Algae are very simple, chlorophyll-containing organisms in a family that includes more than 20,000 different known species. In cosmetics, algae act as thickening agents, water-binding agents, and antioxidants. Some algae are also potential skin sensitizers. For example, the phycocyanin present in blue-green algae has been suspected of sensitization on the basis of patch tests.
Other forms of algae, such as Irish moss and carrageenan, contain proteins, vitamin A, sugar, starch, vitamin B1, iron, sodium, phosphorus, magnesium, copper, and calcium. Most of these are beneficial for skin, as emollients, soothing agents, or antioxidants. However, claims that algae can stop or eliminate signs of aging, or provide other elaborate benefits are unsubstantiated.