The Persistent Pigment Darkening (PPD) rating is a measurement system developed to quantify a sunscreen’s level of protection against ultraviolet A (UVA) radiation. It was originally developed in Japan and is a standardized method for assessing the efficacy of the UVA filters in a sunscreen formulation. The PPD rating provides a numerical value indicating how much longer skin can be exposed to UVA rays while protected, compared to unprotected skin.
What Persistent Pigment Darkening Measures
The PPD rating is specifically concerned with blocking UVA rays. These rays penetrate deeper into the skin than UVB rays, reaching the dermis layer where they damage collagen and elastin fibers. This deep penetration causes long-term skin damage, including premature aging, wrinkles, and an increased risk of skin cancer.
UVA exposure causes an immediate, temporary darkening of the skin pigment, which is the “pigment darkening” referred to in the PPD test. This darkening is due to the oxidation of existing melanin in the skin. Since UVA does not cause immediate skin redness, a separate test is necessary to measure a product’s ability to block these deep-penetrating rays. The PPD test is an in vivo method, conducted on human subjects, to gauge how effectively a sunscreen prevents this UVA-induced darkening.
Calculating the PPD Numerical Rating
The PPD numerical rating is derived from a controlled in vivo experiment involving human volunteers. Small areas of a volunteer’s skin are exposed to measured doses of UVA radiation, both with and without the sunscreen applied. Researchers determine the Minimum Persistent Darkening Dose (MPDD), which is the lowest dose of UVA energy needed to cause a visible, persistent darkening of the skin hours after exposure.
The PPD value is calculated as the ratio of the MPDD on protected skin to the MPDD on unprotected skin. For example, a sunscreen with a PPD rating of 10 means it took 10 times the amount of UVA exposure to cause persistent darkening on the protected skin compared to the unprotected skin.
PPD Versus the SPF Rating
The Sun Protection Factor (SPF) measures a sunscreen’s ability to protect the skin from UVB rays, which are the primary cause of sunburn (erythema). SPF indicates how much longer it takes for protected skin to redden compared to unprotected skin.
In contrast, PPD measures protection against UVA rays, which are associated with photoaging and deep cellular damage. An effective sunscreen must offer “broad-spectrum” protection, meaning it protects against both UVB and UVA radiation. In regions like the European Union, a product can only carry a specific UVA seal if its UVA protection factor is at least one-third of its SPF value.
Reading PPD on Sunscreen Labels
The PPD number itself is sometimes listed on European sunscreens, but it is not the primary labeling standard in North America. Consumers in many Asian markets, particularly Japan and South Korea, commonly encounter the Protection Grade of UVA (PA) system. The PA system is a simplified, categorical representation of the PPD numerical rating.
The PA system uses plus signs to indicate the level of UVA protection, ranging from PA+ to PA++++. A PA+ rating corresponds to a PPD value between 2 and 4. The highest rating, PA++++, indicates a PPD of 16 or greater.