SPF: What does it really mean?

Gabriela Maloney, DO
      edited by Bridget McIlwee, DO

It’s not a secret that UV radiation can cause acute sunburns. But did you know that UV radiation also causes skin cancer and wrinkles (called photoaging)? All individuals, regardless of their skin type or tone, are susceptible to the damaging effects of UV radiation. Lighter skin tones are at highest risk from the sun and should use sunscreen with a high SPF every day, reapplying often.

But what does all of this really mean?

UV_and_Vis_SunscreenSPF stands for “Sun Protection Factor”. SPF measures the sunscreen’s ability to protect skin from UVB radiation. The SPF number is a ratio: it compares the minimal dose of UV radiation required to cause redness (minimal erythema dose, MED) of skin with sunscreen when compared to skin without sunscreen. For example: if a person applies sunscreen with an SPF of 30 as directed, it would take 30 times more UV radiation for that person’s skin to develop redness than it would if he or she had not applied any sunscreen.

SPF testing and determination is performed by having individuals apply sunscreen on their skin and then exposing them to a light source that simulates the natural UV (solar) radiation.

In order to replicate the SPF’s stated effectiveness outside of the laboratory, one must apply the same amount of sunscreen used during testing. An average adult male has a body surface equivalent to 1.9 m2, which would require about 1.3 oz (equivalent to the size of a shot glass) of sunscreen to get the full sun protection denoted by the SPF. And, this amount of sunscreen must be reapplied frequently – usually every 2 hours. Unfortunately, only a small percentage of the population applies sunscreen as recommended. Thus user error – not an ineffective product – is usually the cause of sunburns inflicted while ‘using’ sunscreens.

broad-spectrumphan-biet-kem-chong-nangWhat does “Broad-Spectrum” mean?

The SPF is established based on the ability of sunscreen to protect against UVB rays, but it does not measure protection against UVA radiation.

Ultraviolet (UV) radiation is divided into UVA and UVB spectra, according to ranges of wavelengths. UVB rays encompass only about 5% of the UV radiation reaching the earth’s surface. Approximately 95% of the UV radiation we are exposed to is UVA, which can also cause photoaging and skin cancer.

So, if SPF ratings don’t tell us how protected we are from UVA rays… how do shield ourselves from them? The answer is the use of “broad spectrum” sunscreen. The FDA has established new labeling regulations that only allow sunscreens which have passed various protection tests to be labeled “broad-spectrum”. However, even though many sunscreens are now labeled “broad-spectrum”, the amount of protection against UVA rays is still not rated or quantified.

UV-protective ingredients are separated into two broad categories: physical and chemical. Two commonly used broad-spectrum physical sunscreens are zinc oxide and titanium dioxide. These ingredients work by  reflecting and scattering UV light, and are often called ‘UV blockers’. They are much less likely than chemical ingredients to cause irritant or allergic skin reactions. Broad spectrum chemical sunscreens contain organic compounds – like avobenzone, oxybenzone, octocrylene, and homosalate – that work by absorbing UV light and dissipating it as heat.

What’s in a number? 

A sunscreen must be labeled SPF 15 or higher in order to decrease your risk of skin cancer. There is a common misconception that SPF numbers which are twice as high will give you twice the protection from the sun, which is simply not true. When sunscreen is applied properly, SPF 15 will absorb approximately 93% of UVB radiation. In comparison, SPF 30 will absorb 97% of UVB radiation, and SPF 50 will absorb approximately 98%. (And remember, this protection only lasts as long as that first coat of sunscreen – about two hours, or less if you’ve been swimming or sweating.)

To reduce consumer confusion, the FDA has capped SPF ratings at 50, and is now requiring SPFs greater than 50 to be relabeled 50+.

The bottom line… 

  • Regardless of the SPF, sunscreens must be applied liberally (at least the equivalent of one shot glass full of sunscreen) to all the sun-exposed parts of the body.
  • Sunscreen application should occur 15-30 minutes prior to sun exposure to allow formation of a protective film on the skin.
  • Sunscreen should be reapplied at least every two hours. Even sunscreens labeled “water-resistant” or “very water-resistant” are eventually washed away after sweating or swimming.


Photo Credit: SkinCancer.com & Consumer Reports

Resources: American Academy of Dermatology, Uptodate.com



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