Sunscreen Dosing: Recommendations from a Dermatologist
Fact: Sunburns and sun damage contribute to skin cancer.
Not Fact: Sunscreen contributes to cancer. Stopping there.
Just like avoiding this virus, you can't just rely on one thing alone. Not just masks, not just avoiding poorly ventilated crowds of strangers, not just hand washing. You ideally try all.
So with preventing sunburns, SAAAMEE. You seek shade, avoid sun when it's the strongest between 10am-2pm, wear sun protective clothing, hats, and try to use sunscreen properly.
What is the proper application of sunscreen?
This can get confusing.
Most people apply it incorrectly. When a sunscreen is developed and tested for efficacy, it is assigned a sun protection factor or “SPF” to measure how well it blocks UV radiation, and how long it can protect you in a given amount of time.
The proper dose of sunscreen is applied in the dose of 2mg/cm2 to the body. What the heck does that mean?
Here are a couple concepts to help
The "rule of 9’s" and the "fingertip unit" can help us prescribe the correct amount of topical medication to treat a patient’s skin condition. These two concepts can be extended to help you in practical life when it comes to sunscreen.
With the rule of 9's, the body's surface area is divided into 11 areas, each representing roughly 9% of the total body. Sunscreen can be applied to each of these areas at a dose of 2 mg/cm2 if two strips of sunscreen are squeezed out on to both the index and middle fingers from the palmar crease to the fingertips. The application of these “two fingers” of sunscreen will provide a dose of the product that approximates to that used during the laboratory determination of the sun protection factor. Such a dosage guide is a means of ensuring that users are protected according to their expectations.
Body areas by this system:
Head, neck, and face
Left arm
Right arm
Upper back
Lower back
Upper front torso
Lower front torso
Left upper leg and thigh
Right upper leg and thigh
Left lower leg and foot
Right lower leg and foot
A golf ball sized amount or a shot glass full to sun exposed areas are other helpful visuals. This isn't perfect. People under do it. Try your best!