UVB (think of B for burn) is the cheif culprit behind sunburn while UVA (think of A for aging). They also exacerbate the carcinogenic effects of UVb rays and are increasingly seen as the cause of skin cancer on their own. Broad Spectrum sunscreen was mandated by the FDA in 2014 for manufactures to formulate equal levels of UVA and UVB protection, now protecting both sunburn and skin cnacer.

Fun Fact:

SPF30 protects against 97%

SPF50 protects against 98%

SPF100 protects against 99%