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Ozone is a gas made up of 3 oxygen atoms; it has the chemical formula O3. Ozone found at ground level, where people live and breathe, is formed by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight. For this project, ozone levels are measured in units of parts per billion (ppb) by volume. Ozone concentrations, averaged over the summer season in each region when ozone levels tend to be highest, are used to represent the exposures experienced by human populations in those regions.