The main external damage to skin is primarily caused by:

Sun damage is recognized as the primary external factor that significantly harms the skin. Ultraviolet (UV) radiation from the sun can lead to various skin issues, including premature aging, sunburn, and more severe conditions like skin cancer. When the skin is exposed to sunlight without appropriate protection, it undergoes a series of changes, such as the breakdown of collagen and elastin, which are essential for maintaining skin's structure and elasticity. Moreover, UV damage can lead to uneven pigmentation, DNA alterations in skin cells, and inflammatory responses.

While pollution, aging, and harsh chemicals also contribute to skin damage, they generally do not have the same profound and direct impact as UV radiation. Pollution can exacerbate skin issues, and aging naturally leads to changes in skin health, yet neither of these factors match the intensity of the skin damage caused by sun exposure. Harsh chemicals can irritate and damage the skin barrier, but again, the pervasive and long-term effects of sun damage are more widely recognized in dermatological contexts. Thus, acknowledging sun damage as the primary cause is crucial for understanding skin health and developing effective protective measures.