What Is SPF: How It Works, Uses, Best SPF Level for You, and More

• The SPF factor in a given product is representative of how well that product will protect your skin from the sun’s harmful UVB rays.
• Consistent use of products with SPF prevents your skin from aging prematurely and reduces your risk of developing skin cancer. 
• SPF cannot protect you from UVA rays – only broad-spectrum sunscreen can.

Sunlight is partially made up of ultraviolet B (UVB) and ultraviolet A (UVA) rays – both of which can be damaging to the skin. SPF indicates how long it would take UVB rays to burn your skin when sunscreen is applied correctly, compared to wearing none at all.  

What does SPF stand for?

Sun protection factor (SPF) indicates how long sunscreen will protect your skin from sunburn. So, in theory, if your skin would usually burn after 10 minutes, using a sufficient amount of SPF 15 sunscreen would protect you from UVB rays for 15 times longer, to equal 150 minutes. This means, for the most part, the higher the SPF number, the longer your skin is protected from sunburn. 

In reality, the intensity of the sun’s rays varies throughout the day, and this affects the duration of SPF protection. Other factors, such as the product’s formulation and the quantity you apply, will also affect the level of your exposure to the sun’s rays. 

Aside from sunscreen, other types of skin care products may list SPF when photoprotective agents are built into them, including moisturizers, powders, foundations, hand creams and serums.

What Does SPF Do?

One goal of using SPF sunscreens is to reduce the risk of developing skin cancer. One in five Americans will develop skin cancer in their lifetime, and exposure to UV radiation is one of the major causes of, and risk factors associated with it. Sunscreens and other products with adequate SPF help reduce this risk.

Another goal is to prevent skin from aging prematurely. Long-term exposure to the sun’s UV rays accelerates the skin’s aging process. UV rays cause sunburns and damages vital proteins such as elastin and collagen, which are responsible for making skin smooth and appear youthful. 

This harm is known as photoaging and results in uneven pigmentation, age spots, wrinkles and drooping, leathery skin. Areas of the body such as the face, neck and hands usually show signs of aging sooner, as these are the areas most exposed to the sun.

Lastly, the sun’s rays worsen hyperpigmentation. When exposed to sunlight, the body produces extra melanin, the skin’s natural protection from overexposure to UV rays, resulting in these patches of skin darkening further. 

To avoid these results, SPF sunscreens should be used consistently as a preventative measure.

How does it work?

Protection from UV rays is achieved with either physical blockers or chemical barriers. Physical blockers reflect and scatter UV rays back outward using titanium oxide or zinc oxide as active ingredients. 

Chemical barriers work by absorbing the rays and converting them into heat before they can reach the skin and cause damage. Chemical sunscreens can contain a number of absorbers, but the most common ones are oxybenzone or avobenzone.

Some sunscreens combine both physical and chemical photoprotective agents in their formulations. 

How long does SPF protection last? 

Theoretically, SPF products should protect for as long as the SPF number indicates. If your unprotected skin would typically burn after 10 minutes of exposure to UVB rays, an SPF 30 product should prolong the time to 30 times longer.

The photostability of the product (how long it can stay stable under the effects of the sun), however, will affect how long it lasts. 

While the literature is lacking on the photostability of some commercial SPF products, there is some information available on the photostability of SPF sunscreens. Unfortunately, this information is often inconsistent. 

One study found that using inorganic chemicals to filter sunlight makes sunscreens more stable and longer lasting as opposed to organic filters. Another study found no significant difference between the two, and advocated for organic filters due to the potentially damaging results of inorganic ones.     

To avoid potential risks, most public health agencies and bodies recommend you reapply SPF every two hours. 

Some SPF products protect longer than others. Factors such as formulation and consistency affect longevity and the types of activities you engage in also affect protection; water-resistant products will last longer than other formulations if you go swimming or sweat a lot. 

What is broad-spectrum SPF?

Technically, there’s no such thing as broad-spectrum SPF; broad-spectrum sunscreen is the correct term. Unlike sunscreen with SPF that only protects your skin from UVB rays, broad- spectrum sunscreen protects you from both UVA and UVB rays. 

More UVA rays reach the earth’s surface than UVB. UVA rays penetrate more deeply into the skin than UVB rays because the skin’s natural UV protection is less adept at blocking them. 

UVA rays cause damage to skin cells and fibers, and reduce skin’s collagen production. They are responsible for more long-term skin damage such as wrinkles and loose skin, and they can even harm the body’s immune system.

For maximum protection, it is therefore best to opt for a broad-spectrum sunscreen. These contain not only SPF, but also blockers and absorbers that can shield the skin from UVA rays. 

What SPF Level is Best for You?

One factor in choosing an SPF product—especially an SPF sunscreen—is skin type. An important tool is the Fitpatrick skin type classification which determines sun-sensitivity based on skin colour.

This scale ranges from type I, very fair and likely to burn, to type VI, very dark and unlikely to burn. While it is recommended for all skin types to wear SPF 15 or above daily, a moderate use of these 15 SPF is generally adequate for mid-to-dark skin types. 

However, type I skin absorbs more UV rays than type VI, so while this type can apply low SPF products more frequently, they may also choose to use a higher SPF product that will last longer. 

SPF sunscreen chart 

Different SPF levels block different percentages of UVB rays. Although there is not a large difference between a 15 SPF and 50 SPF, higher SPFs will provide better protection. 

SPF Level	% of All UVB Rays Blocked
SPF 15	93.3%
SPF 30	96.7%
SPF 45	97.8%
SPF 50	98%

Are higher SPF sunscreens better?

While it is better to choose sunscreens with an SPF of 15 or higher, an SPF 50 sunscreen blocks about 98% of UVB rays. However, no sunscreen blocks 100% of UVB rays, therefore products with a higher SPF will not provide additional protection.

Products That Can Contain SPF

A variety  of products list SPF as an ingredient, including makeup and skin care products.

SPF products incorporate at least one active ingredient (blocker or absorber) into a water-, oil- or ethanol-based products.

Products that contain physical blockers tend to be the best choice for those with sensitive skin, as the ingredients are not likely to irritate skin. However, they do tend to leave a white cast and may not be suitable for darker skin tones, except when they are tinted. Chemical sunscreens, on the other hand, are better absorbed and don’t leave a white cast.

Sunscreens

Sunscreens are available in several forms: lotions, creams, gels, sprays, ointments, wax sticks and oils. If you have dry skin, you may opt for a cream sunscreen. For oily skin, lotions tend to be better choices as they are less greasy; gels are better suited for hairy areas of skin such as the scalp and chest.

You would be best served by choosing a sunscreen with a consistency and application that suits your lifestyle. Ultimately, the best sunscreen is the one you’ll actually use. Regardless of your choice, you should make sure you apply it as directed by the manufacturer.

Moisturizers

Moisturizers with SPF are not as effective as sunscreens as they provide less protection even with equivalent SPFs. Moisturizers are more difficult to spread across the skin, areas of the face are sometimes missed, and they are not applied as thickly as sunscreens. Regardless, using a moisturizer with built-in SPF is still better than not using any form of sunscreen at all. 

Makeup

Makeup such as foundation and powders with SPF are likely not sufficient protection because they’re rarely ever applied evenly or in adequate enough quantities. That said, you can use cosmetic products with SPF as long as you use sunscreen underneath.

Sunscreen should not be applied to the lips, but your lips do need protection from UV rays. Use lip balms or lipstick with SPF to protect this sensitive skin and reapply frequently.

Tips for using SPF products

You should develop certain habits around your sunscreen usage:

• Apply sunscreen generously, and at least 15 minutes before you go outdoors
• Apply sunscreen even under cloudy conditions, as UV rays penetrate clouds and can burn your skin
• Remember to apply sunscreen to easily forgotten areas such as the ears, toes and neck
• Apply lip balm with a minimum SPF of 15 and reapply frequently
• Reapply your sunscreen every two hours – more often if sweating heavily or after swimming
• When outdoors when the sun’s UV rays are at their peak (between 10 a.m. and 3 p.m) combine SPF products with protective clothing.

Takeaway

Sun protection factor, or SPF, tells you how long a particular product will protect you from harmful UVA rays, and the percentage of UVB rays that it is able to block. Using SPF products is crucial to prevent skin cancer and premature skin aging, but it can not offer 100% protection. 

If you know you will be in strong sunlight for an extended period of time, support your SPF product with physical coverage. As well, you can layer your protection with skin care products and makeup containing SPF. 
For the best possible protection, look for broad-spectrum sunscreens that protect against UVB and UVA rays, as both play a role in aging skin and increasing the risk of developing skin cancer.