Why? Because it's misleading. For years I've been tracking those numbers, secretly applauding as they rose from a mere 2 on Coppertone back in the 1970's to a whopping 75 on a recent purchase of Neutorgena. I felt virtuous and smart when I purchased a host of different moisturizers over the years both for my face and body with an SPF of 15 or higher. Not only was I moisturizing my paper dry skin,I was protecting it too and saving time in the process.
Unfortunately I was wrong. SPF refers to Sun Protection Index. It's number indicates the amount of time you can stay in the sun after applying the product without burning . It protects only against the UVB rays that cause sunburn but does not protect against the UVA rays that penetrate deep into the skin and cause premature aging and skin cancer. You need what is known as a broad spectrum sunscreen, one that states that it protects against UVA andUVB rays. Most often that protection is afforded by either by trademarked combinations such as Parasol, Helioplex or Mexoryl or ingredients such as Zinc Oxcide,Avobenzone, Homosalate or Octisalate. Read those labels before you purchase.