Sun Protection Factor (SPF) measures a product’s ability to protect skin from the sun’s ultraviolet (UV) radiation. The SPF number indicates how much longer it would theoretically take for skin to redden from sun exposure when using the product, compared to using no sunscreen. This metric focuses specifically on UVB rays, which are the primary cause of sunburn. As sunscreens carry very high SPF values, many consumers wonder what difference a number like 70 actually makes. This analysis clarifies the specific protection offered by SPF 70 and puts the high number into the context of comprehensive sun safety.
Defining SPF and the Blockage Percentage of SPF 70
SPF is primarily a measure of protection against Ultraviolet B (UVB) rays, which are the main cause of sunburn. The SPF number is determined by calculating the ratio of the time it takes for skin to burn with sunscreen applied versus without it. For instance, an SPF of 70 theoretically means it would take 70 times longer to get a sunburn than if the skin were unprotected.
The protection level is best understood through the percentage of UVB rays blocked. Sunscreens with an SPF 70 rating block approximately 98.5% of incoming UVB radiation. This means only 1.5% of the burning rays penetrate the skin when the product is applied correctly. The U.S. Food and Drug Administration (FDA) regulates these claims, allowing manufacturers to market high numbers, even though the practical difference between high SPFs is small.
The Real Difference Between SPF 30, 50, and 70
Analyzing blockage percentages reveals the concept of “diminishing returns” in sun protection. While the SPF number increases significantly from 30 to 70, the actual gain in blockage is minimal. An SPF 30 sunscreen filters out about 97% of UVB rays, which is considered a high level of protection.
Moving up to SPF 50, the protection level increases slightly, blocking about 98% of UVB radiation. The jump to SPF 70, which blocks approximately 98.5% of UVB rays, represents only a 1.5% increase in protection over SPF 30. This small margin illustrates that no sunscreen product can offer 100% protection against the sun’s rays. The marginal difference means SPF 70 is not shielded twice as effectively as SPF 35.
When High SPF is Appropriate
While SPF 30 is sufficient for daily use, high-number sunscreens like SPF 70 serve as a safety buffer in specific situations. The primary reason for choosing a high SPF is to compensate for human error in application. Most people apply far less sunscreen than the amount used in laboratory testing, which immediately lowers the effective protection rating. Using SPF 70 may ensure a person still receives protection equivalent to a correctly applied SPF 30 or 50, even if they under-apply.
High SPF products are also appropriate for individuals facing prolonged, intense sun exposure. This includes:
- Water sports or mountain climbing at high altitudes.
- Tropical vacations near the equator, where UV radiation is concentrated.
- Individuals with medical conditions (e.g., Lupus, history of skin cancer).
- Those using photosensitizing medications (e.g., certain antibiotics or retinoids).
These groups benefit from the slight edge in protection a very high SPF offers.
Why Relying Solely on a High SPF Number is Insufficient
The Sun Protection Factor is only one part of an effective sun safety strategy. The SPF number measures protection only against UVB rays, meaning a high number does not guarantee protection from Ultraviolet A (UVA) rays. UVA rays penetrate the skin more deeply and are associated with photoaging and an increased risk of skin cancer.
For complete defense, consumers must select a product labeled “Broad Spectrum,” which signifies protection against both UVA and UVB radiation. The high SPF rating loses effectiveness if the product is not applied generously and consistently. Experts recommend applying approximately two milligrams per square centimeter of skin, equating to about a full teaspoon for the face and neck alone. Consistent reapplication every two hours is non-negotiable, regardless of the SPF number, especially after swimming or sweating.