Along with fragrances and dyes, UV filters, particularly organic moieties, are known to elicit irritant responses in subjects predisposed to such skin reactions (48,49). It is generally desirable, therefore, to reduce to a minimum the concentration and number of UV filters present in a sunscreen product formulation to minimize risk of these types of incompatibilities.
Aesthetics/Skin Feel
Each type of sunscreen active described above can have a negative impact on sunscreen product skin feel, with higher levels having a corresponding larger effect. Specifically, the general skin-feel tradeoffs of the various types of sunscreen actives are:
• Polar oils tend to make the product feel greasy and oily, especially at high concentrations.
• Oil soluble crystalline solids require high levels of oily solvents/emollients to dissolve them and keep them from crystallizing in the product over time, and hence make the product feel greasy and oily.
• Water soluble salts tend to reduce the capability of most aqueous polymeric thickeners. This, in turn, leads to the use of much higher polymer levels to achieve a target product thickness, and these high polymer levels make the product feel sticky and heavy on the skin.
• Insoluble powders/particulates can make the product feel dry and draggy, and often can lead to an undesirable white appearance on the skin.
Additionally, even beyond the specific aesthetic effects above, there is a further general effect that comes from putting significant levels of sunscreen actives into a product—higher “coated” feel on the skin. Specifically, the single largest component of most non-sunscreen lotions and creams is a volatile carrier, typically water. Thus, when a layer of non-sunscreen lotion or cream is applied to the skin, most of the product evaporates, leaving behind a thin layer of non-volatile material consisting of moisturizers, emollients, thickeners, preservatives, and similar materials. By adding UV filters to a formula to achieve SPF 15 or SPF 30, for example, the level of volatile carrier in the product is significantly reduced. As a result, much more of the applied product is left behind on the skin, and the skin feels “coated.” Thus, even if the greasy, draggy, or sticky effects of the sunscreen actives are reduced by other technologies, the skin will still be left with an unpleasant coated feeling given the high level of non-volatile materials left behind from the sunscreen product. To compensate for this, many consumers apply product at a lower dose or less frequently, which will likely reduce the efficacy (50,51).
As stated, all of these factors—cost, skin compatibility, and aesthetics/skin feel— will influence patient compliance, either directly, viz amount of product applied and frequency of application/reapplication, or indirectly in decisions related to repurchasing the product. Thus, by developing more efficient sunscreens, manufacturers can minimize the amount of sunscreen actives needed to achieve a given efficacy target, and hence deliver lower cost, less irritating, and better-feeling sunscreen products. Sunscreen products consumers will use more regularly will provide a significantly greater degree of protection.