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Background: Skin cancer is the most common type of cancer in the United States. The majority of skin cancer is caused by overexposure to ultraviolet (UV) light. National Environmental Public Health Tracking Program at CDC has collaborated with partners to develop and disseminate county-level daily UV irradiance (2005 to 2015) data for the contiguous United States.Methods: UV dataset was derived based on measurements from the Ozone Monitoring Instrument (OMI) during 2005-2015. UV irradiance data were first calculated at the census tract level; then daily population-weighted UV irradiance were calculated at the county level. We analyzed the spatial distributions and long-term trends of UV irradiance, and ratio of UV irradiance to solar irradiance. We also have used the newly developed dataset in a few applications to improve characterization of UV exposure.Results: The UV dataset is currently the only publicly-available, spatially-resolved, and long-term UV radiation dataset covering the contiguous United States. The national averages across all years available is 2,700 J/m² for erythemally weighted daily dose of UV irradiance (EDD). UV irradiance increased toward the South and the West. The ratio of UV to solar irradiance mainly increased toward the South and in some areas with high altitude. This spatial variation suggests that using solar irradiance as indicator of UV irradiance in studies covering large geographic regions may bias the true pattern of UV exposure. National annual average daily UV irradiance increased significantly over the years by about 0.5% per year. In addition, we identified hotspots of skin cancer vulnerability, which took into consideration demographic, social risk factors and UV exposure intensity. We also examined state laws allowing sunscreen use in schools in the context of state average UV irradiance. Finally, we are currently studying the association between UV irradiance and cutaneous melanoma incidence rates among white US residents.