What Does SPF Really Mean?

Sunscreen is an essential part of any skin care routine, but with so many labels and numbers on bottles, it can be confusing to understand what SPF actually means and which is best. Many people assume that a higher SPF offers significantly better protection, but that’s not necessarily the case. Additionally, SPF only measures protection against UVB rays and not UVA rays, which are also harmful to the skin.

To help you better understand your sunscreen choices when shopping, this guide breaks down what SPF really means, how it’s calculated, and why choosing the right sunscreen is about more than just the SPF number.

SPF stands for Sun Protection Factor. It measures how well a sunscreen protects your skin from UVB rays , the type of ultraviolet radiation that causes sunburn. However, it does not measure protection against UVA rays , which penetrate deeper into the skin and contribute to premature aging and skin cancer.

SPF is essentially a time multiplier for how long you can stay in the sun before burning. For example, if your skin normally burns in 10 minutes, an SPF 30 sunscreen theoretically allows you to stay in the sun 30 times longer (or about 300 minutes) before burning. However, this is not a guarantee, as factors like sweating, swimming, and sunscreen application methods all impact effectiveness.

SPF ratings are based on controlled laboratory testing. To determine SPF, researchers apply a set amount of sunscreen to the skin and measure how much UVB radiation is needed to cause redness compared to unprotected skin. The amount of UVB exposure needed to turn the skin red is called the MED (minimal erythema dose).

Here’s a general breakdown of SPF protection levels:

• SPF 15 : Blocks 93% of UVB rays

• SPF 30 : Blocks 97% of UVB rays

• SPF 50 : Blocks 98% of UVB rays

• SPF 100 : Blocks 99% of UVB rays

As you can see, there is only a small difference in protection between SPF 30 and SPF 50, and even less between SPF 50 and SPF 100. This is why higher SPFs do not necessarily mean better protection.

Ultraviolet (UV) radiation from the sun is classified into two primary types that affect human skin: UVA and UVB. While both contribute to skin damage, they differ in their biological effects, wavelength penetration, and role in skin diseases.

UVA radiation, which has a longer wavelength (320–400 nm), penetrates deeper into the skin, reaching the dermis. This type of UV radiation is primarily responsible for photoaging, as it degrades collagen and elastin fibers, leading to wrinkles and loss of skin elasticity. UVA exposure also generates reactive oxygen species (ROS) , which contribute to oxidative stress and DNA damage. Although less energetic than UVB, UVA is implicated in the development of skin cancer due to its ability to cause DNA mutations through oxidative mechanisms. Notably, UVA rays are present throughout the day and can penetrate glass, making exposure a concern even indoors or while driving.

UVB radiation, with a shorter wavelength (280–320 nm), primarily affects the epidermis and is the primary cause of sunburn. It directly damages DNA by inducing the formation of cyclobutane pyrimidine dimers (CPDs), a type of mutation that can lead to carcinogenesis (3). Unlike UVA, UVB intensity fluctuates based on time of day, season, and latitude, with peak exposure occurring between 10am and 4pm. Chronic UVB exposure is also associated with the development of non-melanoma and melanoma skin cancers (4).

SPF only quantifies protection against UVB radiation and does not account for UVA exposure. This means that an SPF rating alone does not indicate comprehensive sun protection. To reduce the risk of both UVA and UVB-induced skin damage, it is essential to use broad-spectrum sunscreens containing active ingredients capable of filtering both types of radiation, such as zinc oxide, titanium dioxide, and/or avobenzone. 

It is very important that you use a broad spectrum sunscreen to ensure protection against both UVA and UVB rays. Sunscreens labeled as broad-spectrum have been tested to ensure efficacy against both types of radiation , making it more effective than single-ray sunscreens that solely focus on UVB protection.

In fact, an SPF 15 broad-spectrum sunscreen can offer better overall skin protection than an SPF 45 product that lacks UVA-filtering capabilities. Active ingredients such as zinc oxide, titanium dioxide, and avobenzone are among the most effective in providing broad-spectrum protection.

There are many types and formulations of effective broad-spectrum sunscreens to choose from. Some popular options include:

As noted above, the difference in sun protection between an SPF 50 and SPF 100 sunscreen is negligible. While there is nothing wrong with using a high-SPF sunscreen, the problem is that it can give people a false sense of security. In reality, all sunscreens need to be reapplied every 1 to 2 hours or after swimming or sweating, no matter the SPF number listed on the bottle. 

Additionally, you must still use the recommended amount of sunscreen (a shot glass full for the entire body), regardless of the SPF. Many people assume that  a high SPF means they do not have to apply as much. This is not true.

Finally, choosing a broad-spectrum sunscreen is the most important factor when picking a sunscreen, regardless of SPF.

No single sunscreen can protect your skin from 100% of every type of light. While wearing sunscreen is essential for every skin type and can greatly reduce your risk of skin cancer, it is best to combine great sunscreen habits with other sun protection and lifestyle habits for optimal skin health. These include:

• Seeking shade whenever possible, especially if you plan to be outdoors for long periods of time during peak hours. 
• Avoiding peak hours of sunlight between 10am and 4pm when possible. If you have to be outside during this time, find shady areas and wear sun-protective clothing.
• Wearing sun-protective clothing such as wide-brimmed hats, UV-blocking sunglasses, and thin, breathable fabrics over your arms and legs. There are now many brands like Coolibar that make attractive clothes for this specific purpose.
• Eating a diet rich in antioxidants like berries, citrus fruits, and leafy vegetables for added protection against oxidative stress.

SPF is an important factor when choosing a sunscreen, but it’s not the only thing to consider. Remember that SPF only measures UVB protection, so it is vital that you use a broad-spectrum sunscreen that blocks both UVB and UVA rays. Additionally, don’t be fooled into feeling a false sense of security if you are using a high-SPF sunscreen. No matter the type or SPF of sunscreen, it needs to be reapplied regularly when outdoors or inside near a window.

If you are having trouble finding a sunscreen that you like, make sure you know your skin type first. Some skin types do better with certain sunscreen formulations than others. 

1. Baumann L. Antiaging Ingredients in Ch. 37 of Baumann's Cosmetic Dermatology Ed 3. (McGraw Hill 2022)
2. Baumann, L. Ch. Cosmeceuticals and cosmetic Ingredients (McGraw Hill 2015)
3. Portillo-Esnaola M, Rodríguez-Luna A, Nicolás-Morala J, Gallego-Rentero M, Villalba M, Juarranz Á, González S. Formation of Cyclobutane Pyrimidine Dimers after UVA Exposure (Dark-CPDs) Is Inhibited by an Hydrophilic Extract of Polypodium leucotomos. Antioxidants (Basel). 2021 Dec 7;10(12):1961.
4. D'Orazio J, Jarrett S, Amaro-Ortiz A, Scott T. UV radiation and the skin. Int J Mol Sci. 2013 Jun 7;14(6):12222-48. doi: 10.3390/ijms140612222. PMID: 23749111; PMCID: PMC3709783.
5. Binks, B. P., Fletcher, P. D., Johnson, A. J., Marinopoulos, I., Crowther, J., & Thompson, M. A. (2017). How the sun protection factor (SPF) of sunscreen films change during solar irradiation. Journal of Photochemistry and Photobiology A: Chemistry, 333, 186-199.
6. Couteau, C., Paparis, E., El-Bourry-Alami, S., & Coiffard, L. J. M. (2012). Influence on SPF of the quantity of sunscreen product applied. International Journal of Pharmaceutics, 437(1-2), 250-252.