Algin

Algin is the sodium salt of alginic acid, a sugar-based polymer pulled out of brown seaweed.

In cosmetics, it's a workhorse thickener, binder, and film former rather than an active; it's mostly there for texture and slip.

Industry survey data collected by the Cosmetic Ingredient Review put its maximum reported use at 50% in paste masks and mud packs with reported use going all the way down to 0.001% in aerosol products. Most formulas sit at the low end since a little goes a long way as a thickener.

Allergy-wise, this ingredient is about as low-risk as it gets topically:

• A 20% water solution of sodium alginate tested negative for both irritation and allergic contact dermatitis in human patch testing.
• 2% was non-irritating to skin and eyes in animal testing.
• The CIR panel specifically noted the absence of clinically relevant reactions to polysaccharide gums in dermatology practice.

Is it comedogenic?

The "algae causes acne" idea comes from three places that got tangled together:

1. Old case reports of people eating large amounts of kelp tablets and breaking out (that's dietary iodide, and dermatologists note it produces papules and pustules rather than true comedones)
2. The fact that "algae" covers thousands of organisms, some of which yield genuinely oily, lipid-rich extracts that behave like any other oil
3. Guilt by association with carrageenan's bad food-additive reputation.

None of that transfers to a purified polysaccharide like algin. Clogged pores are a lipid problem where an ingredient has to get into the follicle and mix with sebum to form a comedone. Algin is a big, water-loving sugar chain that does neither; it gels and then sits on the surface as a film until it's rinsed off.