What Is SPF in Sunscreen? - Sun Protection Factor Explained

In the world of skincare, few topics get as much attention as the importance of sun protection. Central to this discussion is the concept of Sun Protection Factor, more commonly known as SPF. This metric is not just a number on your sunscreen tube; it's a critical indicator of how well a sunscreen can protect your skin from the harmful effects of the sun's ultraviolet B (UVB) rays. In this guide, we cover: What is SPF? How is determined and How do you choose your sunscreen's SPF?

What is SPF?

SPF stands for Sun Protection Factor and is a measure of how well a sunscreen will protect skin from UVB rays, the kind of radiation that causes sunburn, and photoaging, and can contribute to skin cancer. SPF rating is a theoretical calculation that helps you understand how long you can be exposed to the sun before getting sunburned. For example, if you use sunscreen with an SPF of 50, theoretically, you can stay in the sun 50 times longer than you could without protection before you start to burn.

However, this calculation is based on applying sunscreen generously and evenly, which many people do not do in practice. Real-world effectiveness can be significantly lower, which is why it is recommended to reapply sunscreen every two hours, especially after swimming or sweating.

How Is SPF Determined?

The calculation of the SPF value is achieved through a simple equation. This involves dividing the duration it takes for a designated skin section to start reddening with sunscreen on by the time it takes to show similar redness without sunscreen protection. For example, if it takes 500 seconds for the skin to begin to burn with sunscreen, compared to 10 seconds without it, dividing 500 by 10 gives us 50. Hence, the SPF of the sunscreen is determined to be 50.

Does Higher SPF Matter?

Yes, higher SPF does matter, but with some nuances to consider. SPF is a measure of how well a sunscreen can protect the skin from UVB rays, which are the primary cause of sunburn and contribute to skin cancer. Here’s a breakdown of why higher SPF matters, alongside some important considerations:

Enhanced Protection: Higher SPF's Mean More Time without Being Burnt

• Longer Protection Time: Higher SPF provides longer protection against UVB radiation. For instance, SPF 50 sunscreen blocks about 98% of UVB rays, while SPF 30 blocks approximately 97%. This difference seems small, but it can be significant during extended exposure to the sun.

Things to keep in mind:

• Diminishing Returns: The increase in protection becomes less significant as the SPF value rises. For example, the difference in UVB protection between SPF 30 and SPF 50 is relatively minor compared to the difference between SPF 15 and SPF 30.

• False Sense of Security: High SPF products may lead individuals to believe they can stay in the sun longer without reapplying sunscreen or taking other sun protection measures. This behavior can increase the risk of sun damage.

• Broad Spectrum Protection: It’s important to choose a "broad-spectrum" sunscreen that protects against both UVA (which can lead to premature aging and skin cancer) and UVB radiation. High SPF values do not indicate protection against UVA rays unless the sunscreen is labeled broad spectrum.

• Application Matters: For any SPF ratings to provide adequate protection, proper application is crucial. Sunscreen needs to be applied generously and evenly across all exposed skin and reapplied every two hours, or more frequently if swimming or sweating.

Why a Higher SPF is not necessarily better

While it might seem intuitive to assume that a higher SPF equates to better protection from UV rays, this isn't always the case. The decision to use a higher SPF sunscreen should be informed by understanding its benefits and limitations. Here's why a higher SPF might not necessarily be better:

Diminishing Returns on Protection

• Marginal Increase in UVB Protection: The difference in UVB protection between high SPF sunscreens is relatively small. For example, SPF 30 blocks approximately 97% of UVB rays, while SPF 50 blocks about 98%. The incremental benefit diminishes as the SPF value increases.

False Sense of Security

• Extended Sun Exposure: High SPF sunscreen can lead individuals to believe they can stay in the sun longer without reapplying sunscreen or seeking shade. This false sense of security can lead to more prolonged sun exposure, increasing the risk of sun damage and skin cancer.

• Inadequate Application: People often apply less sunscreen than required, assuming a high SPF sunscreen offers more protection. Proper application is essential; most adults need about one ounce of sunscreen to fully cover all exposed skin.

Broad Spectrum Protection is Key

• UVA Protection Overlooked: SPF does not measure protection against UVA rays, which penetrate the skin more deeply and are associated with aging and skin cancer. High SPF sunscreens might not offer proportionate UVA protection unless they are labeled as "broad spectrum."

Practical Considerations

• Cost and Comfort: Higher SPF sunscreens can be more expensive and might not offer proportional value for the extra protection they claim to provide. They can also be thicker or greasier, making them less comfortable to wear, especially during daily use.

Is it Better to Use SPF 30, 50, or 100?

Choosing between SPF 30, SPF 50, and SPF 100 sunscreen involves understanding how the Sun Protection Factor (SPF) works and what it means for your skin's protection. Each SPF number has its benefits and considerations. Here’s a breakdown to help decide which SPF might be better for you:

SPF 30

• Protection Level: Blocks about 97% of UVB rays.

• Suitability: SPF 30 is often recommended for daily use if you spend a few minutes to a couple of hours outside and have moderate sun exposure. It's suitable for most people under typical conditions.

• Reapplication: Like all sunscreens, it should be reapplied every two hours, especially if you are sweating or swimming.

SPF 50

• Protection Level: Blocks about 98% of UVB rays.

• Suitability: Offers a higher level of protection and might be preferable for those with fair skin, a history of skin cancer, or conditions like photosensitivity. It's also a good choice for activities like hiking, swimming, or skiing, where you're exposed to the sun for extended periods.

• Reapplication: Needs to be reapplied every two hours or after swimming or sweating to maintain its effectiveness.

SPF 100

• Protection Level: Blocks about 99% of UVB rays.

• Considerations: While it offers the highest protection among the three, the difference in protection between SPF 50 and SPF 100 is relatively small. There is a concern that high SPF sunscreens might lead users to feel overconfident and neglect other sun safety measures or fail to reapply sunscreen as needed.

• Suitability: Might be chosen for extended outdoor activities in highly reflective environments, like snow or water, where the sun’s effect is intensified.

SPF measures sunscreen protection from UVB rays only NOT UVA

SPF, or Sun Protection Factor, specifically measures the level of protection a sunscreen offers against UVB rays, which are primarily responsible for causing sunburn and play a significant role in developing skin cancer. The SPF rating indicates how much longer you can be exposed to UV radiation without getting sunburned when using the sunscreen as directed compared to not using any sunscreen. For example, if you use an SPF 30 sunscreen properly, it means you can theoretically stay in the sun 30 times longer than without any protection before you start to burn.

However, it's important to note that SPF does not measure protection against UVA rays, which penetrate the skin more deeply than UVB rays and are also harmful. UVA rays can lead to premature skin aging, and wrinkles, and contribute to skin cancer. For comprehensive sun protection, it's recommended to use broad-spectrum sunscreens that protect against both UVA and UVB

What Is the difference between SPF and the PA+ system? 

The SPF and the PA+ systems are both measures designed to indicate the level of protection a sunscreen offers, but they focus on different aspects of sun damage.

SPF Rating

Focus: Measures protection against UVB rays, which are primarily responsible for sunburn and contribute to skin cancer.

Scale: SPF values range from as low as 15 to as high as 100+. The number indicates how long UVB rays would take to redden the skin when using the sunscreen exactly as directed compared to unprotected skin. For example, an SPF 30 sunscreen theoretically allows you to stay in the sun 30 times longer without getting sunburned.

Global Use: SPF is widely used around the world and is recognized as the standard for measuring UVB protection.

PA+ System

Focus: Measures protection against UVA rays, which penetrate the skin more deeply than UVB rays are primarily responsible for aging, and can contribute to skin cancer.

Scale: The PA system is based on the Persistent Pigment Darkening (PPD) response reading at 2-4 hours of sun exposure. Protection grade is often indicated by plus signs (+), with more plus signs indicating higher protection: PA+ signifies some UVA protection; PA++ indicates moderate protection; PA+++ signifies high protection, and PA++++ indicates extremely high UVA protection.

Regional Use: The PA+ rating system is commonly used in Asia and is a standard for measuring UVA protection.

Key Differences between SPF and PA+

Type of Protection: SPF focuses on UVB rays, while PA+ focuses on UVA rays.

Measurement Scale: SPF uses a numerical scale (the higher the number, the greater the protection), whereas PA+ uses a plus sign system to indicate levels of UVA protection.

Does How Much Sunscreen I Put On Impact the Rating?

Yes, the amount of sunscreen you apply significantly impacts the effectiveness of the SPF rating. The Sun Protection Factor (SPF) rating on sunscreen products is determined under controlled conditions, with a specific amount of sunscreen applied to the skin (2 milligrams of sunscreen per square centimeter of skin). However, in real-life usage, many people apply less than this recommended amount, which can substantially reduce the level of protection.

Key Points on Sunscreen Application and SPF Effectiveness:

• Underapplication: If you use less sunscreen than the recommended amount, you won't receive the full SPF protection advertised on the product. For example, applying half the required amount can reduce the protection exponentially, making an SPF 30 product function more like an SPF significantly lower than 15.

• Proper Coverage: Achieving the advertised SPF requires generous and even coverage over all exposed areas of the skin. Missed spots or thin application can leave skin vulnerable to UV damage.

• Volume for Full Body Coverage: For an adult, approximately 1 ounce (about a shot glass full) of sunscreen is needed to cover the body adequately and achieve the stated level of SPF protection.

• Reapplication: Sunscreen needs to be reapplied every two hours, or more frequently if swimming, sweating, or towel drying, to maintain effective protection. This reapplication is crucial, as sunscreen can wear off, dilute, or be removed through activity.

Additional Ways to Protect Yourself

Besides using sunscreen with an adequate SPF, there are several other strategies you can employ to protect yourself from the harmful effects of the sun's ultraviolet (UV) rays. These methods are crucial for comprehensive sun protection from UV rays, especially during peak sun intensity hours (10 a.m. to 4 p.m.). Here are additional ways to safeguard your skin:

1. Wear Protective Clothing

• Long-Sleeved Shirts and Pants: Opt for tightly woven fabrics that block more sun than thinner materials. Some clothing comes with a UV protection factor (UPF) rating, which indicates how effectively it shields the skin.

• UPF Hats: Wear a wide-brimmed hat that covers not only your face but also your neck, ears, and scalp.

• UV Sunglasses: Protect your eyes and the sensitive skin around them with sunglasses that provide 100% UV protection. Look for wraparound styles that block rays from entering sideways.

2. Seek Shade

• Whenever possible, stay under shade, especially during the midday hours when the sun's rays are the strongest. Use umbrellas, trees, or other shelters to reduce your exposure to harmful UV rays.

3. Avoid Peak Sun Hours

• If you can, plan your outdoor activities for the early morning or late afternoon when the sun's rays are less intense.

4. Use Extra Caution Near Water, Snow, and Sand

• These surfaces can reflect the sun’s rays and increase the risk of sunburn. Be mindful of your exposure and apply protective measures diligently in these environments.

5. Monitor the UV Index

• The UV Index provides a daily forecast of the expected risk of overexposure to the sun. Use this information to plan your outdoor activities in a way that reduces your exposure to harmful UV radiation.

6. Apply Sunscreen Properly and Regularly

• Even on cloudy days, up to 80% of the sun’s UV rays can penetrate the clouds, so apply sunscreen generously to all exposed skin. Remember to reapply every two hours, or more often if swimming or sweating.

7. Use Extra Protection for Children

• Children's skin is more sensitive to the sun. Dress them in protective clothing, hats, and sunglasses, and apply sunscreen regularly. Keep babies younger than six months out of direct sunlight whenever possible. Here are some tips on getting your kid used to sunscreen.

8. Be Mindful of Medications

• Some medications can increase your sensitivity to the sun. Consult your healthcare provider or pharmacist about your medications and take extra precautions if necessary.

By integrating these protective measures with the use of broad-spectrum sunscreen, you can significantly reduce your risk of sunburn, premature skin aging, and skin cancer, ensuring your skin remains healthy and protected under the sun.