On Your Health

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How to Apply Sunscreen

02 August 2022

A trip to the beach or pool isn’t usually complete without getting a whiff of sunscreen being applied by a neighboring vacationer. As common as these experiences are, it is less common for people to consistently use sunscreen – and use it properly. In fact, most people apply less than half of the necessary amount of sunscreen that is required to be effective. 

To help ensure you and your family protect your skin this summer, we provided a guide on how sunscreen works and how to properly apply it.

How does sunscreen work?

The sun is an important part of our planet and solar system. Without it, planet earth wouldn’t have light or heat to survive. That same heat – the hottest part of the sun tops at out 27 million degrees – responsible for providing light energy can also be damaging to your skin. 

There are two types of harmful rays the sun produces, both of which are necessary to understand when discussing sunscreen. Long wave ultraviolet A (UVA) rays contribute to aging and wrinkles, while short wave ultraviolet B (UVB) rays are the main cause of sunburn. People don’t see this type of ultraviolet light, though, because UV rays have shorter wavelengths than visible light.

When shopping for sunscreen, you’ll find one of three types of products to choose from: physical blockers, chemical absorbers and broad spectrum sunscreens (a combination of blockers and absorbers).

Physical blockers

Think of physical blockers as a shield – they reflect UV rays from your skin. This type of sunscreen is made from the following two ingredients:

  • Titanium dioxide
  • Zinc oxide

The particles are ground up finely and mixed with other inactive ingredients, such as water or oils, so they can easily spread over your skin.

Chemical absorbers contain ingredients that absorb UV radiation so it can’t damage your skin. Here are some of the most common chemical absorbers: 

  • Aminobenzoic acid
  • Avobenzone
  • Octisalate
  • Octocrylene
  • Oxybenzone

Broad spectrum sunscreen

Initially, sunscreen was designed to protect you from UVB rays since they’re the light that causes sunburn and increases your risk of skin cancer. As more research was conducted, UVA rays, while not as intense as UVB rays, penetrate the skin deeper than UVB rays where most skin cancer occurs. Thus, UVA rays also increase your risk of skin cancer.

As a result, broad spectrum sunscreens were created to block both UVA and UVB rays by using a combination of physical blockers and chemical absorbers. 

When buying sunscreen, look for a product that provides SPF 30 or higher, broad spectrum protection and is resistant to water and sweat.

What is SPF?

As a consumer, it’s easy to get confused with the acronym “SPF” when reading sunscreen labels. In short, SPF stands for sun protection factor, which is a fancy way of informing you of how well sunscreen protects you from UVB rays (but not UVA rays). Sunscreens are tested to see how much UV exposure it would take to cause a sunburn when wearing sunscreen compared to when not wearing sunscreen.

For example, sunscreen with an SPF of 30 means it will take 30 times longer to receive a sunburn when using that sunscreen compared to if you had used no sunscreen at all.

The American Academy of Dermatology recommends using an SPF of 30. But even an SPF of 15 can help lower your risk of skin cancer. Using an SPF 15 sunscreen daily can lower your risk of squamous cell carcinoma by 40 percent and your melanoma risk by 50 percent, according to the Skin Cancer Foundation.

Still, many people misinterpret SPF’s true meaning and can think the number associated with the SPF indicates how many hours you’re protected by the sun. In other words, some people think an SPF of 30 means one application of sunscreen provides 30 hours of protection. However, SPF is only used to measure the amount of sun exposure instead of the time of sun exposure.

Likewise, some people think more is better when it comes to sunscreens with a high SPF. This isn’t the case, though.

For most people, an SPF of 30 offers protection from 97 percent of UV rays, according to the American Academy of Dermatology. People with fairer skin may benefit from an SPF of 50 for more protection. Anything higher is unlikely to offer drastic benefits as no sunscreen can block 100 percent of UV rays. In fact, a higher SPF sunscreen can give you a false sense of security by thinking you can stay out in the sun without reapplying. 

As a guide, this is how the different types of SPF sunscreens stack up against each other in protecting against UVB rays:

  • SPF 15: Blocks 93 percent of UVB rays
  • SPF 30: Blocks 97 percent of UVB rays
  • SPF 50: Blocks 98 percent of UVB rays
  • SPF 100: Blocks 99 percent of UVB rays

What is the proper way to apply sunscreen?

How often do you see someone squirt a quarter-sized amount of sunscreen in the palm of their hand to rub on their entire body? This isn’t nearly enough to properly reflect and absorb UV rays from damaging your skin.

Typically, you need at least an ounce – the size of a shot glass – for your entire body. Don’t skip any body parts, either, even the lips. It’s common for people to miss the tops of their ears, the back of the neck, the top of the feet, hands and wrists and any areas covered by hair.

For your face, about a teaspoon of sunscreen will usually cover this area. Don’t worry, you don’t need a measuring spoon to get the exact amount. Simply place two fingers together and place enough sunscreen to cover both fingers.

How to use stick and spray sunscreens

While the 1-ounce guidelines apply to lotion-based sunscreens, it’s not as easy to measure spray or stick sunscreens. As a general rule when using stick sunscreen, perform four passes back and forth for each area. When using spray sunscreen, spray enough to where your skin glistens. 

Doing this will help ensure that you’re using enough sunscreen to be protected. You should also rub it in. Many people think spraying sunscreen on their body is enough. However, it’s easy to miss spots or provide uneven coverage.

Regardless of the type of sunscreen used, many people are also guilty of not allowing enough time for sunscreen to absorb. You should apply sunscreen for 15 to 30 minutes before heading outside.

Can you get sunburned through a car window?

Most people associate sunscreen with outdoor activities, but have you ever thought about applying sunscreen on your way to the beach or on a long drive in the car? That may not be high on your agenda, although it should be considered moving forward.

No, you won’t get sunburned through a car window, as glass blocks UVB rays, but UVA rays can still penetrate through and damage your skin. When exposed to these UVA rays for an extended period of time, you put yourself at a higher risk for skin cancer. Any skin damage that occurs will likely come from side and rear windows – front windshields come with window protection to filter out UVA rays. This explains why truck drivers, bus drivers and pilots are more likely to experience skin issues.

A study by the Journal of the American Medical Association found pilots who flew at 30,000 feet for 56 minutes experienced the same amount of UVA ray exposures as a 20-minute tanning session.

Even if it doesn’t lead to skin cancer, the constant exposure to UVA rays can cause a condition called unilaterally photodamaged skin in which the top layers of the skin become thick and inelastic.

While these instances include years and years of exposure, it’s still wise to use sunscreen if you spend long periods of time in your car. A broad spectrum sunscreen will protect against UVA rays. In addition, you can also install UV-blocking window film to prevent UV rays from entering the car.

How long does sunscreen last once applied?

Between water, sweat, friction and the sun’s rays, sunscreen loses its effectiveness in a matter of hours. That’s why reapplying often is key. How often should you reapply? In general, you should put on another layer of sunscreen every two hours. Some activities will require more frequent applications. For example, reapply every 40 to 80 minutes if you’re in the water. You should also reapply after drying off your body with a towel or if you’re sweating.

Keep in mind sunscreen is water resistant and not waterproof. There is no such thing as waterproof sunscreen because it will wash off at some point, especially if you’re in an ocean, lake, river or swimming pool. Sunscreens either fall under the category of water resistant (effective for 40 minutes) or very water resistant (effective for 80 minutes). In either case, you should read the product’s label to be sure of when you need to reapply.

The sun is at its hottest point between 10 a.m. and 2 p.m., which may require more frequent sunscreen applications – sunscreen is more likely to break down and clump when the UV rays are strongest.

Does sunscreen expire?

Sunscreen can expire after a certain date. By law, the U.S. Food and Drug Administration requires sunscreens to last for at least three years.

You should throw out any sunscreen that has passed its expiration date. Some sunscreens may lose their strength or effectiveness before the expiration date if they’re not stored properly. Keep sunscreen out of the direct sun. For trips to the beach or pool, consider wrapping them in a towel or keeping them in a cooler away from the sun.


For more questions on the relationship between sun exposure and skin damage or general inquiries on your sunscreen use, please contact your primary care physician. For more health and wellness content, visit the INTEGRIS Health For You blog.

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