Santa Clara University


Sunscreen Basics

Sunscreen should contain protection against both UVA and UVB rays. UVA rays cause skin changes that result in skin cancer (basal cell, squamous cell, and melanoma) as well as premature aging (wrinkles, skin sagging, and age spots). UVB rays cause sunburns, which increase the risk for developing melanoma. UVA rays can penetrate through glass, so your skin can still be affected when not in direct sun, such as in cars and near windows. UV rays released from tanning booths/sun lamps are also harmful, and should be avoided.


Unless indicated by an expiration date, sunscreen is stable for 3 years. You can generally use sunscreen left over from last year, but keep in mind it may mean you are not using the appropriate amount of sunscreen, since a bottle shouldn’t last very long.

Proper use

When using sunscreen, it is important to apply it properly to be sure it’s effective. Sunscreen should be applied 30 minutes before going outdoors, and should be applied thickly. (Your skin should appear white initially). It typically takes at least 1 oz (the size of a shot glass) to cover the exposed parts of the body. Gel forms work best on hairy areas. Don’t forget to apply sunscreen to your ears, hands, and back of neck.

People of all skin types should wear sunscreen year-round. It is a misconception that people with dark skin do not need sunscreen. People with dark skin can get skin cancer (although less frequently than those with light skin), as well as pigment changes from sun damage. Even on overcast days, 80% of the sun’s rays can penetrate through clouds and fog, so it is important to remember to wear sun-screen year-round.

When outdoors, sunscreen should be reapplied every 2 hours. Use extra caution near water, snow and sand, as they reflect the damaging rays of the sun, and can increase your chance of sunburn. (Sand reflects 25% of the sun’s rays, and snow reflects 80 %.) “Water-resistant” sunscreens can lose their effectiveness after 40 minutes in the water, so reapply it regularly.


Sunscreens typically list SPF as the degree of sun protection. This only reflects the ability of the sunscreen to reflect UVB rays, not UVA. A sunscreen with SPF15 (when applied correctly) reflects 93% of UVB rays, and one with SPF 30 reflects 97% of UVB. None of the available sunscreens reflect 100% of UVB rays, so it is important to limit sun exposure, especially during the daytime hours of 10-4, the time of strongest UV rays. The current recommendation is to use a sunscreen with at least an SPF of 15 year round. The sunscreen should also mention that it covers UVA rays, although there is currently no ranking system for the amount of UVA protection.