Sun protection factor (SPF): The SPF rating is calculated by comparing the amount of time needed to produce sunburn on protected skin to the amount of time needed to cause sunburn on unprotected skin. For example, if a person gets sunburn after 10 minutes in the sun, then by using a sunscreen with SPF 15, this person will get sunburn after 150 minutes. Knowing this, however, can lead to misunderstanding. For example, you may tell me that you normally get sunburn in one hour, and thus an SPF 15 sunscreen allows you to stay in the sun 15 hours without getting sunburn! This is not true because SPF is not directly related to time of solar exposure (UV radiation) but to amount of solar exposure. Although solar energy amount is related to solar exposure time, there are other factors that impact the amount of solar energy. For example, the amount of solar energy you get in one hour at 9:00 am is equivalent to what you get in 15 minutes at midday. Plus, there are other various factors that impact the amount of solar exposure too. So SPF is only a relative measure of the amount of sunburn protection provided by a sunscreen. It only tells you that an SPF 30 sunscreen provides more sunburn protection than an SPF 15 sunscreen.
UVA and broad spectrum: So far, I have kept emphasizing that the SPF is a measure of sunburn protection. This is because the SPF is an imperfect measure, which only measures protection a sunscreen offers against UVB (the ultraviolet radiation that causes sunburn). UVA, however, is that mild killer that causes invisible damage and skin aging. To get UVA protection, you can skip the “broad-spectrum UVA/UVB protection” label on the sunscreen package (FDA doesn’t regulate UVA protection measure). Go directly to the “active ingredients” list looking for Avobenzone (also called butyl methoxydibenzoylmethane), Zinc Oxide, Titanium Dioxide, or Mexoryl SX (also called ecamsule). Outside US, Tinosorb S (also called bis-ethylhexyloxyphenol methoxyphenyl triazine) and Tinorsorb M (also called methylene bis-benzotriazolyl tetramethylbutylphenol) are also approved. Don’t worry about the long names. For most cases, they are listed as those shorter names. Do remember that the SPF number does not reflect how much UVA protection you get.