Skincare Numbers: The Figures With Sources, and What Each One Is Limited To

Every skincare number on this page is quoted from the source that published it, with a link to that source and to the page here that works through what it means. The condition attached to each figure is part of the figure: a concentration ceiling for one product category is not a ceiling for the ingredient, and a timeline measured for one drug does not transfer to a different one.

That is the whole reason to keep them together with their conditions rather than as a list of round numbers.

Permitted concentrations

0.5 to 2 percent, salicylic acid in an over-the-counter acne product. The monograph sets it out as Salicylic acid, 0.5 to 2 percent. This is the range permitted for products sold as OTC acne drugs, not a general limit on the ingredient. See salicylic acid.

2.5 to 10 percent, benzoyl peroxide in the same monograph: Benzoyl peroxide, 2.5 to 10 percent. The AAD's starting advice sits at the bottom of that range: While you'll find products that contain up to 10% benzoyl peroxide, it's best to start with a product that contains 2.5%. See benzoyl peroxide.

3 to 10 percent, sulfur, same list: Sulfur, 3 to 10 percent. Different molecules, different permitted ranges, all fixed in the same regulation.

10 percent and pH 3.5, glycolic and lactic acid in consumer products, with conditions. The Cosmetic Ingredient Review concluded they are safe for use in cosmetic products at concentrations ≤10%, at final formulation pH ≥3.5, when formulated to avoid increasing sun sensitivity or when directions for use include the daily use of sun protection. The concentration is one of three conditions rather than the conclusion on its own. See glycolic acid percentage.

30 percent and pH 3.0, the salon tier of that same conclusion: safe for use in salon products at concentrations ≤30%, at final formulation pH ≥3.0, for brief discontinuous use, rinsed thoroughly, applied by trained professionals. The higher number came attached to a person doing the applying.

8 percent and 20 percent, vitamin C. A review of topical vitamin C puts the lower bound at In most cases, for a product to be of biological significance, it needs to have a vitamin C concentration higher than eight percent and the upper at Studies have shown that a concentration above 20 percent does not increase its biological significance and, conversely, might cause some irritation. Both apply to L-ascorbic acid, the form the measurements were made on. See vitamin C forms compared.

15 and 20 percent, azelaic acid, by prescription and by indication. DermNet records it as available at a concentration of 20% in cream form, and that A 15% azelaic acid gel preparation was also approved for use in the US for rosacea. Cosmetic products carrying azelaic acid sit below both. See azelaic acid.

Up to and including 6 percent, bemotrizinol in an OTC sunscreen. The order's footnote says the FDA interprets a concentration up to 6 percent to mean up to and including 6 percent. The other conditions on that ceiling are in bemotrizinol sunscreen.

How long things take

At least 2 to 3 months before acne treatment can be judged. StatPearls states that at least 2-3 months of treatment compliance are needed to assess the efficacy of the treatment. This is the interval before the question "is this working" can be answered at all. See closed comedones and how long skincare takes to work.

Three to six months for photoaging, on the prescription molecule. A systematic review reports that Aside from the obvious difference in retinol strengths, trial evidence suggests that topical tretinoin treatment improves the clinical appearance of (photo-)aged skin only after three to six months of treatment. It is the benchmark short cosmetic trials should be read against. See the retinoid strength ladder.

Up to 12 weeks for azelaic acid in rosacea specifically: People being treated for rosacea might need to use this medication for up to 12 weeks before their skin starts to get better. The indication is part of the figure.

Two to three weeks for chapped lips, following dermatologist guidance. The AAD states that you should have noticeable improvement in 2 to 3 weeks, and that failing that, the next step is a board-certified dermatologist. See how to choose a lipstick.

Four to six weeks after stopping a pore-clogging product. The AAD: After you stop using acne-causing products, it can take 4 to 6 weeks for the acne to clear. Removal is slower than most people wait. See products for oily skin.

A few weeks to build retinoid frequency, which is a tolerance clock rather than an efficacy one: as tolerance improves, the frequency of application can be increased gradually over a few weeks until daily use is achieved.

What the label is required to say

The 1 percent line. Above it, order is meaningful: Ingredients, other than color additives, present at a concentration greater than 1 percent, in descending order of predominance. Below it, order carries no information at all. See how to read a skincare ingredient list.

SPF 15 as the floor in the FDA's own instruction: Use broad spectrum sunscreens with sun protection factor (SPF) values of 15 or higher regularly and as directed.

40 or 80 minutes, the only two water-resistance claims a label may carry. The regulation permits Water Resistant (40 minutes) or the 80-minute version, and nothing else: waterproof and sweatproof are not permitted claims. See physical versus chemical sunscreen.

Hygiene and product life

Every 7 to 10 days for brushes. The AAD says it is a good idea to wash your brushes every 7 to 10 days.

After every use for sponges, which is a different rule rather than a stricter version of the same one: Sponges should be cleaned after every use. See how often to clean makeup brushes.

79 to 90 percent of used cosmetics carried bacteria in a 2019 study of donated products: About 79-90% of all used products were contaminated with bacteria. The figure describes products people had been using, not products as sold.

93 percent of beauty blenders in that study had never been cleaned: Ninety-three per cent of beauty blenders had not been cleaned and 64% had been dropped on the floor and continued to be used. Behavior, again, rather than a property of the sponge.

Three months for mascara, on a stated reason: A study published in a medical journal reported that staph aureus bacteria are found in mascara after three months of use.

Every 10 to 12 weeks for benzoyl peroxide products: Replace products that contain benzoyl peroxide every 10-12 weeks, with a storage condition attached: Throw out products exposed to higher temperatures (e.g., greater than 78°F).

Numbers about skin itself

pH 5.4 to 5.9 at the surface, with a range of 4 to 6. A review of the acid mantle reports that The pH of the viable epidermis is near neutral, but the skin surface pH is acidic, with a value of 5.4–5.9 (range 4–6) in healthy adult white males and slightly higher in age-matched females. The same review calls the sex difference small and its implications unknown.

Hours, not minutes, for skin pH to return after washing: it is recommended that skin surface pH measurements be made 2–3 hours after washing with tap water, 5 hours after washing with synthetic detergents, and 10 hours after washing with alkaline soaps. See skin pH.

10 to 30 percent water in the stratum corneum: To maintain skin health, the moisture level in the stratum corneum should ideally range from 10% to a maximum of 30%. See dehydrated skin versus dry skin.

Around half of women and 40 percent of men report sensitive skin: approximately 50% of women and 40% of men may report having sensitive skin. Self-report, which is what makes the category hard to pin down. See how to know your skin type.

What this page does not include, and why

Prices. They change, and a reference page is the worst place to freeze one. Costs live on the pages that can be kept current, including what a dermatologist visit costs and teledermatology platforms compared, where the figure is dated and attributed to the provider publishing it.

Numbers without a checkable source. Market-size statistics, industry survey findings and "9 out of 10 users agreed" figures are excluded, because the thing that makes a number worth keeping is that a reader can open the source and see it. What that kind of claim is actually measuring is set out in reading clinical claims.

Disease prevalence. This site covers care and product choice rather than disease, so epidemiological figures belong with the medical references that publish them.

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The pattern across all of these is the same, and it is why the conditions travel with the figures here rather than being trimmed off. Two percent is a ceiling for one product category. Three to six months belongs to one molecule. Ten percent comes with a pH and a sun protection instruction. Strip the condition and the number becomes portable in a way the evidence behind it never was.