What is the difference between UVB and UVA rays?

UVB rays (290-320nm) are the main cause of tan and sunburn because they damage the skin’s superficial epidermal layers. UVB rays play a big part in the development of skin cancer, and contribute to photoaging. UVB rays account for 5% of the sun’s rays and vary according to season and weather conditions; they are more intense in the summer and at high altitudes. 

UVA rays (320-400nm) can pass through glass and clouds and are present every day of the year at equal intensity during all daylight hours. They account for 95% of UV rays that reach the earth. Since UVA rays are longer in wavelength than UVB, they are able to penetrate into the deeper skin layers affecting collagen, elastin and blood vessels. UVA plays a major role in photoaging. It can damage skin cells in the basal layer of the epidermis where most skin cancers arise (e.g. basal cell and squamous cell carcinoma). 

What is SPF?

The SPF number only shows the level of sunburn protection provided by the sunscreen. The SPF test measures the amount of UV ray exposure it takes to cause sunburn when a person is using a sunscreen in comparison to how much UV exposure it takes to cause a sunburn when they do not use a sunscreen. The product is then labeled with a specific SPF value indicating the amount of sunburn protection provided by the product. Higher SPF values provide greater sunburn protection than lower values. Since SPF values are determined by a test that measures protection against sunburn caused by UVB rays, SPF values only describe a sunscreen's UVB protection, not UVA.

What does broad spectrum mean?

Sunscreens that offer protection from both UVA and UVB rays are labeled as broad spectrum. 

What SPF value should I look for in a sunscreen?

Only broad-spectrum sunscreens with an SPF of 15 or higher can claim to reduce the risk of skin cancer and early skin aging if used as directed with other sun protection measures. Any sunscreen not labeled as broad spectrum and those with an SPF between 2 and 14 can only claim to prevent sunburn. (U.S Food and Drug Administration).

Instead of thinking about sunscreens as the percentage of rays they block, we should rather think about the percentage of rays getting through onto the skin. An SPF of 30 blocks 96.7% of UV rays so that only 3.3% will get through, compared with an SPF of 15 which blocks 93.3% of UV rays and lets through 6.7% - twice as much. Therefore, you can clearly see the benefit of using an SPF of 30 over an SPF of 15. Higher-number SPFs block slightly more UV rays, but no sunscreen can block 100 percent of the sun’s rays.