How Does Sunscreen Work to Protect Your Skin?

Rain or shine, winter or summer, any time spent outdoors leaves your delicate skin vulnerable to sun damage. Whether you’re spending a summer’s day at your local pool, gliding down the slopes at your favorite ski resort, or simply sitting outside during your lunch break, it’s important to protect your skin with the right sun protection options.

We’ve heard about the importance of applying sunscreen since childhood, but how does sunscreen work to protect our skin? It’s simple to spray or slather on some SPF, but the science behind sunscreen protection is quite complex. In this guide, we’ll take a look at the ways sun exposure affects skin based on a variety of factors, and help you determine which sunscreen options are best suited to your skin type and lifestyle.

How does sunscreen protect your skin? Let’s find out.

Sun Exposure and Sun Damage

We often associate a bronzy glow with a healthy, youthful appearance, but over time, sun exposure can actually accelerate the signs of skin aging. To understand how sunscreen works to safeguard your skin from the sun’s rays, it’s important to understand exactly how the sun affects your skin.

Sunlight travels to earth in a combination of visible and invisible rays, and many of these rays are made up of ultraviolet (UV) light. When UV rays hit your skin cells, they throw off body processes that affect the appearance of your skin and growth rate of your skin cells. It feels good to soak up those warm rays, but exposure to UV radiation can cause a host of physical ailments; in the short-term, that might mean a sunburn that’s painful to the touch. In the long-term, it might mean wrinkles and skin damage, and in the worst-case scenario, life-threatening skin cancer.

There are two types of UV radiation that can affect your skin; longer UV rays are called UVA rays, while shorter rays are called UVB rays. Excessive exposure to UVB rays can lead to sunburns and superficial damage to the epidermis, or outer layer of the skin; because UVA rays are longer, they can penetrate deeper into the dermis—your skin’s thickest layer—resulting in more permanent damage.

Split image of a woman’s face with one side showing her skin without sun protection and the other with it.

While UVB rays are less prevalent during the winter, mornings and evenings, UVA rays are always present, regardless of season or time of day. UVB rays actually account for almost 95 percent of the radiation that penetrates the Ozone layer. Though UVA rays don’t cause sunburns, they are the main factor behind signs of skin aging and cases of skin cancer.

How does sunscreen work to protect your skin against both of these harmful types of radiation? It depends on the product you use. It’s important to purchase sunscreens that provide broad spectrum (or full spectrum) protection—meaning they protect your skin from both UVA rays and UVB rays.

Factors That Affect Your Photosensitivity

Your skin’s sensitivity to sunlight (or “photosensitivity”) is based on a few different factors, including age, skin tone, and the climate or season in which you’re spending time outdoors.


As you age, it becomes harder for your skin to repair itself, and over time, UV damage can affect the skin’s underlying connective tissue. Thanks to this breakdown in elastin, your skin may begin to develop more wrinkles and lines.

This breakdown isn’t exclusive to those who spend the bulk of their waking hours outside. In fact, most sun exposure is incidental to daily life, including those shorter intervals during which we head outdoors without skin protection—whether that’s to run to the grocery store or walking from your car to the office building. Protective makeup products like this SPF 35 lip shine can help add another level of protection against these daily UV exposures.

These 5-minute increments where our skin goes unprotected can build up over a lifetime, and may prove to cause more damage than summer days spent at the beach (when sunscreen is more liberally applied). Be sure to apply sunscreen every morning, regardless of what your day has in store.

Skin Tone

Your skin tone plays a significant role in determining your UV sensitivity and skin cancer risk. While everyone’s skin can be affected by sun exposure, those with very fair skin are highly sensitive to ultraviolet light, and are more likely to burn; in contrast, those with darker skin pigment have minimal sensitivity to the sun’s rays, and rarely burn, if at all.

Why is this? People with darker skin have a higher number of melanocytes, which are cells that produce melanin. Melanin helps block damaging UV rays, so those with naturally darker skin are less likely to get sunburned, while those with lighter skin are more likely to burn.

Skin tone also plays a crucial role in determining skin cancer risk; fair skin is more susceptible to skin cancer, while those with deeply pigmented dark brown to black skin experience skin cancer relatively rarely.

Close up of three women with different skin tones.

Climate/Time of Year

The climate in your area and time of year both affect the likelihood of sun damage. The Ultraviolet (UV) Index, developed by the National Weather Service (NWS) and the U.S. Environmental Protection Agency (EPA), is designed to help you understand your daily exposure to UV radiation.

Learning how sunscreen works means understanding just how much risk you face of sun damage. The UV Index indicates the level of UV exposure expected on a given day, ranging from zero (lowest risk of UV radiation) to 11 (highest risk of UV radiation).

We’ve taken UV index predictions from the middle of summer (July 2016) and dead of winter (January 2017) to show you the way time of year and climate can affect UV radiation risk.

Woman at beach holding phone up to show the daily UV index.

Sun protection is a must in climates that carry a high risk of sun damage, and this risk increases during hot summer months. As the map indicates, much of the lower half of the United States has a UV index rating of more than 5 during July, meaning there’s a high to extreme level of risk of UV radiation.

So how does this compare to UV index ratings during cooler winter months?

During the winter, UV index ratings across the country shows significantly reduced levels of UV radiation. However, up to 80 percent of the sun’s UV rays can penetrate cloud coverage—meaning you don’t get a hall pass for your sunscreen habits during the winter.

While climate and time of year can affect the likelihood of burning, summer isn’t the only season that sun damage occurs. It doesn’t need to be warm or sunny outdoors for UV rays to wreak havoc on your skin, so be sure to wear SPF year-round.

How Does Sunscreen Work

Sunscreen works by combining organic and inorganic active ingredients that protect the skin against the sun’s rays. Sunscreen may come in a variety of forms, including sprays, liquids, lotions, powders, and creams.

There are two main types of sunblock available: physical sunscreen and chemical sunscreen. Understanding the way these sunscreens are made and the ingredients they contain can help you pick the one that’s best suited for your skin type.

How does physical sunscreen work?

Physical sunscreen, also called “mineral” sunscreen, contains inorganic physical UV filters that reflect, scatter and block the sun’s rays before they penetrate the skin. These active mineral ingredients, also known as physical blockers, are designed to sit on top of the epidermis rather than be absorbed into the skin.

Common Physical UV Filters

If you purchase a physical sunscreen, it likely contains either Titanium Dioxide and/or Zinc Oxide, as these are the only physical UV filters that have been approved by the FDA for sun protection. These two ingredients are the least likely to cause adverse skin reactions, so those with sensitive skin typically elect to use hypoallergenic sunscreens that list one of these as an active ingredient.

  • Titanium Dioxide: This ingredient is a naturally occurring mineral characterized by its white pigment. Titanium Dioxide is a useful addition to cosmetic products, as it remains stable even when exposed to UV radiation and doesn’t degrade in the sun.
  • Zinc Oxide: Zinc oxide is still the only FDA-approved ingredient that protects the skin from both UVA and UVB rays. This physical blocker is naturally occurring, but can also be synthetically produced. It scatters and reflects UV rays, preventing them from penetrating the outer layer of the skin.

The Benefits of Physical Sunscreen

  • Physical sunscreen naturally offers broad spectrum protection
  • Less likely to irritate the skin
  • Reflects heat, making physical sunscreens a great choice for those struggling with rosacea and other skin conditions characterized by excess redness
  • Most physical sunscreens are non-comedogenic, meaning they’re less likely to clog the pores

The Drawbacks of Physical Sunscreen

  • Physical sunscreen can be quick to rub off, and if you’re taking part in activities that involve perspiration or contact with water, you may need to reapply more frequently
  • Some physical sunscreens leave a white-hued residue on the skin
  • Physical sunscreens are often thicker, and take more effort to rub in

Illustration showing how physical sunscreen works.

How does chemical sunscreen work?

Chemical sunscreens function in a different manner; this type of sunblock contains organic (carbon-based) active ingredients designed to absorb UV radiation upon contact. Chemical sunscreens contain organic compounds that catalyze a chemical reaction when exposed to the sun; this reactions transforms UV rays into heat, which is then released from the skin. While physical UV filters block both UVA and UVB rays, chemical UV filters often only protect against one or the other.

Common Chemical UV Filters

  • Avobenzone: This is the most commonly used UVA chemical filter found in chemical sunscreens. This ingredient is unstable, meaning it quickly degrades in sunlight.
  • Octinoxate: This chemical filter is absorbed rapidly into the skin, and is a known endocrine disruptor that can affect thyroid function.
  • Octisalate: While Octisalate helps absorb UVB rays (but not UVA rays), it’s also a penetration enhancer, meaning it increases the amount of other ingredients that pass into your skin. If a chemical sunscreen contains hazardous ingredients, they are more likely to pass into the body when Octisalate is present in the formula.
  • Oxybenzone: This chemical UV filter absorbs UVB and UVA rays, but it is a photosensitizer, meaning it increases the body’s production of free radicals after sun exposure. It’s also been implicated as a hormone disruptor, and may affect the production of estrogen in the body.
  • Octocrylene: This chemical UV filter can absorb both UVB and UVA rays, but like Oxybenzone, it also increases the production of free radicals after being exposed to the sun.

The Benefits of Chemical Sunscreen

  • Chemical sunscreen formulas tend to be thinner, making them easier to spread evenly across the skin
  • Chemical sunscreens rub into the skin, leaving less residue

The Drawbacks of Chemical Sunscreen

  • Chemical sunscreen must be applied at least 20 minutes before sun exposure, as it’s not effective immediately
  • Chemical sunscreens contain chemical ingredients that can irritate the skin
  • When worn in direct UV light, chemical sunscreen protection can be used up more quickly, meaning it requires more frequent application
  • The protection it offers gets used up more quickly when in direct UV light, so reapplication must be more frequent
  • Chemical sunscreens transform UV rays into heat and this heat can increase the likelihood of redness, especially for those struggling with rosacea and similar skin conditions
  • Chemical sunscreens may be more likely to clog pores, which can make breakouts more frequent for those with oily skin
  • The ingredients found in chemical sunscreens can irritate the eyes

Physical Vs. Chemical: Which sunscreen is best?

While both physical and chemical sunscreens can be effective for blocking the sun’s UV rays, sun blocks that contain numerous chemical ingredients can cause skin irritation and redness. Sunscreens that count Zinc Oxide and Titanium Oxide as their active ingredients, like Colorescience Sunforgettable Sunscreen, offer the protection you need without subjecting your skin to harsh side effects.

How Long Does Sunscreen Last?

To determine how long your sunscreen will protect your skin when exposed to ultraviolet light, you first have to understand its ratings. SPF stands for Sun Protection Factor, and this measurement indicates how well and for how long your sunscreen will protect your skin from the sun’s UVB radiation, unlike PA ratings, which are meant to measure a product’s level of protection from UVA radiation.

This chart shows the difference between no sunscreen, SPF15, and SPF30. In this example, going without sunscreen allows 100 photons of radiation to enter your skin. If you use SPF15 sunscreen, which offers 93 percent protection from the sun’s rays, only 7 of those photons would be able to penetrate your skin. That protection is improved by using SPF30 sunscreen, which protects against 97 percent of the sun’s rays; in this case, only three photons would penetrate your skin.

Your skin’s photosensitivity affects the length of time that SPF can protect you from UV rays, with skin tone being the deciding factor.

So how does sunscreen work with different skin tones?

A person with very fair skin should wear at least SPF30 anytime they head outdoors, whether they’re spending 15 minutes or two hours exposed to the sun’s rays. However, to protect their skin from three or four hours of sun exposure, they’d need to apply SPF50.

In contrast, a person with very dark skin is less likely to burn, and may require only SPF15 to protect their skin for up to four hours.

The chart below further details the effectiveness of SPF ratings for different skin tones.

Reapplication is the only way to keep your skin consistently protected, whether you’re spending an hour or four hours in the sun. Consider your skin tone and use this chart to help you determine how often you should reapply different ratings of SPF.

Note: Although this chart highlights how sunscreen works throughout specified periods of time, it’s also important to consider the ways your activity level and lifestyle habits impact its efficacy. If you’re sweating profusely, your sunscreen might wear off more quickly. Likewise, if you’re spending time in the pool, your sunscreen be quicker to wash or rub off. Keep in mind that even the best waterproof sunscreens can't be completely water- or sweat-proof; your formula may be water-resistant and sweat-resistant, but you’ll need to reapply regardless.

Areas of the Body Most Prone to Sunburn in the Summer

How does sunscreen work to protect different parts of the body? Any exposed skin is susceptible to sun damage, and you can battle exposed facial dryness with things like hydrating mists. But there are certain areas of the body more prone to sunburn —if simply for the fact that they see the sun more often.

The chart below indicates the areas in which a man or woman wearing the typical swimsuit is most prone to burn during the summer months.

  • Head and neck: Because your face and neck are almost always exposed (regardless of whether you’re in winter gear or your summer swimsuit), these areas of the body are prone to strong sunburns. Be sure to apply sunscreen to these areas any time you’re going outside, and consider travel size sunscreen options for on the go.
  • Chest: During the summer months, your chest is highly susceptible to sun damage. If you’re wearing a lower-cut shirt or swimsuit, be sure to apply plenty of sunscreen to the delicate skin on your chest.
  • Arms and shoulders: Arms are subject to medium sunburns, while shoulders are more likely to experience strong burns if left unprotected. As summer is the season we’re most apt to sport tank tops and sleeveless tees, pay special attention to your shoulders and arms in terms of SPF.
  • Stomach: If you wear a tummy-baring swimsuit in the summer months, be sure to slather on the sunscreen; because this sensitive skin rarely sees the sun in other instances, it’s more susceptible to strong burns.
  • Legs: The upper portion of your legs is vulnerable to strong burns, while your shins are more likely to receive medium burns. Be sure to coat your thighs in sunscreen and don’t forget to protect the back of your legs.
  • Feet: If you sport sandals or open-toed shoes frequently, the skin found on the top of your foot is often exposed to the sun’s rays, which may translate to strong burns. Wear closed-toe shoes or coat your feet with plenty of sunscreen.

While this chart is an effective guide to use, and can help you anticipate the areas of your body that might need a little bit more protection (or more frequent sunscreen reapplication), always ensure any bit of exposed skin is protected by sunscreen when heading outdoors—regardless of whether these areas of the body are prone to burning or not. Sunscreen only works if it’s applied at the right time, and reapplied when necessary.

Now that you understand how sunscreen works, be sure that you’re also taking advantage of the many sun protection tactics that go beyond SPF. Consider the time you spend outdoors; if the UV index rating is high, you may want to stick to indoor activities. If you do head outside, be sure to wear protective clothing, hats, and sunglasses.

Key Takeaways: How Does Sunscreen Work to Protect Your Skin?

  • There are two types of UV radiation that can affect your skin; longer UV rays are called UVA rays, while shorter rays are called UVB rays.
  • Age, skin tone, climate, and time of year affect your skin’s photosensitivity.
  • Physical sunscreen contains physical UV filters that reflect block the sun’s rays before they penetrate the skin, while chemical sunscreens contain organic compounds that transform UV rays into heat.
  • Both physical and chemical sunscreens are effective for sun protection, but chemical sunscreens may cause excess irritation and side effects.
  • Various levels of SPF rating allow a different percentage of harmful radiation photons to enter the skin.
  • To determine how long your sunscreen works, you have to understand its SPF rating and the way that correlates to your skin tone and photosensitivity.
  • Reapplication is the only way to keep your skin consistently protected from the sun. Reapply every two hours— more frequently if you’re spending time in the water or sweating.
  • There are certain areas of the body that are more prone to sunburns because they’re more often exposed, especially during the summer season.