All About Sunscreen

May is Skin Cancer Awareness Month. 

The first lines of defense are avoidance of UV radiation, finding shade, and using sun protective clothing. Whatever skin is exposed after these measures should get sunscreen.


Why do I need sunscreen?

Ultraviolet (UV) rays corrupt and mutate DNA in skin cells. In addition, the sun’s radiation impairs the ability of cells to repair DNA. This adds up over a lifetime and eventually leads to skin cancers such as basal cell carcinoma, squamous cell carcinoma, and malignant melanoma.


Do you like to play outside?

UV intensity increases 10% for every 1000 meter increase above sea level. Water reflects 10% of UV radiation, sand reflects 15%, and fresh snow reflects up to 80%. Even on a cloudy day up to 80% of UV radiation makes it through to Earth.UV radiation penetrates through glass so damage can happen even while driving in a car or sitting at your desk.


How does sunscreen work?

Many sunscreens contain organic ingredients such as avobenzone, oxybenzone, octinoxate, and octisalate. These sunscreens change shape in response to UV rays, converting radiation into non-damaging wavelengths. Other sunscreens contain the inorganic minerals zinc oxide and/or titanium dioxide. These are basically ground up rocks that UV radiation cannot penetrate. The size of the mineral particle dictates how opaque the sunscreen looks. That is why many sunscreen manufacturers use nanotechnology—so their products will look more clear and natural rather than opaque white when you put them on. Zinc oxide is actually the same ingredient used in diaper paste, and what gives it the white color.  Both types of sunscreen prevent UV rays from contacting the skin and causing DNA mutations.

There are different kinds of UV radiation: UVA and UVB. Your skin needs protection from both, so it is critical to use a sunscreen that is labeled “broad spectrum.” Mineral sunscreens such as zinc oxide or titanium dioxide offer the best protection against both types of radiation. UVA penetrates more deeply into the skin, causing premature skin aging, tans, and skin cancer. Tanning beds emit 2-3 times more UVA than the sun in order to get that tan faster. That means that using a tanning bed puts you at a much higher risk of developing skin cancer. UVB does not penetrate as deeply but does causes sunburns and skin cancer.


There is no such thing as a healthy tan.

A suntan is only produced in response to DNA damage. That means that you only get tan if enough DNA mutations are occurring to trigger an emergency defense response in the skin. A tan demonstrates that your skin is trying to protect itself and that you are increasing your risk of developing skin cancer.


Will sunscreen block my Vitamin D?

Vitamin D is important for the development and maintenance of healthy bones. If you do not have enough Vitamin D you can develop rickets as a child or osteomalacia as an adult. Too much Vitamin D is toxic and can cause a variety of problems including brain damage in the very young, bone growth abnormalities, severe depression, kidney, liver or heart damage. The body gets the Vitamin D it needs through the diet. Foods such as fish, milk, cheese, egg yolks, and fortified foods such as orange juice and cereal are high in Vitamin D.

Vitaimin D is also made in the skin. Most people overestimate how much sun they need to get vitamin D. The skin can only make so much vitamin D and unfortunately it requires UVB radiation to accomplish this. After 20 minutes in the sun vitamin D synthesis machinery becomes overwhelmed and no additional vitamin D can be made. Most people can satisfy the body’s requirement for vitamin D with less than 20 minutes of sun exposure twice a week or less than 10 minutes of sun exposure daily.3


What is SPF?

SPF stands for sun protection factor. It is the measurement of a sunscreen’s protective effect against UVB radiation. SPF does not reflect a sunscreen’s ability to protect against UVA radiation.

The SPF number is calculated as a ratio: if it takes 5 minutes for skin to redden in the sun without sunscreen, and 150 minutes for skin to redden in the sun when using sunscreen (exactly as directed), that sunscreen’s SPF would be 30 (150 minutes divided by 5 minutes). A very thick layer of sunscreen is needed to get consistent results for SPF testing measurements. If you use an SPF 30 sunscreen exactly as directed, it will take 30 times longer to burn than if you were not wearing sunscreen. Unfortunately, it is very difficult to apply a thick enough layer of sunscreen to get the SPF protection indicated on the bottle. This is why I recommend using the highest SPF you can find.


How much SPF do I need and how much do I apply?

A sunscreen that is broad spectrum, SPF 30 or higher (the higher the better), and water-resistant can provide effective protection from the sun. All sunscreens need to be reapplied approximately every two hours, after sweating or swimming, and according to the directions on the bottle.

Adults should be applying 25mL to cover the body when wearing swimwear. That is about a shot glass full of sunscreen every 2 hours.


Sunscreen application basics

  • Choose SPF 30 or higher - use SPF 100 if you can find it.
  • Apply 25mL per application (one shot glass full).
  • Use a broad spectrum sunscreen for UVA and UVB coverage.
  • Don’t forget lip balm with SPF.
  • Reapply at least every 2 hours.
  • Make sunscreen a daily habit.
  • Do not use a sunscreen after the expiration date. FDA regulations require all sunscreens to retain original strength for three years.


If you are burning or significantly tanning through sunscreen:

  • Use a sunscreen with higher SPF
  • Reapply using a more generous amount
  • Use a sunscreen with more UVA protection (select a sunscreen with zinc oxide and/or titanium dioxide)

For more information, visit:

Melanie Bui, MD, PhD, is a dermatologist at the University of Vermont Medical Center and an assistant professor at the Larner College of Medicine at UVM. Cari E. Carpenter, ’22, is a medical student at the Larner College of Medicine at the University of Vermont.



  1. Surveillance, Epidemiology, and End Results (SEER) Program ( SEER*Stat Database: Incidence - SEER 9 Regs Research Data, Nov 2017 Sub (1973-2015) - Linked To County Attributes - Total U.S., 1969-2016 Counties, National Cancer Institute, DCCPS, Surveillance Research Program, released April 2018, based on the November 2017 submission.
  2. INTERSUN: the Global UV Project: A guide and compedium. Geneva, World Health Organization, 2003.
  3. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. The American journal of clinical nutrition. 1999 May 1;69(5):842-56.
  4. Williams J, Maitra P, Atillasoy E, Wu MM, Farberg A, Rigel D. high Spf sunscreen provides significant clinical benefit in actual use conditions: Spf 100+ is more effective than Spf 50+: 4389. Journal of the American Academy of Dermatology. 2017 Jun 1;76(6):AB154.