How Does Sunscreen Work? How Long Does it Last?

Reference Lab

DEC 27, 2022

If there’s one thing that dermatologists, estheticians, and medical professionals alike can agree on, it’s that sunscreen should be a staple product in everyone’s skin health routine. But, how is sunscreen good for your skin? UV radiation accounts for up to 80% of damage incurred on the skin, and the best known way to prevent sun damage to the skin is by using sunscreen. As the time investment for applying sunscreen is so small compared to the benefits that it can bring, it is no wonder that it comes so highly recommended. But despite this resounding endorsement, only 30% of women and 15% of men report wearing sunscreen daily¹.

This may be due to sunscreen’s bad rep as being oily, leaving a white cast, and proving difficult to use with makeup. The skin care and cosmetology industry has been working hard during the past decade to overcome these challenges and make SPF application integral to most skin care routines. From including SPF in makeup products to developing innovative formulations that defy sunscreen’s oily, white cast predecessors, it’s never been easier to make SPF application a regular part of your daily routine.

So what exactly does sunscreen do for your skin and how often should you be applying it? These questions and the science behind sunscreen will be explored in this blog. Due to a variety of factors, including age, location, climate, and skin tone, everyone’s skin photosensitivity is different. UV rays impart their detrimental damage on cells by reaching DNA and causing mutations. While our bodies have the ability, albeit limited, to repair the damage caused by harmful UV rays, this process becomes less efficient and slows down as we age, making it easier for sun damage to become more severe over time.

Our bodies also have the ability to block the UV rays’ ability to reach our DNA through melanin, which is the molecule responsible for skin tone and pigmentation. Darker skin tones have more melanocytes, a type of cell responsible for the production of melanin, which in turn blocks UV radiation from reaching the deeper layers of the skin². This effect is evident in the statistics on skin cancer between skin tones, with lighter skin tones being 20 times more likely to develop melanoma compared to darker skin tones³. Regardless of your skin tone, age, or geographic location, it’s always a good idea to wear sunscreen, as the body’s defenses against those harmful UV rays fall short of being 100% effective. Generally, sunscreen works by providing a chemical or mechanical barrier against UV radiation, limiting the amount of UV rays that reach your skin cells, and supplementing the shortcomings of the natural protective capabilities that your skin already possesses. There are two main classes of sunscreens, dictated by the mechanism of its active ingredient - chemical sunscreens, which contain organic UV absorbers as their active ingredient, and physical or mineral sunscreen, which contains inorganic UV blockers as their active ingredient. Inorganic UV blockers that are frequently included in physical sunscreens include titanium dioxide and zinc oxide, two metallic oxides that physically shield the skin from UV rays. As the name suggests, a physical sunscreen creates a physical barrier on the top of the skin that blocks UV rays from reaching skin cells and their DNA. Common organic UV absorbers include avobenzone or oxybenzone. These molecules work by absorbing the UV radiation into their chemical bonds and releasing that absorbed UV radiation as heat⁴. By absorbing the UV radiation, the chemical bonds within organic UV absorbers are broken down, making chemical sunscreen less effective with time. This explains why reapplication is so critical to an effective SPF strategy. When it comes to the proper use of sunscreen, the key is adequate reapplication. If your skin is left unprotected from sun exposure, a few varying outcomes can occur depending on the class of UV rays that reach skin cells. The 3 major forms of UV radiation are UVA rays, UVB rays, and UVC rays, but UVC does not reach the Earth’s surface as it is blocked by the ozone layer. Of the two remaining forms, UVB has a shorter wavelength and is responsible for sunburns as well as some forms of skin cancer.

UVA has a longer wavelength that penetrates deeper into the skin and dermis⁵, making it primarily responsible for skin maladies that cause premature wrinkling, age spots and can also heighten the risk for some types of skin cancer⁶. Approximately 95% of UV rays that reach the Earth’s surface are UVA rays, and the remaining 5% are UVB ⁷. Regardless, the result of inadequate protection from sun exposure is an increased risk for sunburn, skin cancer, and accelerated physical signs of skin aging. The length of time that an application of sunscreen lasts is highly dependent on the type of sunscreen used, the activities a person engages in while wearing the sunscreen, and the SPF level that they choose to apply.

In general, it is recommended that individuals apply sunscreen at least every 2 hours to remain effective against UV rays⁸. However, experts suggest even more frequent application or using a water-proof formula if sweating and water exposure is expected.

However, we understand that applying sunscreen every two hours may not be feasible everyday, especially on top of makeup. On these days where you have applied a full face of makeup, first apply any topical skin care products, such as a peptide moisturizer, then follow up with a layer of SPF before applying your face makeup. When asked, does sunscreen go on before or after moisturizer, we typically recommend after as the final step in your skincare routine.

SPF stands for Sun Protection Factor, and it's a measure of how much UVB radiation is required to produce a sunburn on the skin - the higher the SPF, the better your skin is protected from sunburn. Unfortunately, there is no standard for quantifying how well a particular sunscreen will protect against UVA radiation. For this reason, anyone wishing to protect against UVA radiation should apply sunscreen labeled as “broad-spectrum” which indicates that a given sunscreen protects against both UVA and UVB.

SPF is also a good indicator of the length of time that a sunscreen will remain effective. It has been found that in average conditions, a sunscreen with SPF 50 will last for approximately 3 to 4 hours⁹. If you’re on the move, sweating, or in a place with a particularly strong UV index, it is recommended to apply SPF 50 sunscreen every 2 to 3 hours. Just like sunscreen prevents the development of sunburns, it will also increase the time required to develop a tan. Tanning is due to the increase in melanin content in the skin, which is a direct reaction to UV exposure. When sunscreen blocks the UV exposure to your skin cells, less melanin is produced, and the development of a tan is delayed.

In general, the greater the SPF value, the more sun exposure will be required to develop a tan. Though tan skin is commonly thought to improve appearance, it is developed at the expense of your long term skin health. Not to mention, sun exposure accelerates the visible signs of aging. We suggest opting for healthy skin and trying out methods of “fake tans” to achieve a natural-looking glow without the damage. Vitamin D enables critical processes in the body, particularly bone growth and maintaining bone density¹⁰. The primary way that the body produces Vitamin D is through a process that occurs when skin cells are exposed to sunlight, specifically due to the interaction between UVB rays and a protein in skin known as 7-DHC¹¹.

So will the use of sunscreen lead to a Vitamin D deficiency? The short answer is probably not. Since the amount of UVB necessary for the production of vitamin D is so minimal, the small amount that manages to get through the sunscreen will likely maintain adequate Vitamin D levels. For context, the recommended daily amount of Vitamin D can be produced by exposing your arms and legs to the sun for 15 minutes 3-4 times per week ¹². Additionally, Vitamin D can be easily supplemented, with most multivitamins containing 100% of your recommended daily value of Vitamin D. While sunscreen limits your production of vitamin D, there are also certain environments that will lead to less of the vitamin being synthesized. For example, people living in more extreme latitudes will experience less direct sun exposure and more hours of darkness during winter months, impacting the amount of UVB radiation that reaches their skin. It’s especially important for people experiencing limited sun exposure to supplement Vitamin D¹³. Research has found that in order to achieve the recommended daily intake value of Vitamin D from sunlight, people should spend anywhere between 9 and 45 minutes in the sun per day, depending on how much skin is exposed as well as the season¹⁴. Individuals worried about their vitamin D levels can check their blood for vitamin deficiencies and supplement as necessary.

• The amount of UV radiation that reaches the skin depends on many factors including skin tone, season, pollution, sunscreen application, and length of exposure.
• Sunscreen has two large categories of ingredients; inorganic blockers (physical sunscreen) and organic absorbers (chemical sunscreen).
• UVB radiation is responsible for tans and sunburns and some skin cancer while UVA radiation increases the appearance of wrinkles, age spots, also increasing the risk for some other forms of cancer.
• Sunscreen should be regularly applied as it is frequently washed or rubbed off.
• Sunscreen effectiveness increases with SPF and decreases with activity and time.
• Vitamin D levels are likely not impacted by the value of SPF sunscreen applied, as the skin absorbs more than enough UVB radiation for vitamin D synthesis.
• The length of sunlight exposure required to produce enough vitamin D can depend on the season and the amount of skin exposed.

Sources:

1. https://www.cancer.gov/news-events/cancer-currents-blog/2015/no-sunscreen
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3400707/
3. https://khn.org/news/article/skin-cancer-risk-overlooked-in-dark-skin/
4. https://www.cosmeticsbusiness.com/news/article_page/What_is_organic_and_inorganic_sunscreen/153042
5. https://www.fda.gov/radiation-emitting-products/tanning/ultraviolet-uv-radiation
6. https://www.skincancer.org/risk-factors/uv-radiation/
7. https://www.who.int/news-room/questions-and-answers/item/radiation-ultraviolet-(uv)
8. https://www.skincancer.org/skin-cancer-prevention/sun-protection/sunscreen/
9. https://www.colorescience.com/blogs/learn/how-does-sunscreen-work-to-protect-your-skin
10. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
11. https://www.skincancer.org/blog/sun-protection-and-vitamin-d/
12. https://healthyeating.sfgate.com/much-skin-needs-exposed-vitamin-d-8162.html
13. https://www.hsph.harvard.edu/nutritionsource/vitamin-d/
14. https://pubmed.ncbi.nlm.nih.gov/30992519/