What Does the SPF Number Mean?

Let’s clarify a prevalent misunderstanding regarding the Sun Protection Factor (SPF) in sunscreen. A common belief is that if it takes one hour for unprotected skin to burn, then an SPF of 15 allows for 15 hours of sun exposure without burning. Thus, a sunscreen with SPF 100 would theoretically permit nearly three and a half days in the sun without risk of sunburn. This assumption is incorrect.

The SPF rating does not indicate how long an individual can remain in the sun without burning. It's crucial to remember that all sunscreens lose effectiveness after about two hours and require reapplication. Essentially, SPF measures the level of protection against sunburn; the higher the SPF, the less UV radiation penetrates the skin.

The filtering capabilities of various SPF levels are as follows:

• SPF 15: filters 93% of UVB rays
• SPF 30: filters 97%
• SPF 50: filters 98%
• SPF 100: filters 99%

In simple terms, an SPF 50 sunscreen allows approximately 2% of UVB rays to reach your skin, while an SPF 100 lets in about 1%. Although these differences may seem minor, they can have significant implications, especially considering that UVB intensity fluctuates based on factors like the time of day and geographical location.

For example, someone with fair skin may burn after about 40 minutes of sun exposure at 9 a.m. but could burn within 10-15 minutes around noon. On particularly sunny days, that 1% difference can become quite meaningful over several hours. This is especially true for individuals with sensitive skin or a history of skin cancer.

Another critical point to consider is that SPF testing involves applying a specific amount of sunscreen—2 milligrams per 2 square centimeters of skin. This standardization ensures that results are comparable across different products.

To illustrate:

• The average male requires roughly 36 ml (about 1.2 ounces) to cover his entire body.
• The average female needs around 32 ml (approximately 1 ounce).

However, if a person applies the recommended amount of sunscreen (1 ounce) and re-applies it every two hours, a standard 3-ounce bottle would only last half a day at the beach. In reality, most individuals apply only about half of the suggested amount, which can drastically reduce the SPF’s effectiveness.

Higher SPF: An Effective Choice?

Some experts propose that using a higher SPF may compensate for the common issue of insufficient application, which aligns with my own perspective. Nevertheless, this has not been extensively studied. A 2011 summary statement from the U.S. Food and Drug Administration (FDA) raised concerns about higher SPF values, stating that there is currently insufficient evidence to demonstrate a clinical benefit of SPFs above 50.

A research study sought to determine if SPF 100 sunscreens outperform those with lower ratings in real-world scenarios. The findings, published in the Journal of the American Academy of Dermatology, involved 199 participants engaged in skiing and snowboarding, averaging six hours of sun exposure. Half of their faces received SPF 100, while the other half received SPF 50. The results indicated:

• More than half of the subjects experienced greater sunburn on the side with SPF 50 compared to SPF 100.
• Only 5% had worse sunburn with SPF 100 than with SPF 50.

The authors concluded that SPF 100 sunscreen demonstrated a notable clinical advantage over SPF 50 in preventing sunburn under actual usage conditions. Similar findings from other studies have shown that SPF 85 offers superior protection compared to SPF 50.

If you're going to the effort of applying sunscreen, opting for the highest SPF is a sensible choice, especially since the cost difference is often minimal and some evidence suggests enhanced protection—though further research is needed. However, if SPF 100 remains unused on your shelf because you dislike its texture or appearance, then SPF 50 is perfectly acceptable.

Historical Context of Sunscreen

For those who find sunscreen unpleasant, consider the experiences of U.S. soldiers during World War II. Benjamin Green, a military soldier, invented a thick, petrolatum-based sunscreen known as Red Vet Pet, which soldiers used to shield themselves from harsh sun exposure. Although Coppertone later refined this product, it was far from comfortable.

The American Academy of Dermatology recommends using broad-spectrum sunscreen with an SPF of 30 or higher. While they acknowledge that higher SPF sunscreens provide slightly better protection, SPF 30 is deemed the minimum effective level for adequate sun safety when applied correctly.

UVA vs. UVB: The Other Side of Protection

Regardless of the SPF you choose, remember that this number only reflects UVB protection and does not account for UVA radiation. Both types of rays can damage DNA in skin cells, contributing to skin cancer and premature aging.

UVB rays are responsible for sunburn and play a critical role in developing skin cancer, while UVA rays penetrate deeper into the skin, causing aging and wrinkles.

Although the SPF rating system for UVB was established in 1974, it wasn’t until 1986 that the FDA approved an ingredient for UVA protection, avobenzone. In 1997, this ingredient was allowed in sunscreen products. However, it wasn’t until 2011 that the FDA introduced new regulations requiring verification of UVA protection in sunscreens, allowing them to be labeled as “Broad Spectrum SPF (value)” if they passed the FDA’s tests.

UVA rays are often referred to as the “silent killers” because they don’t cause sunburn, leading many to underestimate their impact. Sunscreens manufactured prior to 2011 may have given a false sense of security, allowing individuals to stay in the sun longer without burning while being exposed to harmful UVA rays.

In Europe and Asia, a PA grading system indicates UVA coverage levels:

• PA+: Some protection
• PA++: Moderate protection
• PA+++: High protection
• PA++++: Extremely high protection

For optimal skin protection against UVA damage and photoaging, selecting a sunscreen with a PA+++ or PA++++ rating is advisable. In Europe, sunscreens with a UVA symbol in a circle signify that UVA protection is at least one-third of the UVB protection. Higher SPF ratings correspond to better UVA protection, further reinforcing the importance of choosing higher SPF products.

Ultimately, the most crucial factor is consistent sunscreen application. If you have a history of skin cancer, possess sensitive or fair skin, or are engaging in high-altitude activities or vacations near the equator, opt for higher SPF sunscreens.

Additionally, take other sun safety measures: wear long sleeves when feasible, don a hat, seek shade, and avoid sun exposure between 10 a.m. and 2 p.m. when UV radiation is strongest. If you find sunscreen disagreeable, remember, you’re not alone—at least you're not using a mix of rice bran, jasmine, and lupine like the ancient Egyptians did.