Ultraviolet Radiation (UV)
The sun gives off different types of UV (UVA, UVB, and UVC). The UVC is mostly filtered out by the Earth’s ozone layer before reaching the Earth’s surface. When we think of photodamage, the main players are UVA and UVB.
UVA classically thought of as the aging ray”, makes up roughly 95% of the UV reaching Earth’s surface. It has a longer wavelength and thus penetrates deeper into the skin to affect the main structural component of skin - collagen. It is the principal culprit for UV-induced fine lines and wrinkles.
UVB has a shorter wavelength than UVA and thus does not penetrate as deep as UVA. UVB is the main mediator of erythema of sunburn and the pain associated with sunburn. UVB creates more damage to the skin cell’s DNA compared to UVA, which is a key step in photocarcinogenesis (skin cancer production).
Prolonged, unprotected exposure to both UVA and UVB can lead to signs of photoaging and skin cancer. Therefore, it is paramount to choose a sunscreen that protects the skin from both UVA and UVB.
Figure 1: Radiation in relation to Altitude and Ozone concentration. UVC is filtered out while UVB and UVA reach the surface.
What is SPF?
Sun Protection Factor (SPF) tells one how long to redden one’s skin if using sunscreen appropriately vs. how long it would take to cause sunburn without sunscreen. I.e., SPF 50 it would take 50x longer than if wearing no sunscreen.
Stated another way, SPF 50 and SPF 30 allow 2% and 3% block 98% and 97% of the sun’s UV, respectively. This difference may not seem significant, but SPF 50 blocks 50% more of the UV than SPF 30.
Who should wear sunscreen?
EVERYONE! The FDA recommends not applying sunscreen to infants less than six months old. In this population, sun avoidance or other sun-protective measures are important. Look for clothing with the marking “UPF” (Ultraviolet Protection Factor). This is the SPF for clothing. Other good options include wide-brimmed hats, sunglasses, long sleeves/pants, seeking shade, and avoiding peak sun. Sun protective clothing applies to those older than six months as well!
How to properly wear sunscreen?
Choose a broad-spectrum sunscreen. Broad-spectrum means a sunscreen will protect against both UVA and UVB. Luckily, most commercially available sunscreens are broad spectrum. If you are planning to be outside (sunny or cloudy), driving during the day, working near windows during the day, wear sunscreen. It is recommended to apply an SPF of at least 30 if planning to be outside and using an SPF 15 for daily use. Apply 20-30 minutes before anticipated sun-exposure. A lot of people have to park their cars and walk outside to their office/place of work. This is sun exposure too! If planning to swim/sweat, look for a water-resistant formulation. One ounce of sunscreen (roughly the size of a standard shot glass) is the recommended amount to cover your entire body. Reapplication is just as important. Make sure to reapply at least every two hours while outside.
Chemical vs. Physical Sunscreen
The active ingredients in sunscreen can be chemical or physical. All active ingredients in sunscreen are technically chemicals. The “physical” active ingredients are inorganic compounds (zinc oxide/titanium dioxide) while the “chemical” active ingredients (benzophenones, cinnamates, salicylates, octocrylene, etc.) are organic compounds. Chemical sunscreens work by absorbing UV radiation and converting it to a lower, less damaging energy. Physical sunscreens act as a shield and reflect UV rays. Many commercially available formulations combine both chemical and physical sunscreens.
Figure 2: Physical vs Chemical Sunscreens
There are pros and cons to each. Both chemical and physical sunscreens are broad spectrum. Benefits of chemical sunscreens include decreased comedogenicity (won’t clog pores), easier to rub in, and less likely to stain your clothing. The drawback to chemical sunscreens includes allergic contact dermatitis and irritation in atopic dermatitis or eczema-prone skin.
Physical sunscreens are less irritating and better suited for sensitive skin. However, they are harder to rub in, may stain your clothing and cause or worsen acne breakouts.
Other things to consider
Your skin type and family history of skin cancer help determine your risk for developing skin cancer. If you have fair skin and a personal or family history of skin cancer, be diligent about your sun protection. Additionally, certain over-the-counter and prescription medications are photosensitizing. Talk with your provider/dermatologist if you are experiencing photosensitivity. Lastly, the type of sunscreen you use is not a one size fits all. There are sunscreens better suited for acne-prone skin, sensitive skin, and oily skin.
About the author
Steven received his M.D. degree from the Medical College of Georgia. He is currently in the final year of his Dermatology residency and is applying for a fellowship in Micrographic Surgery and Dermatologic Oncology. He is active on social media at https://www.instagram.com/stevenkentmd/.