Understanding Sun Protection Factor

SPF stands for sun protection factor. Simply put, an SPF rating tells you how long you can stay in the sun without getting burned while wearing that sunscreen, compared with how long you can stay in the sun before you burn without wearing that sunscreen. For example, if it typically takes you 15 minutes to burn without sunscreen and you apply an SPF 10, it will take 10 times longer (2.5 hours) to burn in the sun.

How SPF Is Determined

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). Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the SPF. 

What the SPF Number Means

A higher SPF doesn't indicate superior sun protection—it indicates that you will remain protected in the sun for a longer amount of time.

For example, an SPF 2 product protects your skin just as effectively as one with an SPF of 30. However, SPF 2 sunscreen will need to be applied more frequently.

To be safe, no matter what SPF you choose, it's best to reapply sunscreen at least every two hours, as well as after swimming or sweating. In fact, the American Academy of Dermatology recommends daily application of SPF 30 to all exposed skin.

UVA vs. UVB Rays

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.

What Else You Wear Matters, Too

Wearing sunscreen with an adequate SPF, and doing so properly, is key to protecting your skin. But the clothing you wear is also important.

Regular Clothing

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 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 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.

Sun-Safe Clothing

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 skin from the sun. UPF stands for ultraviolet protection factor, a rating that indicates what fraction of the sun’s UV rays can penetrate fabric. For example, if you are wearing a shirt with a UPF rating of 50, that shirt allows for 1/50^th of the sun’s UV radiation to reach 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. 

A Word From Verywell

While a quality sunscreen and 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 peak. You are more likely to get skin damage during these hours, especially with sensitive skin. Even in the winter, sun protection is still important. You should also protect yourself indoors and while driving because the sun can penetrate through windows. In your car, transparent window film screens can block out the sun rays; at home, draw blinds closed during peak sunlight hours.