Measuring Sunscreens and UV Protection

Posted on March 5, 2015

Measuring Sunscreens and UV ProtectionHow do sunscreens get evaluated?

SPF is for UVB

SPF stands for sun protection factor.  You need a level of 15 or higher for daily use, and 30 or higher for active use.

 There are two common methods for UVA.

Critical Wavelength test: Is a lab test using special tape to measure UVtransmission with and without sunscreen.  The wavelength where 90% of the absorption is is called the “critical wavelength.”  The stronger the sunscreen, the longer the wavelength.

Persistent Pigment Darkening (PPD) test:  Humans are exposed to UVA with and without sunscreen.  Then they are evaluated for appearance of pigment darkening from 2-24 hours after exposure.  Something with a PPD rating of 10 should allow you to have 10 times as much UVA exposure before you darken as you would with no protection.

Which products can you trust?

The Skin Cancer Foundation has started a program to evaluate sunscreens with a “rigorous standard for sun protection.”  For this, they validate the SPF number, want a critical wavelength of 370, PPD of 5 for daily use or 10 for active protection, proof of photostability (it doesn’t break down in the sun), and substantiation of claims of water or sweat resistance.

Look for this seal on products: