What is SPF? Breaking down sun protection, UV light, sunscreen strength & more

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“Sun protection factor” is something we’re all familiar with, if only in its acronym form: SPF. But how does SPF work? And what level of SPF do you really need to protect yourself?

To find out, WellTuned spoke with Dr. Suzanne Corrington, a medical director for BlueCross BlueShield of Tennessee.

What does SPF mean?

Dr. Corrington: Sun protection factor, or SPF, is really a measure of how effective the product is in protecting skin from solar radiation in the form of ultraviolet (UV) light.

What is ultraviolet light?

Dr. Corrington: UV light is energy produced by the sun that your eyes can’t see but your skin can feel. It’s the same kind of radiation produced by tanning beds.

Types of UV radiation

There are three types of UV radiation:

  1. UVA rays are often less intense than UVB rays, but they penetrate deeper into the skin and are present throughout the day
  2. UVB rays penetrate and damage the outer layer of skin, which is why they’re known as the “burn rays”
  3. UVC rays are blocked by the ozone layer, so they don’t affect skin

UV radiation can cause problems from premature wrinkles to skin cancer. The effects of UV radiation depends on the intensity of the rays where you are (location) and the length of time skin is exposed to the sun without protection.

  • If you live somewhere the sun is strong year-round — such as Venezuela, Colombia or anywhere near the equator — your exposure level and risk is probably higher.
  • If you live in Canada, Alaska or Washington state, for example, where there is less sun year-round, your risk is likely lower.

One important thing to understand about UV damage is that it’s cumulative. While your body can repair some damage caused by radiation, it can’t repair it all. That damage can build up over time and cause skin cells to multiply, leading to tumors.

What factors affect how SPF works?

Skin type

Dr. Corrington: Melanin is the pigment in skin that absorbs UV radiation before it can damage DNA. The darker the skin, the more protection is “built in,” so to speak. People with very light skin may not be able to produce much melanin to protect DNA, which is why it’s important to take skin type into account when making decisions about sunscreens.

Suntan and sunburn

Dr. Corrington: A tan is a visual sign of UV skin damage — Your skin is reacting to the damage and trying to protect itself from more. Coco Chanel helped turn tanning into a status symbol, but the “glow” you see on skin today can turn into skin cancer tomorrow.

Sunburn is a radiation burn resulting from overexposure to UV radiation. We grow up hearing the term “sunburn,” so it doesn’t always have the impact it should, but a “radiation burn” really is as bad as it sounds. Your skin is having an inflammatory response to sun damage, and if it’s severe enough, the skin cells are killed and have to be replaced (peeling). This DNA damage is irreversible and increases the risk of skin cancers and photoaging (the damage that causes wrinkles, dark spots and loose or coarse skin over time).

How does SPF work?

Dr. Corrington: SPF is a measure of how much UV radiation it would take to burn protected skin compared with bare skin. The numbers give you an idea of how long you can stay in the sun without burning, though the math is a little more complicated than people think.

What you really need to know is that:

  • Higher SPF numbers do mean more protection.
  • The American Cancer Society recommends using a sunscreen of SPF 30 or higher.
  • The higher you go, the smaller the difference is between them. For example, the amount of UV rays filtered out by SPF 30 and SPF 50 is only a 1% difference.
  • Any sunscreen with an SPF below 15 only protects against sunburn, not aging or skin cancer.
SPF Percent of UVB rays filtered
15 93%
30 97%
50 98%
100 99%

Maximize the effectiveness of your sunscreen:

  • Choose broad-spectrum sunscreens.
    Broad spectrum means the product will protect you from both UVA and UVB rays.
  • Use an SPF of 30+.
  • Apply sunscreen at least 20-30 minutes before sun exposure. 
  • Apply sunscreen liberally.
    An average person should use at least an ounce, and that number should increase as body size and exposed skin area increase.
  • Reapply sunscreen every 2 hours.
  • Reapply after swimming, sweating or toweling off. 

No sunscreens are truly waterproof or sweatproof. At most, water-resistant sunscreens will protect you for 40-80 minutes while swimming or sweating, and they will come off when you towel off.

  • Wearing SPF-rated clothing.
    It’s an easy way to decrease how much skin you need to cover with sunscreen.

Remember: It’s not really possible to be too protected.

How does mixing two sunscreens work?

Dr. Corrington: Many people think adding SPF 15 to SPF 30 equals SPF 45, but that isn’t true. SPF is not additive. Mixing sunscreens that work differently and have different SPF values only confuses your ability to predict protection. You may even be diluting the effectiveness of the stronger SPF, so it’s best to stick with one sunscreen at a time.

Are there any people who need to be more careful of sun exposure?

Dr. Corrington: There are some medical conditions (lupus, porphyria) and medications (tetracyclines, some antidepressants) that can increase the risk of serious effects from solar radiation. Read warnings on medication bottles and educate yourself about any chronic medical conditions you may have.

It’s especially important to protect children since sun damage builds up over time. But people of all ages benefit from better sun protection. Even if you already have skin cancer, sunspots or scaly patches (actinic keratoses) from years of sun exposure, you don’t want to keep adding more damage to the skin.

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Ashley Brantley

Ashley Brantley has been writing about food, culture and health for more than a decade, and has lived in three of Tennessee’s four major cities (Memphis, Nashville and Knoxville).

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Get more information about specific health terms, topics and conditions to better manage your health on bcbst.com. BlueCross BlueShield of Tennessee members can access wellness-related discounts on fitness products, gym memberships, healthy eating and more through Blue365®. BCBST members can also find tools and resources to help improve health and well-being by logging into BlueAccess and going to the Managing Your Health tab.

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Ashley Brantley has been writing about food, culture and health for more than a decade, and has lived in three of Tennessee’s four major cities (Memphis, Nashville and Knoxville).