The ubiquitous presence of solar UV radiation in human life is essential for vitamin D production but also leads to skin photoaging, damage, and malignancies. Photoaging and skin cancer have been extensively studied, but the effects of UV on the critical mechanical barrier function of the outermost layer of the epidermis, the stratum corneum (SC), are not understood. The SC is the first line of defense against environmental exposures like solar UV radiation, and its effects on UV targets within the SC and subsequent alterations in the mechanical properties and related barrier function are unclear. Alteration of the SC's mechanical properties can lead to severe macroscopic skin damage such as chapping and cracking and associated inflammation, infection, scarring, and abnormal desquamation. Here, we show that UV exposure has dramatic effects on cell cohesion and mechanical integrity that are related to its effects on the SC's intercellular components, including intercellular lipids and corneodesmosomes. We found that, although the keratin-controlled stiffness remained surprisingly constant with UV exposure, the intercellular strength, strain, and cohesion decreased markedly. We further show that solar UV radiation poses a double threat to skin by both increasing the biomechanical driving force for damage while simultaneously decreasing the skin's natural ability to resist, compromising the critical barrier function of the skin.T he stratum corneum (SC), as the outermost layer of the epidermis, is the body's first line of defense against solar UV radiation. Solar UV radiation plays a dual role in human life: it is pivotal for vitamin D production (1) while also a potent and ubiquitous carcinogen responsible for much of the skin cancer in the human population (2). Although progress has been made in understanding the role of UV radiation in causing skin cancer (3), the role of solar UV radiation in altering the mechanical barrier function of the SC remains unknown.The SC provides both critical mechanical protection and a controlled permeable barrier to the external environment. Although the SC is typically a highly efficient barrier, exposure to harsh conditions can alter its function, leading to severe skin damage such as chapping and cracking. Such damage can cause detrimental skin responses including inflammation and infection caused by compromised barrier function, scarring, and abnormal desquamation, and further aggravate the effects of skin disorders such as atopic dermatitis, ichthyosis vulgaris, and chronic xerosis (4-7).UV radiation is divided into three main types based on wavelength: UVC radiation (200-280 nm) is predominately filtered by the ozone layer in the stratosphere, UVB radiation (280-320 nm) is mainly absorbed by the epidermis, and UVA radiation (320-400 nm) penetrates deeper into the dermis but interacts with both the SC and epidermis as well (8) (Fig. 1). The penetration of UV radiation into the skin can initiate detrimental photochemical reactions, causing both acute conditions such as erythem...