June 20, 2002 -- The number on your favorite bottle of sunscreen may be in the double digits, but you may only get about half that amount of protection unless you know how much to use. That's why some researchers are calling for a simple "two finger" method to help you get the most out of your sunscreen.
The sun protection factor, or SPF, that a sunscreen delivers is determined by testing skin that's been generously slathered with a thick coat of sunscreen. But studies have shown that most people apply only 25% to 75% of this amount.
No international standard on sunscreens specifies how much people should use, although it's assumed to be the same amount that's originally used to test the product, according to the researchers. "So it is not surprising that consumers do not know how to apply sunscreens effectively."
Their dosage guide is based on a "rule of nines" that divides the body's surface area or skin into 11 areas that each account for about 9% of the total. The areas are:
- Head, neck, and face
- Left arm
- Right arm
- Upper back
- Lower back
- Upper front torso
- Lower front torso
- Right upper leg and thigh
- Left upper leg and thigh
- Left lower leg and foot
- Right lower leg and foot
The authors say the right amount of sunscreen can be applied to each these 11 areas if people use the "two finger" method. That means two strips of sunscreen should be squeezed from the tip to the base of the index and middle fingers and applied to each of these areas.
But the researchers note that many people may be unwilling to cover themselves or their families with such a copious layer of sunscreen.
An alternative would be to apply one finger's worth of sunscreen initially -- knowing that the amount of protection will only be about half of the SPF written on the bottle -- and then reapply another fingertip worth of product within a half hour of the initial application.
The proposal from Steve Taylor, a general practitioner with the Sunset Road Family Doctors in Auckland, New Zealand, and Brian Diffey, professor of medical physics at Newcastle General Hospital, in Newcastle, England, is published in the June 22 issue of the British Medical Journal.