Sunscreen Usage

Problems with usage are arguably the biggest problem with sunscreen.  Put simply, if sunscreen is not used properly, it doesn’t work very well.

One of the biggest problems is the thickness of sunscreen actually applied.  When a sunscreen is given a SPF rating it is based on using 2 mg/square cm.  That’s about 1 oz of sunscreen (a shot glass) per application to cover an adult.  So 2 adults at the beach for 6 hours (needing 3 applications each) would completely use a 5 to 6 oz bottle or tube.

If you don’t put on this much sunscreen, there can be a significant loss in protection.  For example, Stokes and Diffey (1997) found that a SPF 25 sunscreen only gave SPF 5 protection when one-third the recommended thickness was used.

The other most common problem with usage is reapplication.  Some sunscreen ingredients actually break down when exposed to UVR.  Sunscreen can also easily be rubbed off or wiped off.  You have to reapply sunscreen throughout the day to maintain protection.

The worst combination of usage errors is to not apply enough then forget to reapply.  Unfortunately, this is a common pattern and is the reason many sunscreen users get sunburned.

So in order to use sunscreen effectively and not get sun burn, several common mistakes must be avoided:

• Apply sunscreen 20 minutes before you go outside. This allows time for it to penetrate or bind to the skin.
• Use the recommended amount of sunscreen in order for it to be effective against sun burn. Studies show that most people under apply sunscreen – only using 20% to 50% of the recommended amount.  To achieve effective sun protection, an adult should use approximately 1 oz of sunscreen to cover the entire body every 2 hours. Cover all exposed areas liberally. Pay special attention to ears, nose, feet, hands, bald spots, back of neck, arms and legs.
• Reapply sunscreen every 2 hours until sunset - even if the label promises “all-day” protection. It should be reapplied after swimming or sweating, and it should be reapplied if it has been rubbed off.
• Always check the label for an expiration date. Sunscreen ingredients lose their effectiveness over time, and expired sunscreen must be replaced. So do not use sunscreen that was left over from last summer. Throw it away and get a new bottle.