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How Does SPF Protect Against Skin Cancer?

Dermatologists always recommend sunscreen as an important part of any skin care routine, insisting it can prevent concerns like sun damage and skin cancer, but how exactly do sunscreens do that?

What does SPF mean and how is it determined?

Sunscreens are graded by how long and how effectively they protect skin from sun damage using a measurement called the Sun Protection Factor, or more commonly as “SPF.”

The SPF of a product is determined by comparing how long it takes radiation damage to occur on unprotected skin versus protected skin. The basic theory is, if your sunscreen has an SPF of 10, your skin is protected from approximately 10 times more radiation than if you weren’t using sunscreen. Keep in mind that the measurement is not in minutes. The SPF of a product is determined in the United States by redness of the measured area after a certain amount of UVB exposure. The way they determine SPF means there are a few things to look out for when you’re choosing your sunscreen.

What are the different kinds of UV Radiation?

UVB radiation is a light frequency that causes redness in skin, but it does not permeate very far into or cause as significant damage to the skin as UVA does. UVA radiation is actually more dangerous, mutagenic, and carcinogenic than UVB radiation, but because it causes a delayed tan instead of immediate redness, it is not accounted for in the United States measurements of SPF. Many sunscreens have a very high SPF, but only protect from UVB radiation, meaning your skin is still vulnerable to UVA.

One way sunscreens can bolster their SPF rating is by including anti-inflammatories into their products. Anti-inflammatories reduce redness, which makes products appear to offer greater protection to sun damage. Inflammation is bad and does cause aging, but just because you’re stopping inflammation does not mean you are stopping genetic damage that can lead to skin cancer. The best sunscreens for protecting from all kinds of UV radiation are broad spectrum sunscreens, which are usually chemical sunscreens with multiple active ingredients, each capable of absorbing different wavelengths of light.

How does radiation cause skin cancer?

When your skin isn’t protected by an SPF ingredient, it is at greater risk for sun damage and possibly even skin cancer. Skin cancer happens when UV radiation damages DNA strands in your skin cells, causing mutations that result in cancers like melanoma, basal cell carcinoma, or squamous cell carcinoma. Cells with damaged DNA can lose the ability to control their reproduction, causing uncontrolled growth and eventually cancerous tumors. When sunscreens interfere with UV radiation by either reflecting or absorbing it, this damage and mutation is prevented. There are two main categories of sunscreens that dissipate UV radiation in different ways; physical sunscreens and chemical sunscreens.

Physical Sunscreens

The first category of sunscreens are Physical sunscreens, made up of metals, generally Zinc oxide, Iron oxide, or Titanium oxide. When applied to the face as part of a sunscreen, these products reflect UV light away from your face like a mirror. Physical sunscreens protect from a smaller spectrum of radiation away from the skin than chemical sunscreens do. They are also less likely to cause allergic reactions than chemical sunscreens are. Because they do not deteriorate or oxidize quickly in the sun, these mineral sunscreens are recommended for repeated daily sun exposure. Physical sunscreens do not have to be made with oils like chemical sunscreens are. This makes them generally safe for use on oily skin as they do not often contain acne causing ingredients. Dermatologists usually recommend physical sunscreens as your daily sunscreen because even though they protect from fewer kinds of radiation than chemical sunscreens, they do not contribute to skin aging and do not clog pores.

Chemical sunscreens

Unlike physical sunscreen that reflects radiation away like a mirror, chemical sunscreens absorb UV Radiation. Chemical sunscreens usually contain multiple active ingredients to absorb various frequencies of light, making them much better at protecting the skin from genetic damage than physical sunscreens. Though they are more effective at absorbing UV, chemical sunscreens often oxidize, form free radicals, and break down in the sun, meaning they need to be reapplied every hour or so in intense sunlight to avoid long term skin aging. To avoid skin aging from free radicals when using chemical sunscreens, use them on top of a vitamin C serum like (Derma Made) or (SkinCeuticals CE Ferulic), followed by a chemical sunscreen (ELTA MD) Then cover with a mineral sunscreen like (from the list). Chemical sunscreens are better at sun protection than purely mineral, physical sunscreens, but because of the previously mentioned concerns, they are often combined with mineral ingredients.

Combination Sunscreens

In many cases, physical and chemical UV protective ingredients are combined in sunscreens to overcome their individual deficits. Products with combinations of physical and chemical UV protectors are able to reflect light away from the skin, and chemically process any radiation that manages to break through that barrier. Combination sunscreens like these can be very effective in protecting from UV radiation across a broad spectrum of damaging light.

The body’s natural antioxidant defenses

Whether you use sunscreen or not, the body has systems in place to try to defend against sun damage. The skin’s antioxidant defense system keeps it safe from UV based oxidative damage, gathering free radicals that cause cancer automatically. That being said, the sun is much more powerful than our skin so we need all the help we can get staying safe. (Antioxidant product) That’s why even though the body has measures to stay safe from sun damage, we still get sunburns when we don’t use sunscreen. Even darker skin tones which are more naturally resistant to UV need to use sunscreens to protect their skin from damaging UV rays. Due to misconceptions about who is and who isn’t at risk of skin cancer, more people experience the consequences of sun damage today than ever before. This is magnified by the confusing notation of SPF in skin care products. Many people apply sunscreens in proper amounts, but do not realize their mineral sunscreen does not provide broad spectrum protection. If you are going out in the sun, your body’s natural UV defenses are not strong enough to protect from significant sun damage.

What to keep in mind this summer

It’s not always easy to remember something like applying your sunscreen every day, but it is crucial to skin health and prevention of diseases like skin cancer. As you work to figure out your summer plans, keep in mind how long you plan to be in the sun and find a sunscreen that’s right for your goals. Skin cancer can be deadly, and it is only becoming more common every year. The SPF of a product as seen on the label lets you know how much longer your skin is safe in the sun. Understanding what SPF means, what a broad spectrum sunscreen is, the differences between mineral and chemical sunscreens, and how they all play into your skin care is critical in keeping your skin healthy and safe.


Editors. Baumann L.S., & Rieder E.A., & Sun M.D.(Eds.), (2022). Baumann’s Cosmetic Dermatology, 3e. McGraw Hill.