The efficacy of a given sunscreen formula depends on proper application. All sun-screens, regardless of the ingredients, must be applied prior to exposure if they are to be maximally effective. For example, an SPF 15 applied to a person who would normally burn in 20 minutes will allow that individual 300 minutes of exposure before they burn. That SPF 15 sunscreen would now only provide 150 minutes of protection instead of the 300 minutes it would have. Hence, the almost universal recommendation that sunscreens be applied liberally and often, usually every 2 hours.
More frequent application is advised if swimming or engaging in strenuous physical activity.
Always apply sunscreen at least 20 minutes prior to exposure. It takes approximately one ounce for a single full adult body application. I can safely say that very few people apply this much sunscreen. It is crucial that consumers be educated on this point. Chronic effects of sun exposure include photo aging and photocarcinogenesis.