The SPF number on a bottle is produced at an application density of 2 milligrams per square centimeter, the rate the FDA writes into its sunscreen labeling rule at 21 CFR 201.327, paragraph (i)(4)(iii), and the FDA's consumer page turns that rate into a household quantity: at least one ounce for an average-sized adult or child, about the amount it takes to fill a shot glass. Spread a third of that over the same skin and the number on the front stops describing what you are wearing. The condition that changes everything after application is water: the rule allows exactly two water-resistance claims, 40 minutes and 80 minutes, and then requires the label to tell you to reapply.
Three phrases on the front come from three separate tests
Each claim printed on a sunscreen is generated by its own procedure inside 21 CFR 201.327, and passing one of them says nothing about the others.
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| Phrase on the label | Where the rule defines it | The figure it carries |
|---|---|---|
| Broad Spectrum SPF | SPF test in paragraph (i); broad spectrum test in paragraph (j) | Applied at 2 mg/cm²; critical wavelength of 370 nm or greater |
| Water Resistant (40 minutes) or (80 minutes) | Immersion procedure in paragraph (i)(7) | 40 or 80 minutes of water immersion, and nothing beyond it |
| Skin Cancer/Skin Aging Alert | Warning in paragraph (d)(2) | Required below SPF 15 and on any product that fails the broad spectrum test |
The SPF procedure in paragraph (i) of the rule applies both the test product and a reference standard at 2 milligrams per square centimeter and waits at least 15 minutes before the test sites see ultraviolet light. A product that also passes the broad spectrum test prints "Broad Spectrum SPF" with its number; one that fails prints "SPF" and the number alone, which is the difference between two bottles that look identical on a shelf.
Broad spectrum means a critical wavelength of 370 nanometers
The broad spectrum test is arithmetic on an absorbance curve. Under paragraph (j)(7) of the rule, the critical wavelength is the point at which the integral of the spectral absorbance curve reaches 90 percent of the integral over the ultraviolet spectrum from 290 to 400 nanometers, and a mean critical wavelength of 370 nanometers or greater is classified as broad spectrum protection. That single threshold is the whole of the UVA claim in the United States; there is no graded UVA rating on the American label.
What the label is allowed to say about disease follows from the same two tests. Paragraph (c)(2) lets a broad spectrum product of SPF 15 or higher add that, used as directed with other sun protection measures, it decreases the risk of skin cancer and early skin aging caused by the sun. Paragraph (c)(3) of that same section makes any labeling implying that sunscreen alone prevents skin cancer a misbranding violation, and paragraph (g) names three claims as false or misleading outright: "Sunblock," "sweatproof" and "waterproof." A rating produced under one laboratory condition rarely survives contact with weather, which is the same gap that sits between a rain jacket's bench figure and an afternoon of rain, or between a sleeping bag's tested limit and a cold night.
Water resistance ends at 40 or 80 minutes, and the directions say so
The immersion procedure in paragraph (i)(7) is run in an indoor fresh water pool, whirlpool or hot tub held between 73 and 90 °F (23 to 32 °C), and the labeled SPF is measured after the stated period of immersion. The directions that follow are fixed text: reapply after 40 or 80 minutes of swimming or sweating, immediately after towel drying, and at least every 2 hours. A product with no water-resistance claim carries the shorter version, reapply at least every 2 hours and use a water resistant sunscreen if swimming or sweating. A phrase on a package is only as strong as the body that owns the test behind it, which is why the American Dental Association's seal plays the same role for an electric toothbrush.
Two figures that decide what a bottle is still good for sit outside the rule. The FDA's consumer page states that a sunscreen without an expiration date is expired three years after purchase, and the same page answers the marketing question directly: there is no such thing as waterproof sunscreen.
Example: a 3.0 fluid ounce tube against its own directions
As an example, the label Kenvue Brands files for Neutrogena Ultra Sheer Dry-Touch Sunscreen Broad Spectrum SPF 55 lists avobenzone 3 percent, homosalate 10 percent, octisalate 5 percent and octocrylene 10 percent, which is 28 percent active ingredients, with "Water resistant (80 minutes)" on the principal display panel and a tube size of 3.0 fl oz (88 mL).
Run the manufacturer's tube against the FDA's own application figure. One ounce per full-body application divides a 3.0 fluid ounce tube into three applications. The directions on that same label call for reapplication at least every 2 hours, so six hours outdoors is three applications, at the start, at two hours and at four hours. Three applications empty the tube. A family of four working from one 3.0 fluid ounce tube on a beach day is applying a quarter of the tested amount each, and the SPF 55 on the front was never measured at that rate.
Before you buy a sunscreen
- The front panel: "Broad Spectrum SPF" with a number, not "SPF" alone, since only the first passed the 370 nm critical wavelength test.
- The water resistance figure in minutes, 40 or 80, and whether your activity runs longer than it.
- The Drug Facts panel's active ingredients with their percentages, which is the only place the formula is stated.
- The net contents in fluid ounces, divided by one ounce per application, to see how many full applications the container holds.
- The expiration date on the tube, or the purchase date if there is none, against the FDA's three-year figure.
- The warning panel: a Skin Cancer/Skin Aging Alert means the product is below SPF 15 or did not pass the broad spectrum test.
- The directions line for reapplication, which names 40 minutes, 80 minutes or every 2 hours for that specific product.