What is SPF and how much sunscreen do you really need? Experts answer

The sun has given us so much. Flowers, for example. Also sunsets, vitamin D and the 2002 Sheryl Crow hit Soak Up the Sun.
And yet, despite its brilliant generosity, humans must also diligently shield themselves from the sun lest its ultraviolet rays inflict damage on our soft, fallible bodies. Among our most powerful defenses? SPF products.
SPF, or sun protection factor, is a measure of how well sunscreen protects our skin from the sun’s rays, specifically its UVB rays, says Dr Heather Rogers, a board-certified dermatologist and founder of the Seattle-based practice Modern Dermatology. This protection helps shield sun-seekers from skin cancer and premature ageing, Rogers explains.
While sunscreen abounds in pharmacy aisles and beach totes, many people aren’t applying it correctly, experts say.
And the consequences are dangerous. Skin cancer is the most common cancer in the United States, according to the American Academy of Dermatology (AAD). In the UK, rates of melanoma, one of the most serious forms of skin cancer, reached a record high in 2023.
“Your melanoma risk doubles with only five sunburns,” Dr Matthew Block, medical oncologist, and Dr Tina Hieken, surgical oncologist, the leaders of the Stand Up To Cancer Melanoma Research Team explained over email. “Even if you don’t burn, UV exposure can still raise skin cancer risk.”
So how much SPF do we really need? How much sunscreen should we be wearing, and how often do we need to reapply it? What’s the difference between chemical and mineral sunscreens? We asked experts.
A sunscreen’s SPF, or sun protection factor, indicates how much protection a product offers.
When sunscreen is being developed in laboratories, the SPF rating is determined by how long a product protects against sunburn, explains Dr Arash Akhavan, a board-certified dermatologist and owner of The Dermatology and Laser Group in New York City. This means that if untreated skin usually takes 10 minutes to burn, skin treated with SPF 30 would take 30 times longer to burn.
But that’s not how it works in practice, experts say. According to the US Food and Drug Administration, “SPF is not directly related to time of solar exposure but to amount of solar exposure”. The amount of solar exposure depends on the time of day. For example, solar exposure will be greater at midday than in the early morning. It also depends on geographic location – places closer to the equator get greater solar exposure than those closer to the poles, for instance.
Other factors that affect how much protection a sunscreen will offer are the person’s skin type and how often and how thoroughly they reapply it.
Levels of SPF are determined by the percentage of UVB rays that they block, Rogers explains. SPF 15 blocks 93% of UVB radiation, SPF 30 blocks 97%, and SPF 50 blocks 98%. SPF 100 blocks 99% of UVB radiation. So a sunscreen with SPF 50 blocks more UVB rays and therefore offers greater sun protection than one with SPF 30.
As a general rule, you can’t overdo it with sunscreen.
“I would recommend using the highest SPF rating you feel comfortable applying,” says Akhavan.
Both the AAD and the UK’s National Health Service (NHS) recommend using an SPF level of 30 at minimum.
In addition to providing greater sun protection, higher SPF levels help make up for some human error, Rogers says. Many people use far less than the recommended amount of sunscreen for each application, which reduces the level of protection it offers. If you apply SPF 30 too lightly, for example, you may only be getting the coverage of an SPF 15.
“We want to have the number be high enough that even if you’re using less than the recommended amount, you’re still going to have significant protection,” Rogers says.
While SPF refers to the percentage of UVB rays that a sunscreen blocks, UVA rays can also damage our skin. According to the University of Texas’s MD Anderson Cancer Center, “UVA radiation accounts for 95% of all the UV rays that make it to Earth.” These rays penetrate deeply into our skin and result in tans. They also damage collagen and produce wrinkles.
UVB rays don’t penetrate the skin as deeply, but they’re very powerful. The damage they do can lead to skin cancer and cataracts.
In order to protect oneself from both UVB and UVA rays, the FDA recommends using broad spectrum sunscreens, which protect against both.
Physical, or mineral, sunscreens and chemical sunscreens operate at different levels of the skin.
Chemical sunscreens contain sun protective ingredients that get absorbed into the skin, Akhavan says. These generally include ingredients like avobenzone, homosalate, octisalate and octocrylene. These ingredients “absorb the UV radiation and undergo a chemical reaction that removes the harmful energy from the UV rays before it can damage skin cells”, Akhavan explains.
Physical sunscreens, also known as mineral sunscreens or sunblocks, usually contain zinc oxide and/or titanium oxide. These minerals sit on the top layer of the skin and reflect the sun’s rays, preventing them from penetrating into the skin. Mineral sunscreens are often recommended for those with sensitive skin, says the AAD.
There’s no reason to use different SPF levels on your face and body. SPF 30 is the minimum recommended level for both, Akhavan says.
But the best sunscreen is one you will use consistently and different situations may call for different types. “I recommend having multiple different sunscreens,” Rogers says. She refers to these as your “sunscreen staples”.
An everyday face sunscreen – one that you wear to work and under makeup – might be tinted and less water resistant, since water-resistant sunscreens are “less cosmetically elegant”, Rogers says. A heavier and more water-resistant formulation would be better suited for outdoor and water use.
“There isn’t a single sunscreen that’s going to make you happy in all those different situations,” she explains.
Basically everyone over six months of age should wear sunscreen. For babies under six months, the AAD recommends minimizing sunscreen use – protect them from the sun by keeping them in shade and dressing them in sun-protective clothing instead.
For everyone else, SPF 30 or above is standard. Rogers notes that people with darker skin tones have more innate sun protection and therefore take longer to tan or burn – but they are also susceptible to skin cancer.
“People of all skin colors are at risk for skin cancer, skin ageing and sunburn from spending time in the sun,” the FDA’s guidance reads.
Probably more than you think.
For each application, Block and Hieken recommend about 1.5 ounces (roughly one shot glass worth) of sunscreen on your body, and half a teaspoon altogether for your face and neck.
For spray sunscreens, they say to apply “until an even sheen appears on the skin”.
One should fully reapply sunscreen (the entire shot glass and half teaspoon) at least every two hours, experts say. FDA guidance advises additional applications “if you’re swimming or sweating”.
Not reapplying is where many people fall short on their sun protection, Rogers says. “Sunscreen only does as good a job as your application.”
While sunscreen is an important part of sun protection, experts agree that the best way to avoid harmful ultraviolet rays is to stay out of the sun when possible.
Block and Hieken suggest avoiding the outdoors during the middle of the day when the sun’s rays are at their strongest: “Take it inside between 10am and 4pm.”
Rogers also recommends sun-protective clothing, which she argues can be more effective than sunscreen because there is less room for human error. “You don’t have to buy expensive UPF clothing either,” she says, referring to ultraviolet protection factor, a measure of how much protection a piece of clothing offers from UVA and UVB rays.
Dark-colored clothing offers significant sun protection, Rogers says. White clothes, she warns, don’t offer nearly as much.
“A white cotton T-shirt is going to give you a UPF of nine,” she explains. “A gray cotton T-shirt is going to give you a UPF of 98.” There are no formal UPF requirements for clothes, but the Skin Cancer Foundation says that clothing with a UPF of 30 to 49 offers “very good protection”, and that with a UPF 50 or above rates as “excellent”.
Ultimately, the best sun protection is a combination of all of these tools. “Everyone has a different relationship to the sun,” Rogers says.