The correct dose of sun protection factor (SPF) as stated on a sunscreen bottle or tube is achieved if you apply 2 mg/cm2. Typically, consumers apply less than this — 0.5 to 1.5 mg/cm2 — which lowers the SPF to about one-third of that stated on the product.
Drs. Steve Taylor from Auckland, New Zealand and Brian Diffey from Newcastle, UK borrow from a commonly used approach for estimating a patient’s area of injury in hospital burn units — the “rule of nines.”
With the “rule of nines,” the body’s surface area is divided into 11 areas, each representing roughly 9% of the total.
1. Head, neck, and face
2. Left arm
3. Right arm
4. Upper back
5. Lower back
6. Upper front torso
7. Lower front torso
8. Left upper leg and thigh
9. Right upper leg and thigh
10. Left lower leg and foot
11. Right lower leg and foot
To measure the correct amount of sunscreen, squeeze 2 strips onto both the index and middle fingers and then rub onto one of the 11 areas. Do this 11 times to cover each area, and you will have the full SPF protection listed on the label.
This is a lot of sunscreen! And people tend to balk at using that much. As a compromise, the authors suggest that people spread half the amount — one finger of sunscreen — with the understanding that they are using half the recommended amount and will get half the SPF on the label.
To get the full level of protection, they can then apply another finger’s worth within half an hour of the initial application.
If you are still put off by using this much goop, consider wearing a hat.
More information on the proper use of sunscreen and other protective options are here.
John Russo, Jr., PharmD, is president of The MedCom Resource, Inc. Previously, he was senior vice president of medical communications at www.Vicus.com, a complementary and alternative medicine website.