QWhat is the “dilutional effect” as it pertains to SPF?

Judicious use of broad spectrum sun protection factor (SPF) is imperative to reduce signs of accelerated skin aging, pigmentation, and cancer risk. Therefore, it is essential that we as dermatology clinicians counsel our patients on the regular and appropriate use of SPF.

When speaking with my patients about sunscreen, I discuss the “dilutional effect.” I tell patients that one ounce of SPF, or one shot glass, is necessary to cover the entire body surface area. I proceed to tell them that consumers apply anywhere from 20-50% of the recommended allotment, let alone apply sunscreen at the recommended frequency.¹ With this said, I instruct patients to select an SPF of 50 or greater despite the American Academy of Dermatology’s recommendation to select an SPF of 30 or greater. The rationale behind using a higher SPF is logical when framed appropriately to patients.

The “dilutional effect” refers to the concept that despite using a higher SPF, one will not ultimately derive the benefit of the SPF indicated on the packaging due to inadequate application.¹ Consumers are thus effectively “diluting” the protective power of their sunscreen, let’s say from an SPF of 50 to 30, for instance. This is when patients have that rewarding “light bulb moment.”

Rather than waiting for patients to inquire about SPF, I initiate the conversation. My very astute patients ask me if there is really a difference in protection between an SPF of 15, 30, and 50. What I tell them is that in a lab setting, there is a marginal difference in protection afforded at each level. However, in real life, due to the improper dosage and frequency by which sunscreen is applied, there is a meaningful difference and this is key. I like to reference a study conducted by Williams et al² that demonstrated that the use of higher level SPFs in real life were more effective than lower value SPFs in protecting against sunburns.

The concept of the “dilutional effect” highlights the need to understand the real-world application of sunscreens in order to better inform our patients and improve their understanding of sun protection.

References:

1. Petersen B, Wulf HC. Application of sunscreen− theory and reality. Photodermatology, photoimmunology & photomedicine. 2014 Apr;30(2-3):96-101.
2. Williams JD, Maitra P, Atillasoy E, Wu MM, Farberg AS, Rigel DS. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: Results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol. 2018;78(5):902-910.e2. doi:10.1016/j.jaad.2017.12.062