Short answer: SPF stands for sun protection factor, and it is measured under standardized test conditions. SPF primarily measures protection against sunburn, which is driven mainly by UVB. The SPF number alone does not tell you whether a sunscreen provides adequate UVA protection; in the United States, look for the words Broad Spectrum for that additional information. SPF is not a measure of time: SPF 30 does not mean 30 minutes, or 30 times longer in the sun. The number also assumes a generous, even layer. Whatever the SPF, apply as the label directs and reapply as the label directs.
It is often the largest number on a sunscreen label, and easy to misread.
How the number is made
The SPF test is run on volunteers. Small areas of skin receive measured doses of light from a lamp that imitates sunlight, some with sunscreen applied at a set, generous amount and some without. The next day, testers compare the doses that produced a faint redness. If protected skin takes 30 times the dose to redden that unprotected skin takes, the product is SPF 30.
What SPF tells you
It tells you about sunburn protection, and sunburn is mostly a UVB effect, so SPF is primarily a measure of UVB protection. A higher number means more sunburn protection when the product is applied as tested. Dermatology guidance generally recommends SPF 30 or higher for everyday use.
What SPF does not tell you
How well it covers UVA. UVA is the longer-wave part of UV. It plays a smaller part in sunburn but is linked to visible skin aging and skin cancer risk, and the SPF number alone does not tell you whether UVA protection is adequate. In the United States, the words Broad Spectrum mean a product has passed FDA’s broad spectrum test, which requires its UVA protection to rise in proportion to its UVB protection. UVA vs UVB: What Each Does to Your Skin explains the difference.
How long you can stay out. SPF is not minutes of protection. SPF 30 does not mean 30 minutes, and it does not mean you can stay in the sun 30 times longer than without it. FDA describes SPF as related to the amount of solar exposure, not the time. The same hour delivers far more UV at midday than in the early morning, and sunscreen rubs off, sweats off and washes off over the day.
How much you applied. The number assumes the test amount, applied evenly. Many people apply less, and applying too little can reduce the protection you get below what the label states. How Much Sunscreen to Use, and When to Reapply covers application and reapplication.
Reapplying, whatever the number
Reapplication follows the label, not the SPF. US sunscreen labels say to reapply at least every two hours, and they add directions for swimming and sweating. That holds for SPF 15, SPF 30 and SPF 50 alike. A higher number does not stretch the interval, and applying a thicker layer in the morning does not replace reapplying later.
Choosing a higher SPF is also no substitute for applying enough. Whatever the number, apply generously and evenly as the label directs, then reapply as it directs. Do You Need Sunscreen Every Day? covers the daily side of the habit.
Higher numbers
Protection rises with SPF, but not in a straight line. Higher numbers filter somewhat more UVB, each step up adds less than the one before, and no sunscreen filters out all UVB. Some people choose a higher number for long days outdoors; the label’s application and reapplication directions still apply in full.
The words that go with the number
Sunscreen is an over-the-counter drug in the United States, so the wording around SPF is set.
“Helps prevent sunburn.” Any compliant sunscreen may say this.
The longer claim. Only a Broad Spectrum sunscreen with SPF 15 or higher may also say that, if used as directed with other sun protection measures, it decreases the risk of skin cancer and early skin aging caused by the sun. The label lists those measures: limiting time in the sun, especially from 10 a.m. to 2 p.m., and wearing long-sleeved shirts, pants, hats and sunglasses.
The alert. A sunscreen that is not Broad Spectrum, or has an SPF below 15, carries a warning that it has been shown only to help prevent sunburn, not skin cancer or early skin aging.
Victoria’s approach
Victoria Nash, founder of Esenté Skincare, has been a licensed aesthetician since 2003. She also owns and operates Younger You Clinique and is an experienced skincare educator. She draws on her professional experience at Younger You Clinique when evaluating skincare products and their performance. The sunscreen label asks for “other sun protection measures”, and Victoria’s own habits include one. She applies daily broad-spectrum sunscreen to the backs of her hands, and when driving she wears lightweight white linen gloves to help reduce continued sun exposure. That is her personal preventive practice, not a requirement for everyone.
Esenté offers two sunscreens: Derma-Defense SPF 30 in the Legacy Skincare collection and Derma Defense Z-Cote SPF30 in the Elite Skincare collection. Both are labeled Broad Spectrum SPF 30. They are examples among many suitable sunscreens, and the choice depends on your skin, your preferences and your budget. Follow the label directions for whichever you use, including the amount to apply and when to reapply. For a question about reading a sunscreen label, try the Ask Victoria page.
Frequently asked questions
Sun protection factor. It describes how much more UV it takes to cause sunburn on skin wearing the product than on unprotected skin, measured under standardized test conditions. Because sunburn comes mostly from UVB, SPF is mainly a measure of UVB protection.
No. SPF is not a measure of time. FDA describes it as related to the amount of solar exposure, and UV intensity changes through the day. Whatever the SPF, reapply as the label directs, including its directions for swimming and sweating.
It filters somewhat more UVB, but the difference is smaller than the numbers suggest. Neither number lets you apply less or reapply less often, so follow the label for both.
No. Applying too little can reduce protection below what the label states, at any SPF. Choose a Broad Spectrum sunscreen with a suitable SPF, then apply the amount and reapply as its label directs.
Not by itself. SPF mainly measures UVB-related sunburn protection. Look for the words Broad Spectrum, which mean the product passed FDA’s test requiring its UVA protection to rise in proportion to its UVB protection.
If it is labeled as a sunscreen, with a Drug Facts panel, it was tested the same way. The number assumes a generous, even layer, and moisturizer often goes on thinly, so follow its label or wear a separate sunscreen.
About the author. Victoria Nash, founder of Esenté Skincare, has been a licensed aesthetician since 2003. She also owns and operates Younger You Clinique and is an experienced skincare educator.
Reviewed by Victoria Nash, licensed aesthetician, founder of Esenté Skincare and owner of Younger You Clinique. This article is educational and is not medical advice. Ask your dermatologist or licensed skincare professional about products appropriate for your own skin and medical history.
