Carrageenan

Carrageenan is a gel-forming sugar chain pulled out of red seaweed. In skincare, it's a texture ingredient rather than an active that:

• thickens water-based formulas
• stops emulsions from separating
• leaves a light film to help gels and masks feel slippery and cushiony

Formulators typically use it around 0.1-2% but go up to 5% in thick gel masks or toothpaste-style products.

It's a very large molecule that doesn't meaningfully get through intact skin and the CIR Expert Panel has concluded it's safe at current use levels.

True allergy is rare and there are only two published cases from internal/mucosal exposure rather than anything topical. Most of the scary internet content about Carrageenan is actually about poligeenan (also called "degraded carrageenan").

Poligeenan is a deliberately acid-broken down version that isn't food or cosmetic grade.