Understanding Sunscreen & Sunblock

November 03, 2011

You should always try to get the most out of your sun-protection product, but sometimes all the terms that go in to choosing a product can make things difficult. This guide explains what UVA, UVB, and SPF mean, as well as the differences between sunscreen and sunblock. All of this information will help you choose the best sun protection for your skin.

The Differences between UVA & UVB Rays

Both UVA and UVB rays have harmful effects on human skin, but there are some differences between the two. UVA is short for ultraviolet A rays. UVA rays have a long wave-length. There are two types of these rays: UVA I and UVA II. The difference again is wave-length. All UVA rays:

  • Can travel through shade, clouds, and glass.
  • Penetrate to a deeper layer of skin than UVB.
  • Causes the skin to release existing melanin, which contributes to skin-tanning.
  • Can cause skin cancers, skin aging, and wrinkling.

UVB stands for ultraviolet B rays. These have a short wave-length. UVB rays:

  • Cause skin reddening and sunburn.
  • Damage the top layer of skin.
  • Do not penetrate clouds and glass.
  • Lead to skin aging (but at a much slower rate than UVA rays).
  • Causes the skin to produce new melanin (the primary cause of skin-tanning).

How SPF Works

Sun protection factor (SPF) indicates how long it will take for your skin to turn red. For instance, if a sunscreen/sunblock has an SPF of 15, it will take 15 times longer for your skin to redden than it would without the sunscreen/sunblock. Three of the most common SPF numbers are 15, 30, and 50:

  • SPF 15 protects against 93% of the sun's rays.
  • SPF 30 protects against 97% of the sun's rays.
  • SPF 50 protects against 98% of the sun's rays.

The Differences between Sunscreen & Sunblock

Many people use the terms sunscreen and sunblock interchangeably, but there are some significant differences between the two.


  • Sunscreen is transparent when applied to the skin.
  • It absorbs the sun's UV rays before they can damage the skin.
  • Sunscreen must be reapplied throughout the day, because the active ingredients break down once exposed to sunlight.
  • Some sunscreens only protect against UVB rays. Look for the terms broad-spectrum, full-spectrum, or multi-spectrum in the title if you want to find a sunscreen that protects against both UVA and UVB rays.


  • Sunblock is opaque (not transparent) and does not need to be reapplied as frequently as sunscreen.
  • It blocks the sun's UV rays (like a wall).
  • Sunblock is generally used on sun-sensitive areas such as the nose, lips, ears, and shoulders.
  • It contains titanium dioxide or zinc oxide, which block almost all of the sun's rays.

Protect Your Skin

All the acronyms associated with sun protection can make the decision making process confusing, but once you know what everything means, it's really not that hard to make purchasing decisions. Now that you know what to look for when buying sunscreen or sunblock, you can make better choices for your needs.

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