Most people apply sunscreen when participating in outdoor activities during the summer. However, some experts suggest wearing sunscreen on a daily basis to prevent cumulative damage, and to lower the risk of skin cancer. It is recommended that sunscreen be applied 20 minutes before exposure to the sun.

The SPF (skin protection factor) of a sunscreen is a measure of its effectiveness. The higher the SPF, the more protection a sunscreen offers, and the longer a wearer can remain outdoors in the sun. For example SPF 15 means that the user can remain in the sun 15 times longer than would otherwise cause them to burn. In real life the protection from a particular sunscreen depends on the sensitivity to the user and the amount applied. Due to consumer confusion over the protection of sunscreen, sunscreens labelled as having a SPF higher than 30 are illegal to market in Australia. In the United States in 1999 the FDA decided to institute the labelling of SPF 30+ for sunscreens offering more protection. This was done to discourage companies from decreasing the amount of sun protection in their products while encouraging consumers to apply sunscreen more often (by not indicating that they could stay in the sun more than 30 times as long).

The hormone, alpha-melanocyte stimulating hormone, is made when the body is exposed to sunlight and is responsible for the development of the pigment melanin. Research is being done to create stable artificial forms of the hormone. A promising candidate, melanotan, might be useful for skincancer prevention, by causing tanning without exposure to sunlight.

The following are the FDA allowable active ingredients in sunblocks:

• p-Aminobenzoic acid (PABA) up to 15 percent.
• Avobenzone up to 3 percent.
• Cinoxate up to 3 percent.
• Dioxybenzone up to 3 percent.
• Homosalate up to 15 percent.
• Menthyl anthranilate up to 5 percent.
• Octocrylene up to 10 percent.
• Octyl methoxycinnamate up to 7.5 percent.
• Octyl salicylate up to 5 percent.
• Oxybenzone up to 6 percent.
• Padimate O up to 8 percent.
• Phenylbenzimidazole sulfonic acid up to 4 percent.
• Sulisobenzone up to 10 percent.
• Titanium dioxide up to 25 percent.
• Trolamine salicylate up to 12 percent.
• Zinc oxide up to 25 percent.