Scientists need to measure solar ultraviolet radiation (UV) because increased exposure to UV has been associated with adverse health impacts to humans. These impacts include skin cancer, especially the most virulent form (melanoma), and diseases of the immune system. There are ecological impacts as well, such as potential harm to amphibians and other sensitive organisms, bleaching of coral reefs, and so on. The amount of UV reaching the Earth's surface increases over time if there are no physical interactions or chemical reactions to absorb it as it passes through the atmosphere. In particular, reduction in stratospheric ozone (O₃) concentration reduces the amount of UV absorbed in the upper atmosphere, which increases the UV intensity at the Earth's surface.