Singlet oxygen is an excited, highly reactive form of molecular oxygen that differs from the ordinary oxygen we breathe in the arrangement of electrons around its two oxygen atoms. Normal atmospheric oxygen has two unpaired electrons with parallel spins, which actually makes it relatively unreactive despite being essential for life. Singlet oxygen forms when that electron configuration is disrupted, typically through energy transfer from a light-absorbing molecule (a photosensitizer) that has itself absorbed UV or visible light, most often UVA in the context of skin. Because it carries paired electrons in an excited state, singlet oxygen is far more reactive than ground-state oxygen and readily attacks lipids, proteins, and DNA. In skin, singlet oxygen generation is one of the primary ways UVA exposure creates oxidative stress even though UVA does not directly damage DNA the way UVB does. Certain natural compounds in skin, including carotenoids like beta-carotene and lycopene, can help quench singlet oxygen by absorbing its excess energy and dissipating it harmlessly as heat.