We know that sunscreens with varying SPFs numbers are available everywhere — but what does “SPF”actually mean?

Here’s a look at the science that goes into determining a sunscreen’s skin protection factor, and how to choose the best one for your needs.

sunscreen SPFs

How much protection are you getting?

Sunscreens protect your skin by absorbing and/or reflecting UVA and UVB rays. The FDA requires that all sunscreens contain a Sun Protection Factor (SPF) label. The SPF reveals the relative amount of sunburn protection that a sunscreen can provide an average user when correctly used.

SPF is defined as the minimum sun radiation dose (mainly UVB) required to produce sunburn after application of 2mg per square centimeter, divided by the dose of sunlight to produce the same effect on unprotected skin, according to research published in The Lancet by Dr Stephan Lautenschlager, Outpatient Clinic of Dermatology at the Triemli Hospital in Zurich, Switzerland, and colleagues. This translates to a protection factor of 50% for SPF2, 87.5% for SPF8, 93.6% for SPF16, and 96.9% for SPF32.

SPF vs. UVB protectionSunscreens with an SPF of at least 15 are recommended, and higher SPF values (up to 50) provide greater sunburn protection. So what about tubes and bottles marked SPF 70 or 100?

In June 2011, the FDA proposed a regulation that would require sunscreen products that have SPF values higher than 50 to be labeled as simply “SPF 50+.” That’s because the FDA “does not have adequate data demonstrating that products with SPF values higher than 50 provide additional protection compared to products with SPF values of 50.”

>> How does sunscreen work?

You should be aware that an SPF of 30 is not twice as protective as an SPF of 15; rather, when properly used, an SPF of 15 protects the skin from 93 percent of UVB radiation, and an SPF 30 sunscreen provides 97 percent protection (see chart SPF vs. UVB protection to the right).

Furthermore, as manufacturer Banana Boat points out, “SPF numbers don’t add up the way you might think. Using an SPF 8 and SPF 15 together won’t allow you to remain in the sun 23 times longer than without protection.”

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“Wearing a higher sunscreen SPF does NOT mean you do not need to reapply,” add the folks at Coppertone. “You should reapply sunscreen at least every two hours, regardless of the SPF.”

SPF testing

All sunscreens must be tested according to an SPF test procedure. The test measures the amount of ultraviolet (UV) radiation exposure it takes to cause sunburn when a person is using a sunscreen in comparison to how much UV exposure it takes to cause a sunburn when they do not use a sunscreen.

Because SPF values are determined from a test that measures protection against sunburn caused by ultraviolet B (UVB) radiation, SPF values only indicate a sunscreen’s UVB protection. However, sunscreens that pass the broad spectrum test will have demonstrated that they also provide ultraviolet A (UVA) protection that is proportional to their UVB protection. Therefore, a higher SPF value for sunscreens labeled “Broad Spectrum SPF [number]” will indicate a higher level of protection from both UVA and UVB radiation.

Although the SPF ratings found on sunscreen packages apply mainly to UVB rays, many sunscreen manufacturers include ingredients that protect the skin from some UVA rays as well. These “broad-spectrum” sunscreens are highly recommended.

>> Sun protection in childhood prevents skin cancer in adults

Only broad spectrum sunscreens with an SPF value of 15 or higher can claim to reduce the risk of skin cancer and early skin aging if used as directed with other sun protection measures. Non-broad spectrum sunscreens and broad spectrum sunscreens with an SPF value between 2 and 14 can only claim to help prevent sunburn.

That said, all products do not have the same ingredients; if your skin reacts badly to one product, try another one or call a doctor.

Sunscreen not always the answer

Unfortunately, using sunscreen does not mean it is safe to spend extra time in the sun, especially when the UV Index is high. (Check here to see the current UV index numbers from the NOAA/National Weather Service.)

Although a sunscreen with an SPF of 15 or higher offers protection from sunburn, it does not block all of the sun’s damaging rays. In fact, there is no evidence that sunscreens protect you from malignant melanoma, the deadliest form of skin cancer, even though sunburns have been linked with the development of melanoma. There is only limited evidence that sunscreens protect you from several other types of skin cancer.

To reduce your risk, regularly use sun protection measures including:
  • Limit time in the sun, especially between the hours of 10am and 2pm, when the sun’s rays are most intense.
  • Use extra caution near water, snow and sand — all reflective surfaces that can increase your sun exposure.
  • Wear clothing to cover skin exposed to the sun — for example, long-sleeved shirts, pants, sunglasses, and broad-brimmed hats.
  • Use sunscreens with broad spectrum SPF values of 15 or higher regularly and as directed.
  • Reapply sunscreen at least every 2 hours, more often if you’re sweating or jumping in and out of the water.

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