30, 40, or 50: What SPF Value Should I Use?

Everyone should wear sunscreen daily, regardless of age or race. Learn about the differences between SPF values and how to choose the best one for daily wear.

By Valerie George | 9 Min. Read

February 5, 2023

Everyone, regardless of age or race should wear sunscreen daily. Even darker skin tones are at risk for skin cancer and premature aging. While sunscreen is typically only thought of during trips to the beach or sunny days at the park, wearing an SPF sunscreen daily is proven to reduce the risk of skin cancer.1 Our skin is not just exposed to ultraviolet light from the sun during warm summer days vacations—our skin sees significant amounts of ultraviolet light that impacts our skin just by innocent, daily outings, called incidental exposure.2 Understanding what SPF factor means and how it will protect skin from incidental exposure will help you select the right SPF number to protect your skin over daily use.

What is sun protection factor?

SPF stands for Sun Protection Factor, the level of solar energy required to produce a sunburn on protected skin versus unprotected skin. Simply put, it is a measurement of how well an SPF sunscreen is working to prevent skin from burning from the sun. The SPF value is usually indicated by a number on the front of the bottle. The higher the sun protection factor number, the more protection skin has from burning.

What is the difference between SPF 30, 40 and 50?

People often make the mistake of thinking the SPF number is the amount of time one can stay in the sun without getting burned. If you typically burn in 10 minutes without sunscreen, wearing an SPF 30 will protect your skin for thirty times that—300 minutes. That’s simply not the case.

According to the US Food and Drug Administration, one hour of solar exposure at 9:00AM is the same as 15 minutes of solar exposure at 1:00PM, when the sun is the brightest.3 Since SPF is a measure of how much protection there is from solar energy reaching skin, we have to think of SPF in this way, not in time.

For example, an SPF 30 only allows 1/30th of UVB rays to reach skin, compared to skin that’s not protected. This means 1/30th, or 3.33%, of sun’s rays reach sunscreen-wearing skin, as opposed to 100% of rays reaching unprotected skin. Likewise, an SPF 40 allows 1/40th, or 2.5% of UVB rays to reach skin, and an SPF 50 allows 1/50th, or 2% of UVB rays to reach skin.4

What about UVA rays?

The SPF value only takes into consideration protection from the sun’s rays in the UVB spectrum. Since the sun produces three types of rays, UVA, UVB and UVC (which we don’t need to worry about), we need to look for another indicator of full sun protection: broad-spectrum.

A broad-spectrum sunscreen protects skin from both UVB and UVA rays, whereas a sunscreen that is not rated broad spectrum only protects from UVB rays. Look for the words “broad spectrum” on a sunscreen bottle, next to the SPF number, to know if a sunscreen provides this broad protection.

Alternatively, check if the sunscreen contains a broad-spectrum SPF UV filter, like zinc oxide, since these protect skin from both UVB and UVA rays. Since SunsolveMD only uses zinc oxide, all SunsolveMD sunscreens offer broad protection.

How is SPF tested?

In the United States, SPF testing is typically conducted in vivo, or on living tissue. This is usually in the form of human volunteers, which are financially compensated to test various sunscreens for their sun protection factor. Volunteers typically have skin phototypes of I, II, or III, which are typically Caucasians or people of Northern European descent. This lighter skin allows erythema, or redness, which is an indicator of UV exposure, to be easily visualized, which is a key metric for this test.

In a laboratory setting, the test SPF product is applied to the back of 10 or 20 volunteers in a small area, applying the sunscreen in a thickness of 2mg/cm2 of skin. In a neighboring spot, next to the sample, a control SPF is applied, which is a sunscreen formulation with a known SPF value. Finally, a third neighboring area does not have any SPF sunscreen applied, and its bare skin is simply marked.5

The skin is then exposed to a specific radiation of UV light, which includes both UVA and UVB rays, for specific time intervals. At each interval, a trained observer evaluates the three test areas for redness, assigning an SPF value once the SPF areas appear burned. The test SPF product’s value is compared to the control SPF to determine the validity of the test. The test SPF product finally is assigned a number.

Is there a test for broad-spectrum sunscreen?

In addition to requiring use of a broad-spectrum SPF UV filter, broad-spectrum sunscreen also needs to undergo its own testing. Since UVA rays don’t cause burning, the in vivo testing for UVB protection can’t be used. In the US and EU, the sunscreen will undergo a test called critical wavelength. This is an in vitro, or lab test, that measures the ability of the sunscreen to work at the same wavelength as UVA rays; this is called the critical wavelength and occurs at 370nm. If a sunscreen passes irradiation with this wavelength, it can earn the label broad spectrum.

What are the pros and cons of each SPF factor?

The benefits of wearing sunscreen are indisputable. Without SPF, our skin would be exposed to the sun’s harmful rays, causing DNA damage and—eventually—skin cancer. The American Academy of Dermatology projects that 1 in 5 Americans will developer skin cancer in their lifetime.6 That’s over 66 million Americans based on the 2020 census! Wearing an SPF sunscreen reduces the risk of various skin cancers and the damage done to our skin cell’s DNA.

While the best sunscreen is one that you will wear, the AAD recommends a minimum SPF 30 for daily use.6 Sunscreens rated SPF 30 are generally lightweight and feel less greasy than higher SPF sunscreens, so they may be aesthetically pleasing. But remember our SPF calculations? This minimum-recommended sun protection allows 3.33% of the sun’s rays to reach our skin, or conversely, only protects our skin from 96.7% of the sun’s rays.

On the other hand, SPF 40 offers slightly more protection at filtering out 97.5% of the sun’s rays. Unfortunately, this SPF value is not as common as SPF 30 or SPF 50, which are more common.

An SPF 50 is the highest numerical SPF rating allowed for consumers and it protects skin from 98% of sun’s rays. While this doesn’t seem like a big deal compared to the 96.7% or 97.5% that SPF 30 and SPF 40 offer, respectively, every little bit makes a difference. This makes SPF 50 a great choice for high risk individuals, and those with very light skin.

One negative aspect people think of with SPF 50 is that it’s too heavy or opaque due to the zinc oxide. This is now a myth! Modern sunscreen formulations have come a long way. Some, like SunsolveMD, have been formulated to no longer leave the white cast or residue that’s historically undesirable with physical sunscreens.

How much do you need to apply?

No matter the SPF worn, application is key—even higher SPF values need proper application and reapplication. Since SPF values are determined by applying 2mg/cm2 to skin, this means approximately 30 grams, or 1 ounce of sunscreen, should be applied to the whole body. If you’re just applying sunscreen to the face, ears and neck, this is approximately two finger lengths’ worth of sunscreen.

If you aren’t sure if you applied enough sunscreen, try applying a second coat, which will guarantee you’ve covered any missed spots or thin areas. One study found that some consumers apply around 50% of the 2mg/cm2 application that was used during testing.7 This means they’re not benefiting from the full SPF value that sunscreens offer. Proper application (or a second coat!) will improve the protection being offered.

Sunscreens also need to be applied 15 minutes before sun exposure, as well as reapplied every two hours, per FDA guidelines.3 Since sunscreen wears off or becomes inactive over time, reapplication is key to maintain sun the advertised sun protection factor.

Does lifestyle play a role?

How active you are outdoors certainly plays a role in sunscreen selection and the application process. When doing physical activities such as exercising or swimming, perspiration or water could dissolve some of the sunscreen application and deplete the SPF being provided. This is why after swimming SPF should be reapplied.

Some sunscreens have a water-resistant claim in the packaging, which means the advertised sun protection factor remains during 40 or 80 minutes of activity. After the activity, SPF should still be reapplied.

Even if you don’t have an active lifestyle, wearing SPF during unobvious times is important, such as when indoors, on cloudy days and in winter, and at high altitudes, regardless of the weather or season.

Do I have to wear SPF at night?

While there is nothing wrong with wearing SPF at night, per se, it is clearly most beneficial during daytime, when UV light exposure is at its highest. However, new research into blue light exposure shows that blue light emitted from electronic devices may disrupt skin’s circadian rhythm.8 Wearing a broad-spectrum SPF may provide some protection in the blue light range. Once you’re ready for sleep, you can remove the SPF and use other repairing actives and properly moisturize the skin while you sleep, as this is when skin barrier repair is at its worst.

What other ingredients should be included in an SPF sunscreen?

Ultraviolet rays don’t just target DNA and damage it; they generate free radicals on skin that can wreak havoc. Choosing an SPF that protects from free radicals, like SunsolveMD Detox And Shield Mineral SPF50 is perfect when exploring the outdoors. It not only provides broad spectrum SPF 50, but offers anti-pollution benefits as well, thanks to the free radical quenching SunsolveMD TheSolve™ technology. Repairing the damage that occurs as it’s happening will also reduce the stress your skin is under. The Detox and Shield Mineral SPF50 also contains SunsolveMD SolveDNAReverse™, which utilizes a complex repair enzyme to reverse photo sun damage as it occurs.


Green A, Williams G, Neale R, et al. Daily sunscreen application and betacarotene supplementation in prevention of basal-cell and squamous-cell carcinomas of the skin: a randomised controlled trial [published correction appears in Lancet 1999 Sep 18;354(9183):1038]. Lancet. 1999;354(9180):723-729. doi:10.1016/S0140-6736(98)12168-2

Godar DE. UV doses of American children and adolescents. Photochem Photobiol. 2001;74(6):787-793. doi:10.1562/0031-8655(2001)074<0787:UDOACA>2.0.CO;2

Center for Drug Evaluation and Research. (n.d.). Sun Protection Factor (SPF). U.S. Food and Drug Administration. Retrieved February 8, 2023, from

Consumer Reports. (2015, May 15). What does SPF stand for? Retrieved January 25, 2023, from




Valerie George is an award-winning cosmetic chemist and speaker, science communicator, educator, leader, and avid proponent of transparency in the beauty industry.