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  Your Skin and the Sun - Choosing the Right Sunscreen

Your Skin and the Sun

Choosing the Right Sunscreen

by D. R. Thomas, MD, FRCPC

The skin has evolved to protect us from the harmful effects of ultraviolet light. Sunscreens were first developed to prevent sunburns by blocking UVB; they allowed us to prolong our time in the sun, but that resulted in increased exposure to UVA. Modern sunscreens attempt to block the whole spectrum of UV light, so are called broad spectrum. Not all so-called broad spectrum sunscreens protect skin from the whole range of UVA.

Negative Effects on the Skin:

Most of us know that sun exposure has immediate positive and negative effects on the skin. The positive effects include a sense of warmth and pleasure and Vitamin D production. The medium and longer term effects are negative and must also be recognised.

Short Term Effects:

  • Sunburn, or tan
  • Photosensitive rashes
  • Drug and chemical photo toxicity and allergy reactions
  • Light aggravated conditions

Negative Medium Term Effects:

  • Photodamage (Click here to learn more.)
  • Photoaging

Negative Longer Term Effects:

  • Skin cancer (Click here to learn more.)
  • Photoaging

A Comparison on UVA and UVB

UVA and UVB light have different characteristics.

UVA* (320-400nm)

  • Levels are constant throughout the year
  • Penetrates into the lower dermis
  • Penetrates glass
  • 95% of UVL is UVA
  • May be important in causing melanoma
  • Tans the skin
  • Causes most of the aging effects seen in the skin
  • Immunosuppressive
  • Phototoxic reactions to drugs and chemicals
  • Responsible for many photodermatoses

*It should be noted that sun-tanning beds use mainly UVA light. There is no such thing as a "safe" suntan. Recently the US FDA began investigating whether suntan beds should be illegal for anyone under the age of 18 years.

UVB (290-320nm)

  • Amounts vary and increase in the summer, at noon, and on the equator
  • Most only penetrates the epidermis
  • Does not go through glass
  • SPF of sunscreens only measures UVB blockage
  • More carcinogenic than UVA
  • Sunburns the skin
  • Needed for Vitamin D production

Sunscreen Use

Key Points:
  • Broad spectrum only should be used.
  • SPF is only related to UVB protection and does not provide a reference to the UVA protection in the product.
  • All sunscreens will have UVB protection, which is reflected in the SPF.
  • If a skin sunburns in 10 minutes, a properly applied sunscreen SPF 15 means they will burn in 150minutes
  • Physical screens reflect light whereas chemical screens absorb UV converting the energy into heat
  • SPF15 blocks 87.5% of UVB and SPF 50 blocks 98% of UVB.
To get more tips about taking care of your skin, click on

About the author:

Richard Thomas, MD, FRCPC is Assistant Clinical Professor of Dermatology, Department of Dermatology and Skin Science, University of British Columbia, Vancouver, Canada.