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Generally, it is used to hold water and soften skin in a formula. Algae are rich in polysaccharides, pigments, and amino-acids that act as humectants, emollients, and skin-conditioning agents.

It isn't on standard cosmetic-patch test panels and "seaweed dermatitis" is actually an acute irritant reaction to algal toxins in seawater and not the cosmetic ingredient.

You'll see algae listed as a 5/5 on a lot of pore-clogging charts and that number doesn't come from anywhere; it's not in the original rabbit-ear studies that most comedogenic ratings trace back to.

The reasoning behind the reputation is that seaweed is high in iodine and iodine can trigger acne-like breakouts when eaten. This traces back to a 1976 letter in Archives of Dermatology describing two women whose breakouts cleared after they stopped taking high-dose kelp supplements orally.

And the reaction of these two women wasn't clogged pores at all: it was an iodine-triggered pustular rash. Later research found normal dietary iodine doesn't meaningfully affect acne either and nobody has shown topical algae delivers enough iodine to matter.

What we do have is dozens of brown and red algae ingredients found to be non-irritating and non-sensitizing, and a handful of studies that found algae compounds may actually help acne.

However, plenty of people do report breaking out from algae-containing products. Skin is individual and it's worth avoiding if yours has reacted to algae before.

Typical use levels are very low since the total fragrance load in a product is usually well under 1% and citronellol is only a small fraction of that.

The EU asks brands to name it on the label once it goes above 0.001% in leave-on products of 0.01% in rinse-off ones.

It is one of the EU's labeled fragrance allergens but real-world patch testing has shown it to be fairly weak and an infrequent allergen; positive reactions land around 0.1-0.9% of patients across recent European dermatitis clinic studies.

The bigger nuance is oxidation. Citronellol forms hydroperoxides when it reacts with air. These hydroperoxides are more sensitizing than the fresh ingredient, and patch test studies confirmed oxidized Citronellol causes more reactions than purified.

This means good packaging and freshness matter more than the molecule itself.

Fragrance can be irritating to very reactive or compromised skin, so people with sensitive skin may still prefer to skip it.

Geraniol can be found in many essential oils including Rose Oil and Citronella Oil. The scent of Geraniol is often described as "rose-like". Many foods also contain Geraniol for fruit flavoring.

Geraniol can irritate the skin when exposed to air. However, irritation depends on the ability of geraniol to penetrate into the skin. In general, geraniol is not able to penetrate skin easily.

Geraniol is colorless and has low water-solubility. However, it is soluble in common organic solvents.

Like citronellol, it is a natural insect repellent.

2,6-Octadien-1-ol, 3,7-dimethyl-, (2E)-