Part 3 of a 4 part series on sunscreen
Broad spectrum means that a sunscreen protects against two types of ultraviolet sun rays, UVB and UVA. UVB is the shorter wavelength of sunlight that leads to sunburn and protection against UVB is represented by the sunscreen's SPF. When you are exposed to UVB your skin starts to turn pink. So, it is relatively easy to measure whether a sunscreen is protecting you - it will take longer for your skin to turn pink with the sunscreen when exposed to UVB light than without.
UVA on the other hand is a longer wavelength of sunlight that penetrates into the skin and causes slower deeper damage like wrinkles, thinning of the mid layer of skin and discoloring of the skin. Because its effects are over longer periods of time, it is more to difficult to measure the value of UVA protection that a sunscreen provides. Instead, we rely on the properties of the ingredients in the sunscreen to tell us whether it will protect against UVA. Zinc and Avobenzone are the most effective ingredients to protect broadly and should be in any daily sunscreen used to help prevent photo aging. The most effective way to protect your skin from the sun is to wear sun protective clothing, which protects against the broadest spectrum, and apply Zinc or Avobenzone-based sunscreen on parts of the skin that are not covered by the clothing.
Image source: skincancer.org