Which factor does NOT affect the thickness of the stratum corneum?

The stratum corneum, the outermost layer of the epidermis, is influenced by various factors, but air quality does not directly impact its thickness. The thickness of the stratum corneum can vary based on environmental conditions, physiological factors, and lifestyle choices.

Age plays a significant role, as the skin tends to become thinner and less resilient with time, leading to a decrease in the thickness of the stratum corneum. Sun exposure can thicken the stratum corneum due to the skin's natural response to protect itself from UV damage, resulting in increased keratinization. Hydration levels also affect the stratum corneum; well-hydrated skin maintains a normal thickness, while dehydrated skin can lead to an increase in thickness as the skin compensates for moisture loss.

In contrast, while air quality can affect overall skin health and exacerbate certain skin conditions, it does not have a direct impact on the thickness of the stratum corneum itself. Therefore, among the listed factors, air quality is the one that does not influence the thickness of this outer skin layer.