A good sunscreen should protect from sunburn, skin aging and skin cancer.
Most importantly, a sunscreen must provide a high degree of lasting protection against the two key types of ultraviolet radiation, UVA and UVB. (See our article on UV radiation for details.) Therefore, when selecting a sunscreen, you must be able to estimate how much UVA and UVB radiation it blocks after a typical application.
SPF as a measure of UVB protection
It is relatively easy to figure out a sunscreen's effectiveness against UVB: if you spent some time in the sun and have no signs of sunburn whatsoever, then you probably have a good UVB sunscreen. An even better way, at least in theory, is to look at the sunscreen's SPF, a number indicating the degree of protection against UVB.
SPF stands for Sun Protection Factor and is a rough measure of how well a sunscreen protects the skin from UVB rays. For example, if you get a sunburn after an 30 min of sun exposure, then wearing an SPF 5 sunscreen will extend the time until sunburn of the same severity to 5 x 30 min, which equals 150 min or two and a half hours). Keep in mind that SPF scale may be confusing because it is not linear relative to how much UVB is blocked. For example, SPF 30 only blocks about 4% more UVB radiation than SPF 15 (SPF 30 blocks 97% whereas SPF 15 blocks 93% of UVB). SPF has other limitation as well. It does not take into account stability of the sunscreen, how long it stays on the skin, water-resistance and so-forth. Still, SPF is a useful ballpark measure of a sunscreen's UVB protection. The biggest deficiency of SPF is that it fails to take into account UVA, the major contributor to skin aging and some types of skin cancer.
A sunscreen with SPF 15 or higher should provide a good UVB protection as long as it stays on the skin long enough and does not degrade. If you have sun sensitive skin or are exposed to a high peak of UV radiation (e.g. daytime in tropics), use SPF 30 or higher.
Measuring UVA protection
Protection against UVA is critical if you are to minimize skin aging and reduce the risk of skin cancer. Yet, it is much harder to judge how well a given sunscreen protects against UVA than UVB because SPF gauges only UVB.
Many sunscreens don't even mention UVA. Others say that they provide UVA protection but do not specify its strength. You cannot detect UVA damage yourself without special equipment (such as Wood's lamp). And there is no universally recognized SPF-like number to grade UVA protection.
One emerging UVA protection standards is Persistent Pigment Darkening (PPD), which is a biological measure of UVA radiation absorbed by the skin. A sunscreen with PPD 15 or higher should provide good UVA protection (again, as long as it stays on the skin long enough). Unfortunately, relatively few sunscreen manufacturers provide PPD values. Some others provide the percentage of UVA a sunscreen blocks. (Anything above 90%, which is close to PPD=10, should be satisfactory.) Unfortunately, many sunscreens provide no quantitative UVA-blocking information at all. For those, you can contact the manufacturer and try to obtain the UVA protection information. Unless you get a satisfactory response, you would want to avoid such a product.
Sun protection ratings alone are not enough
If is important that your sunscreen has high ratings for UVB and UVA protection. However, high SPF, PPD and other ratings alone are not enough. It is at least as important that you apply your sunscreen frequently and generously. Also, keep in mind that some sun-blocking ingredients and/or their combinations gradually degrade and lose their protective capacity when exposed to sunlight. (See our information on individual sun-blocking agents.) Sunscreens prone to degradation should be applied more frequently, especially during peak sun activity.
