Which factor does NOT influence skin penetration?

The presence of sweat does not significantly impact the skin's ability to absorb topical products or chemical ingredients. Skin penetration is primarily influenced by characteristics of the skin barrier, such as the thickness of the stratum corneum, which is the outermost layer of skin. A thicker stratum corneum acts as a barrier to penetration, while a thinner stratum corneum can facilitate better absorption of substances.

Additionally, temperature plays a key role in skin permeability. When the skin is warmed, it increases blood flow and circulation, potentially enhancing the rate of absorption for topically applied ingredients. Large buildups of dead skin cells, or a significant amount of keratin, can also impede penetration, as these layers can block the pathways through which substances would normally permeate.

In contrast, while sweat can create a moist environment, it does not provide a significant barrier or aid in the absorption of products in the same way that the other factors do. Thus, the presence of sweat is not a primary factor influencing skin penetration.