Several recent publications have shown that most people apply only 20% to 60% of the amount of sunscreen needed to achieve the sun protection factor (SPF) rating of their applied product.
No international standard on sunscreens specifies dosage criteria for users. So, it is not surprising that consumers do not know how to apply sunscreens effectively. We suggest the following methods that would allow users to apply a quantity of sunscreen that would result in closer agreement between the expected and delivered protection.
Clarissa Yang, the Harvey B. Ansell Professor of Dermatology at Tufts University School of Medicineand chief of dermatology at Tufts Medical Center, shares her advice for making the most of sunscreen and other sun blockers.
“You’ll need at least a teaspoon for each leg, another for your chest, and a third for your back. Then it’s a half teaspoon each for your face, your neck, and each arm and rub it in well.”
A 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, 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.