Citric acid in skincare: what it does and is it bad for your skin?

Citric acid shows up on so many labels because it is a handy formulation helper. In small amounts it lowers and steadies a product's pH and binds trace metals that can spoil a formula. At higher strengths it works as an alpha hydroxy acid (AHA) that exfoliates. At normal levels it is not bad for your skin.

Why is citric acid in so many cosmetics?

Citric acid is the acid that gives lemons and limes their sour taste. For cosmetics it is usually produced by fermentation, the same way food-grade citric acid is made. In food it is the additive E330, one of the most common entries in our guide to E-numbers and food additives.

The US Cosmetic Ingredient Review (CIR) Expert Panel, which assesses cosmetic ingredient safety, reports that citric acid functions in cosmetics as a pH adjuster, chelating agent or fragrance ingredient. Those jobs explain why you see it in cleansers, shampoos, moisturizers, serums and even makeup.

• pH adjuster: many ingredients and preservatives work best, or stay stable, within a certain pH range. A small amount of citric acid brings the formula down to that range, often to a level close to the skin's naturally slightly acidic surface.
• Buffer: paired with its salt, sodium citrate, it helps keep the pH from drifting while the product sits on the shelf.
• Chelating agent: it binds traces of metals, such as iron or calcium from water or raw materials. Left free, those metals can speed up spoilage, discoloration and rancid smells.
• Stabilizer for other ingredients: by binding metals that drive oxidation, it helps protect oils and antioxidants in the formula. A review of hydroxy acids notes that citric acid is widely used in topical products as an antioxidant.

In all of these roles, citric acid isn't there to change your skin. It's there to keep the product working as intended until the last drop.

Citric acid as an exfoliant (AHA)

At higher concentrations and a low pH, citric acid behaves like other exfoliating acids. The FDA lists it among the alpha hydroxy acids, next to the better-known glycolic and lactic acid. Chemists sometimes also group it with beta hydroxy acids because of its structure, but don't confuse it with salicylic acid, the BHA that works inside pores. In skincare terms, citric acid acts as a water-soluble surface exfoliant, like the AHAs in our AHA, BHA and PHA guide.

You'll mostly find it as an exfoliant in "fruit acid" blends, peel pads and some toners, often combined with glycolic, lactic or malic acid. It is rarely the star of a product on its own.

How strong is the evidence?

Weaker than for glycolic and lactic acid. Most AHA research has focused on those two, and the US safety panel's concentration and pH limits were set for them. For citric acid there are only a few studies. In one small study from 1997, a 20% citric acid lotion used for three months increased the thickness of the living outer skin layer and the moisture-binding molecules in sun-damaged skin, changes similar to those seen with glycolic acid. That is promising, but it is far from the body of evidence behind glycolic acid, retinoids or vitamin C.

So if smoother skin is your goal, citric acid in a blend is fine, but there is no reason to seek it out over a well-studied AHA.

Is citric acid bad in skincare?

No, not at the levels used in normal products. The CIR Expert Panel concluded that citric acid, its inorganic salts (such as sodium and potassium citrate) and its alkyl esters (such as triethyl citrate) are safe in the present practices of use and concentration. Several of them are also generally recognized as safe as direct food additives.

When citric acid is only a pH adjuster, there is so little of it that it is very unlikely to bother your skin. Irritation is mostly a question for exfoliating products, where the acid is present in larger amounts at a low pH. Possible reactions include:

• Stinging or burning, especially on freshly shaved, sunburned or broken skin
• Redness, dryness or peeling if you use it too often
• Increased irritation when combined with other acids or retinoids in the same routine

If you have easily irritated skin, rosacea or eczema, introduce any exfoliating acid slowly or skip it. Our sensitive skin guide explains how to test new products, and skincare ingredients not to mix covers how to space out acids and retinoids.

Citric acid and sun sensitivity

This applies to citric acid used as an exfoliant, not to the trace amounts in a moisturizer. The FDA advises that AHA products can make skin more sensitive to the sun. In a study the FDA cites, four weeks of AHA use made skin more prone to UV reddening, and the effect faded within a week of stopping. The FDA recommends a "Sunburn Alert" on AHA products: use sunscreen, wear protective clothing and limit sun exposure while using the product and for a week afterwards.

In practice, use exfoliating acids at night and wear a broad-spectrum sunscreen every morning. Our sunscreen guide explains how much to apply and when to reapply.

Why you shouldn't put lemon juice on your skin

If citric acid can exfoliate, rubbing lemon on your face might seem like a natural shortcut. It isn't. A cosmetic is formulated to a set strength and pH; fresh citrus juice is strongly acidic, varies from fruit to fruit and contains many other compounds besides citric acid.

Citrus juice can also cause phytophotodermatitis. Some plants, including lime, contain chemicals called furanocoumarins. When they get on your skin and you then go into the sun, UV light triggers a reaction. DermNet describes red, often blistering patches that appear about 24 to 48 hours after sun exposure, sometimes in drip or streak shapes, followed by brown marks that can last a long time, sometimes for years. Lime juice is a classic cause, which is why the condition is common in bartenders.

How to tell from the label what citric acid is doing

Ingredient lists don't give percentages, but the position of citric acid gives a strong hint. As our guide to reading cosmetic ingredient labels explains, ingredients above 1% appear in descending order, and those at 1% or less can follow in any order.

The product's purpose matters as much as the position. A cream or shampoo that isn't sold as an exfoliant almost always uses citric acid only as a helper. If the packaging carries a sunburn warning or talks about AHAs or exfoliation, treat it as an active and follow the sun advice above.

What about sodium citrate?

Sodium citrate is the sodium salt of citric acid. It is not an exfoliant. Formulators use it to buffer pH and, like citric acid, to bind trace metals. It's common in cleansers, shampoos and micellar water, and the CIR panel includes it in the same safe-as-used conclusion. You may also see potassium citrate, another citrate salt covered by the same review.