like Sargassum may contribute to irritation, the primary cause is the specific toxin released by Lyngbya.
Allergic Responses Following Ingestion
Skin reactions following the consumption of seaweed, such as nori, kelp, or wakame, are systemic. They are categorized as true food allergies or iodine-induced eruptions.
A genuine seaweed food allergy is a Type I hypersensitivity reaction, where the immune system mistakenly identifies a seaweed protein as a threat. This involves the production of Immunoglobulin E (IgE) antibodies, leading to the release of histamine and other inflammatory mediators. Symptoms include hives, angioedema (swelling of the lips, face, or throat), and in severe cases, gastrointestinal or respiratory distress.
Specific proteins like chitinase, found in some red seaweeds used to make nori, are potential allergens. A more common dermatological issue is iododerma, a non-allergic skin eruption caused by an excess of iodine in the body.
Seaweed, particularly brown varieties like kelp, is an extremely rich source of iodine. When sensitive individuals or those with impaired kidney function consume large amounts, the body struggles to excrete the massive iodine load. This accumulation triggers an inflammatory response in the skin, often presenting as an acne-like rash with pustules or nodules.
Management and Prevention of Seaweed-Related Rashes
If a rash develops after contact with ocean water, the immediate priority is to decontaminate the skin and all exposed items. Thoroughly washing the affected area with soap and fresh water is the first step to remove any lingering toxin fragments. Rinsing the skin with isopropyl alcohol may also help neutralize the toxic compounds before applying a soothing agent.
Mild irritant rashes can often be managed with over-the-counter topical treatments, such as a one percent hydrocortisone cream to reduce inflammation and itching. Calamine lotion or cool compresses can provide symptomatic relief from the burning sensation.
If the rash is severe, covers a large area, or shows signs of infection like increased warmth, pus, or persistent pain, medical attention is warranted. Systemic symptoms following ingestion, such as difficulty breathing, throat tightness, or widespread hives, require immediate emergency care.
To avoid contact dermatitis, swimmers should heed warnings about algal blooms and shower immediately after leaving the water, thoroughly washing all swimwear and gear. Individuals with thyroid conditions or a history of iodine sensitivity should monitor their intake of high-iodine species like kelp. Checking commercially available seaweed products for heavy metal contamination is also a sensible precaution.