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It acts as a skin-conditioning agent and the research behind it at this time is mostly lab-dish rather than human trials.

Isolated Chlorella peptides inhibit collagenase and elastase (the enzymes that chew up collagen and elastin) and mop up free radicals in test-tube assays. Another study found a Chlorella gel sped up wound healing in mice. Many of the firming or dark-circle claims come from the ingredient supplier's own studies.

Suppliers suggest using 1-2% of the trade version (Water/Glycerin + Chlorella Vulgaris Extract) in a finished formula which means the actual algae solids are a fraction of a percent.

Allergy risk is low and topical reactions are rare; the 2020 anaphylaxis case came from eating Chlorella supplements and not from putting it on skin.

However, patch testing is sensible if you have reacted to algae before.

The "chlorella is high in iodine so it breaks you out" line borrows from marine seaweed. Freshwater Chlorella isn't a notable iodine source unless it's deliberately grown in iodine-enriched medium.

Just so you know, many lists give algae a comedogenic rating of 5 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.

However, this exfoliating effect only happens at high concentrations (20%) which can be hard to find in cosmetic products.

Due to this, citric acid is usually included in small amounts as a pH adjuster. This helps keep products slightly more acidic and compatible with skin's natural pH.

In skincare formulas, citric acid can:

While it can provide some skin benefits, research shows lactic acid and glycolic acid are generally more effective and less irritating exfoliants.

Most citric acid used in skincare today is made by fermenting sugars (usually from molasses). This synthetic version is identical to the natural citrus form but easier to stabilize and use in formulations.

Rosacea skin has a lower threshold for acids generally, with 62.5% of rosacea subjects in one study reacting to lactic acid in a sting test. This is why we mark citric acid as a potential rosacea trigger.

However, this ingredient shows up as a pH adjuster or chelator at roughly 0.1-1% in many products. These low levels are unlikely to be an issue for rosacea skin and becomes a potential trigger at exfoliating concentrations (20%). Because everybody's skin is unique, we recommend a patch test if you are unsure.