From the Guidelines
Sun Protection Factor (SPF) in sunscreen indicates how effectively it blocks UVB rays that cause sunburn, and according to the U.S. Food and Drug Administration, broad-spectrum sunscreens with a sun-protection factor of 15 or greater protect against both UV-A and UV-B radiation and reduce the risk for skin cancer and early skin aging 1.
How SPF Works
SPF works by using chemical absorbers that convert UV radiation into heat, or physical blockers like zinc oxide that reflect and scatter UV rays away from the skin.
- An SPF 30 sunscreen blocks about 97% of UVB rays, allowing you to stay in the sun 30 times longer than without protection before burning.
- Higher SPF offers marginally better protection (SPF 50 blocks 98% of UVB rays), but no sunscreen blocks 100% of UV radiation.
Key Considerations for Sunscreen Use
When choosing a sunscreen, look for "broad spectrum" protection that guards against both UVA and UVB rays, and opt for a water-resistant sunscreen if you plan to be in the water or sweat heavily.
- Apply 1 ounce (about a shot glass full) of sunscreen to exposed areas 15-30 minutes before sun exposure.
- Reapply every two hours or after swimming or sweating.
Importance of Broad-Spectrum Protection
Remember that SPF primarily measures UVB protection, so it's crucial to select "broad spectrum" sunscreens that also protect against UVA rays, which contribute to skin aging and cancer risk 1.
- The U.S. Food and Drug Administration has determined that broad-spectrum sunscreens with a sun-protection factor of 15 or greater, when used as directed, protect against both UV-A and UV-B radiation and reduce the risk for skin cancer and early skin aging.
From the Research
How Sun Protection Factor (SPF) Works in Sunscreen
- SPF is a measure of how well a sunscreen protects against ultraviolet B (UVB) rays, which are a primary cause of sunburn 2.
- Sunscreens with higher SPF values provide greater protection against UVB rays, but the difference in protection between SPF 30 and SPF 100 is not as significant as one might expect 3.
- Broad-spectrum sunscreens, which protect against both UVA and UVB rays, provide better protection from solar ultraviolet-simulated radiation and natural sunlight-induced immunosuppression in human beings 4.
Factors Affecting SPF Efficacy
- The efficacy of sunscreen can be affected by various factors, including the amount applied, the frequency of re-application, and the presence of sweat or water 5.
- Re-applying sunscreen can provide additive protection, with two applications resulting in higher SPF values 5.
- The type of sunscreen filter used can also impact its efficacy, with some filters providing better protection against UVA rays 4.
Comparison with Other Sun Protection Methods
- Sun-protective clothing can provide superior UV protection compared to sunscreen, with fabrics blocking erythemogenic UVR better than sunscreens 6.
- The combination of SPF or UPF with % UVA-blocking may provide a more suitable measure for broad-spectrum protection 6.
- While sunscreen remains an important photoprotective modality, clothing should be considered the cornerstone of UV protection 6.