SPF is one of the most important skincare ingredients for every skin type. If left unchecked, ultraviolet (UV) rays from the sun cause an array of skin problems, from accelerated signs of aging to skin cancer. However, many people do not know what SPF means, how it is measured, and what SPF number to use.
This article will explain the details of SPF measurements, broad-spectrum and water-resistant sunscreens, chemical vs. physical sunscreens, and tips for choosing the best sunscreen for you.
How SPF Is Measured
Sun protection factor, or SPF, measures how much UV exposure is needed in order to burn the skin. This number is the MED, or minimal erythema (redness) dose. Therefore, a higher SPF value generally equates to more sun protection.
The SPF number of a product is determined by applying 0.4 ounces or 2 mg/cm2 of sunscreen on the skin. Next, artificial UVB light is shined on the skin. Researchers then measure the level of sunburn on the skin after this protected exposure. The SPF value indicates how much more UVB light can be exposed to the skin before redness occurs.
For example, someone wearing SPF 15 can be exposed to 15 times the amount of UVB light than someone wearing no SPF at all before redness will occur. Compare this to someone wearing SPF 30, which means that they can withstand 30 times the amount of UVB light before burning than someone wearing no SPF.
However, studies have found that SPFs higher than 50 offer a negligible amount of added UVB protection. For example, SPF 50 blocks 98% of UVB rays, while SPF 100 blocks 99% of UVB rays. Thus, SPF 100 does not offer double the protection as SPF 50, as one might assume. The problem with this is that people who use very high SPFs may have a false sense of security, believing that they can stay out in the sun for twice as long than if they were to wear SPF 50.
I recommend that you wear at least SPF 15 on a daily basis, and SPF of 30 or higher when you will be outdoors for 30 minutes or more.
How Much Sunscreen Do You Need to Use?
Another common problem with sunscreen use is that many people do not apply enough to reach the SPF number listed on the bottle. You must use at least ¼ teaspoon of SPF on your face, and one ounce (a shot glass) on your body in order to get the proper amount of SPF.
Additionally, you must reapply sunscreen at least every hour and after swimming or sweating profusely. Otherwise, the amount of sun protection on your skin will significantly decrease throughout the day, leaving your skin unprotected.
What Does “Broad-Spectrum Sunscreen” Mean?
The measurement for SPF outlined above only applies to UVB protection. There is not currently a standard of measurement in the United States that applies to UVA protection. This is why you want to look for sunscreens that say they offer “broad-spectrum protection,” which includes both UVA and UVB protection.
UVB light has a shorter wavelength, so it does not penetrate as far into the skin as UVA light. Therefore, UVB light is responsible for the superficial symptoms of a sunburn that you see, such as redness and peeling. UVA light, on the other hand, can penetrate deeper into the skin, causing accelerated aging, dark spots, and skin cancers. This is why it is crucial that your sunscreen offers both UVA and UVB protection.
Do You Need Water-Resistant Sunscreen?
Water-resistant sunscreens are tested differently than non-water-resistant sunscreens. You only need to use a water-resistant product if you will be submerged in water, such as swimming or in a hot tub, or if you will be excessively sweating such as running or playing sports outside. Keep in mind that you still need to reapply water-resistant sunscreen – testing for these products labels them as either “water-resistant for 40 minutes” or “water-resistant for 80 minutes.”
If you will not be swimming or sweating profusely, you do not need to use a water-resistant sunscreen. You can read more about the testing process for water-resistant sunscreens in this blog.
Chemical vs. Physical Sunscreen
There are two main types of sunscreens: chemical and physical. Chemical sunscreens work by absorbing UV rays, then converting them to heat. This prevents the UV rays from penetrating the skin and causing cellular damage.
Physical sunscreen works by creating a physical barrier between your skin and UV rays. These ingredients reflect the UV rays off your skin, preventing them from penetrating the skin and causing damage.
Chemical sunscreen ingredients such as oxybenzone and octinoxate have recently come under fire for possibly causing damage to coral reefs and having unwanted hormonal effects on the user. Another ingredient commonly found in chemical sunscreens, avobenzone, can cause stinging and irritation.
Physical sunscreens, on the other hand, do not come with these risks, but they can be less spreadable and may leave a white film on the skin. Zinc oxide and titanium dioxide are the active ingredients used in most physical sunscreens.
My personal recommendation is to use a physical sunscreen such as EltaMD Physical Broad-Spectrum SPF 41 whenever possible. However, if chemical sunscreen is all that is available, it is still safer to use than going without any sun protection at all.
Bottom Line
Using the right type, amount, and SPF of sunscreen is a vital step in your skincare regimen, no matter your skin type. Never rely solely on SPF added to makeup or moisturizers to protect your skin – these can be used in conjunction with a standalone sunscreen.
Choose a sunscreen that will work for your budget and personal preferences so that you will use it daily and will apply the proper amount. Do not choose an expensive sunscreen that you will want to use sparingly. There are many options available, so find one that you like and use it regularly!
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