SPF stands for Sun Protection Factor. The SPF number indicates the level of protection against ultraviolet (UV) B rays. Sunscreens with a higher SPF offer greater protection from the sun.
SPF refers to the amount of UV radiation it takes to produce sunburn when wearing sunscreen in comparison to how much UV exposure it takes for unprotected skin to get sunburned. But, contrary to popular belief, SPF does not tell you how long you can stay in the sun without getting burned. Sunscreen Material Skylight Shades Fabric
This article explains what SPF numbers mean. It discusses how SPF is measured and how to determine the right level of protection for your skin. It also offers advice for other ways to protect your skin from UV damage.
The SPF number is determined through indoor experiments that expose human subjects to a light spectrum meant to mimic the noontime sun (when the sun's rays are at their most intense).
Scientists first determine a test subject's minimal erythema (mild sunburn) dose (MED)—the amount of UV radiation (solar energy) exposure it takes to induce erythema within a few hours of exposure. The test is repeated at a later date with the subject wearing sunscreen.
The SPF value is determined by dividing the amount of UV radiation needed to cause sunburn on skin with sunscreen by the amount of UV light it takes to cause sunburn on unprotected skin.
It's a common misconception that SPF numbers refer to how long you can stay in the sun. A higher SPF value increases your protection, but it's not a straightforward mathematical equation.
SPF measures the quantity of UV radiation, not exposure time. This is because the intensity of solar energy varies throughout the day. For example, you can get roughly the same amount of UV radiation between 9 and 10 a.m. as you would in 15 minutes of early afternoon sun.
Wearing SPF 30 doesn't mean you can stay in the sun for twice as long as an SPF 15. SPF 30 will offer twice as much protection from UV radiation as SPF 15, but that does not translate to double the time.
In addition, an individual's threshold for UV exposure varies from person to person. Pale skin absorbs more UV light than dark skin and, therefore, burns more easily.
This is due to a pigment (melanin) in the outer layer of skin that shields against UV rays. The more melanin in the skin, the darker the skin tone—and better protection from the sun.
While pale skin burns faster and more noticeably than darker skin tones, Black skin can still get sunburns that result in warm skin, pain, peeling, and skin damage.
Regardless of your skin type, the American Academy of Dermatology recommends applying an SPF 30 or higher on all areas of exposed skin daily. Reapply sunscreen every two hours when outdoors, as well as after swimming or sweating.
The SPF only indicates the level of protection against the sun's ultraviolet B rays, which are called UVBs for short.
Initially, UVB rays were thought to be the only UV rays to worry about, since they are shorter in length and cause sunburn. However, ultraviolet A (UVA) rays also pose risks. They age the skin and contribute to skin cancer. SPF alone does not protect against UVA rays.
In order to get the best sun protection possible, look for a sunscreen that provides both UVA and UVB protection.
Look for a sunscreen that says "broad spectrum" or "full spectrum" on the label and that has an SPF of at least 30 to protect your skin from both UVA and UVB rays.
Yes, higher SPF does matter, but it is unclear where the threshold lies. A sunscreen with a higher SPF rating blocks a greater percentage of UV light.
However, as SPF ratings increase, the added benefits shrink. For example:
Sunscreens with higher SPF ratings are typically more expensive than those that offer less protection. SPF 30 is generally considered the breakpoint, where higher SPF provides only marginally better protection. Depending on your skin, this may not be meaningful enough to justify its cost.
When used properly, SPF 30 should provide adequate sun protection for most people. This means applying about a shot-glass amount of sunscreen 15 minutes before going outside and reapplying it every two hours and after swimming.
However, few people use sunscreen correctly. Under typical-use conditions, research shows SPF 100 provides significantly more protection against sunburn than SPF 50.
Wearing sunscreen with an adequate SPF, and doing so properly, is key to protecting your skin. But the clothing you wear is also important.
Generally speaking, long sleeves and pants are more protective than tank tops and shorts. But even if you're covered in clothing from head to toe, there is a chance that the sun can make its way right through fabric fibers.
When some fabrics are examined under a microscope, especially if they are made from fibers knitted or woven together, space between fibers is visible. UV rays can penetrate through those spaces and reach the skin. The more tightly knit a fabric is, such as denim, the less likely UV rays can get through. The less tightly knit a fabric is, such as linen, the more likely UV rays can get through.
If you opt for lightweight, summery fabrics, you should still apply sunscreens even on parts of the body that are covered up.
Wearing sun-safe clothing is a great additional measure you can take beyond wearing sunscreen to help protect your skin from the sun. It's especially a great idea for young, active children who may have trouble sitting still for a few minutes while a parent tries to apply sunscreen, and who are often in and out of a pool, lake, or ocean.
UPF clothing is made with fabric that protects the skin from the sun. UPF stands for ultraviolet protection factor, a rating that indicates what fraction of the sun’s UV rays can penetrate the fabric. For example, if you are wearing a shirt with a UPF rating of 50, that shirt allows for 1/50th of the sun’s UV radiation to reach the skin underneath the shirt.
One example of a brand that specializes in this type of UPF clothing is Coolibar, which makes clothing and accessories for adults and children. UPF clothing is becoming increasingly more common, especially in kids' clothing and beachwear items like rash guards.
Be picky about which type of sun-safe clothing you buy. A long-sleeved shirt covers more skin and provides more sun protection than a tank top, and a floppy, wide-brimmed hat shields the face better than a baseball hat.
While using quality sunscreen and wearing appropriate clothing can protect you from the sun, you should also avoid working outside during peak daylight hours.
Seek shade between 10 a.m. and 4 p.m. when the sun is at its strongest. You are more likely to get skin damage during these hours, especially with sensitive skin.
Sun protection is not for summer only. Sunscreen should also be worn in the winter, spring, and fall.
You should also wear sunscreen indoors and while driving because the sun can penetrate through windows. In your car, transparent window film screens can block out the sun's rays. At home, draw the blinds closed during peak sunlight hours.
Heckman CJ, Chandler R, Kloss JD, et al. Minimal erythema dose (MED) testing. J Vis Exp. 2013;(75):e50175. doi:10.3791/50175
American Cancer Society. How Do I Protect Myself From Ultraviolet (UV) Rays?
U.S. Food and Drug Administration. Sun protection factor.
Fajuyigbe D, Verschoore M. Sun exposure and black skin. Curr Probl Dermatol. 2021;55:62-71. doi:10.1159/000517594
American Academy of Dermatology. Sunscreen FAQs.
U.S. Food and Drug Administration. Sunscreen: how to help protect your skin from the sun.
Environmental Working Group. The trouble with SPF.
Williams JD, Maitra P, Atillasoy E, Wu MM, Farberg AS, Rigel DS. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: Results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol. 2018;78(5):902-910.e2. doi:10.1016/j.jaad.2017.12.062
Skin Cancer Foundation. Sun-Protective Clothing.
American Academy of Dermatology. Prevent Skin Cancer.
By Heather L. Brannon, MD Heather L. Brannon, MD, is a family practice physician in Mauldin, South Carolina. She has been in practice for over 20 years.
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