What SPF should I use?

Sunblock protects your skin by absorbing and/or reflecting UVA and UVB radiation. All sunblocks have a Sun Protection Factor (SPF) rating. The SPF rating indicates how long a sunscreen remains effective on the skin. A user can determine how long their sunblock will be effective by multiplying the SPF factor by the length of time it takes for him or her to suffer a burn 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 use 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. will also help protect your skin from overexposure and minimize sun damage.

The American Association of Dermatology (AAD) recommends that a "broad spectrum" sunblock with an SPF of at least 15 that is applied daily to all sun exposed areas, then reapplied every two hours. However, in some recent clinical trials, sunblocks with SPF 30 provided significantly better protection than sunblocks with SPF15. Therefore at UCSF, we recommend sunblocks with SPF of at least 30 with frequent reapplication.

What is the best type of sunblock to purchase?

The best sunblock varies from individual to individual. We recommend broad spectrum sunblock with UVA and UVB protection, a SPF rating of at least 30, in a form that is gentle enough for daily use.

Active ingredients of sunblock vary from manufacturer to manufacturer and can be divided into chemical versus physical agents. Chemical sunblocks work by absorbing the energy of UV radiation before it affects your skin. Physical sunblocks reflect or scatter UV radiation before it reaches your skin. Some sunblocks combine both chemical and physical sunblocks.

The two types of physical sunblocks that are available are zinc oxide and titanium dioxide. Both provide broad spectrum UVA and UVB protection and are gentle enough for everyday use. Because these are physical blocking agents and not chemicals, they are especially useful for individuals with sensitive skin, as they rarely cause skin irritation.

Most chemical sunblocks are composed of several active ingredients. This is because no single chemical ingredient blocks the entire UV spectrum (unlike physical sunblocks). Instead, most chemicals only block a narrow region of the UV spectrum. Therefore, by combining several chemicals,with each one blocking a different region of UV light, one can produce a sunblock that provides broad spectrum protection. The majority of chemical agents used in sunblock work in the UVB region. Only a few chemicals block the UVA region. Since UVA can also cause long-term skin injury, dermatologists at UCSF routinely recommends sunblocks that contain either a physical blocking agent (e.g. titanium dioxide or zinc oxide) or Avobenzone (also known as Parsol 1789).

Sunblocks comes in a variety of forms. Lotions, oils, sticks, gels, sprays and creams can all be effective sunscreens. However, sunblocks are only effective if they are used. We encourage you to try several types and find the one which works and feels the best to you. All sunscreens should be applied 15-20 minutes before sun exposure to allow a protective film to develop, then reapplied after water contact and sweating. Some sunblocks can lose effectiveness after two hours, so reapply frequently.

In general, spray lotions and gels are the least oily but also the ones that wash off more easily and need to be reapplied more frequently. If you develop a rash or other type of allergic response to a sunscreen, try a different brand or form (lotion vs. oil, for example) to see if you can better tolerate it. The most common allergic reactions occur with sunscreens that contain PABA-based chemicals. If you develop a rash to a sunblock, check the label to see if PABA is an ingredient. If so, consider avoiding sunblocks that contains this in the future. Alternatively, try a titanium dioxide or zinc oxide containing sunblock as they rarely cause skin irritation and provide very good broad spectrum UV protection.

Water resistant sunblocks are available for active individuals or those involved in water sports. It's important to check the label to ensure they say "water-resistant" or "very water-resistant."

• Water-Resistant sunblock maintains the SPF level after 40 minutes of water immersion
• Very Water-Resistant sunblock maintains the SPF level after 80 minutes of water immersion

Common active ingredients of sunblock and the type of UV light blocked: