**1. Introduction**

In living organisms, photosensitizing reactions using photosensitizers (involving photocatalysts in this review) are used in a wide variety of ways. Beneficial uses include treatment of skin disease [1], elimination of cancer cells [2], and construction of tough collagen structures [3]. On the other hand, the photosensitizing reaction by amino acids and vitamin components in the living body through exposure to ultraviolet rays can cause skin aging. In this chapter, we reviewed the photosensitizing reactions in living organisms related to photoaging.

The important role of the skin is to protect the body from various external environmental factors. In other words, the skin has the role of preventing physical, chemical, and bacteriological invasion into the body and preventing water loss due to evaporation. The skin is composed of three layers: the outermost layer of the epidermis, the dermis, and the subcutaneous adipose tissue. Keratinocytes and melanocytes are well-known cells that make up the epidermis. Keratinocytes contribute to the barrier function of the skin by differentiating, and melanocytes produce melanin pigment to protect the epidermis and dermis from ultraviolet rays. In the dermis, the extracellular components, produced by dermal fibroblasts, have collagen fibers, elastic fibers, hyaluronic acid, and proteoglycan as the main constituents. These extracellular components have a high water retention effect and contribute to the maintenance of the hydrophilic environment of the dermis. Subcutaneous adipose tissue is rich in mature adipocytes and has the role of reducing external pressure. In addition to those essential capabilities, the skin has also a role in thermoregulation, immune response, and social communication [4–8].

Like many other organs, the skin undergoes adverse changes over time in response to changes in lifestyle and hormonal balance. However, unlike most other organs, the skin receives major changes due to exposure to the environment, especially UV rays from the sun. Chronic exposure to UV rays causes an early aging phenotype (photoaging) that resembles the aging caused by the passage of time (chronological aging) [9]. As a result, areas of the body that are routinely exposed to the sun, such as the face, neck, and forearms, show the visible manifestation of aging (senile lentigo, wrinkles, sagging, etc.) faster than other areas of the body [10].

The effects of chronological aging and photoaging induce serious alteration in the dermis with detrimental changes to the extracellular matrix [11]. Collagen accounts for the majority of the dermal matrix. However, with age, normal collagen content decreases, and the ratio of collagen degenerated by oxidation, carbonyl modification, and glycation increases [12, 13]. In addition, the ability of fibroblasts to generate collagen is diminished by environmental factors in addition to chronological aging. In particular, the upper layers of the dermis on the face, neck, and back of the hands, which have been exposed to the sun's rays, are characterized by the accumulation of glycated elastic fibers (solar elastosis) [14]. Furthermore, photoaging is mainly induced by long-term UV exposure. UVA, a long-wavelength UV rays, causes serious damage to the skin due to ROS produced by the reaction with photosensitizers in the skin (**Figure 1**) [15]. However, due to the wide variety of *in vivo* photosensitizers associated with ROS production, understanding the mechanisms of ROS production and effective quenching methods is very complicated. In this chapter, we focused on the hydrogen peroxide generated by the photosensitization reaction by multiple photosensitizers and UVA in the living body. We also introduced a simple screening method for discovering active ingredients that are effective against the photosensitization reaction through UVA.
