What Do SPF and PA Numbers Measure? | Rules for Sunscreen Labeling

“SPF50+ PA++++” on sunscreen packaging. This is about how those numbers and plus signs are determined.

The two look at different types of ultraviolet rays

• SPF (Sun Protection Factor) -- an index for UVB, with wavelengths of about 280-320nm.
• PA (Protection Grade of UVA) -- an index for UVA, with wavelengths of about 320-400nm. It is expressed in 4 levels, from PA+ to PA++++.

Because ultraviolet rays are not all the same, a single index is not enough.

The measurement method is set by international standards

According to the Japan Cosmetic Industry Association, the measurement methods are based on ISO international standards. SPF is ISO 24444, and PA is ISO 24442. The labeling method itself is set as the association’s voluntary standard. The PA++++ label was added through a standards revision announced in 2012.

The assumption of “2mg/cm²”

This may be the most important point in practical use. In testing, products are evaluated with 2mg (2µL) applied per square centimeter of skin. The number on the package is the value under that amount of application.

Even with the same product, if you apply less than the assumed amount, the conditions are not the same as during testing. Before comparing the size of the numbers, it helps to know under what conditions those numbers were obtained. That is what reading a label means.

There are also rules for the order of ingredients. How to Read the Full Ingredient List is also available.

References: Japan Cosmetic Industry Association; ISO

—The Value Village Editorial Team