Sunscreen protects your skin by absorbing and reflecting ultraviolet rays from the sun. All sunscreens have a sun protection factor (SPF) rating that indicates how long a sunscreen remains effective on the skin.
You can determine how long sunscreen will be effective by multiplying the SPF factor by the length of time it takes you to develop a sunburn without sunscreen. For instance, if you normally develop a sunburn in 10 minutes without wearing a sunscreen, a sunscreen with an SPF of 15 will protect you for 150 minutes — 10 minutes multiplied by the SPF of 15.
Although sunscreen helps minimize sun damage, no sunscreen completely blocks all wavelengths of UV light. Wearing sun protective clothing and avoiding sun exposure from 10 a.m. to 3 p.m. also will help protect your skin from overexposure and minimize sun damage.
The American Association of Dermatology (AAD) recommends that a "broad spectrum" sunscreen (meaning it protects against ultraviolet A (UVA) and ultraviolet B (UVB) rays with an SPF of at least 15 that is applied daily to all sun exposed areas, then reapplied every two hours.
UVA are longer or weaker rays and UVB are medium or stronger rays.
In some clinical trials, sunscreens with SPF 30 provided significantly better protection than sunscreens with SPF 15. At UCSF Medical Center, we recommend sunscreens with an SPF of at least 30. Adults should cover their bodies in enough sunscreen that would fill a shot glass and reapply every two hours.
The best sunscreen to use varies by individual. We recommend broad spectrum sunscreen, which provides UVA and UVB protection, and an SPF rating of at least 30, in a form that is gentle enough for daily use.
Active ingredients of sunscreen vary by manufacturer, and may be a chemical or physical agent:
Water-Resistant sunscreen maintains the SPF level after 40 minutes of water immersion/
Very Water-Resistant sunscreen maintains the SPF level after 80 minutes of water immersion
If you develop a rash or other allergic response to a sunscreen, try a different brand or different form, such as lotion rather than oil. The most common allergic reactions occur with sunscreens that contain PABA-based chemicals.
Try a sunscreeen without PABA if you develop skin irritations. Sunscreens with titanium dioxide or zinc oxide provide very good broad spectrum UV protection and rarely cause allergic reactions.
Reviewed by health care specialists at UCSF Medical Center.
This information is for educational purposes only and is not intended to replace the advice of your doctor or health care provider. We encourage you to discuss with your doctor any questions or concerns you may have.
Dermatologic Surgery and Laser Center
1701 Divisadero St.
San Francisco, CA 94115
Phone: (415) 353–7878
Fax: (415) 353–7838
High Risk Skin Cancer
1701 Divisadero St., Third Floor
San Francisco, CA 94143-0316
Phone: (415) 353-7830
Fax: (415) 353-7860
1600 Divisadero St., Fourth Floor
San Francisco, CA 94115
Phone: (415) 353-9900
Fax: (415) 885-3802