The U.S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background O₃ level as a baseline in its O₃ risk assessments. This background is defined as those concentrations that would exist in the absence of North American emissions. Lefohn et al. [2001] have argued that frequent occurrences of O₃ concentrations above 50–60 ppbv at remote northern U.S. sites in spring are of stratospheric origin, challenging the EPA background estimate and implying that the current O₃ standard (84 ppbv, 8-hour average) may be unattainable. We show that a 3-D global model of tropospheric chemistry reproduces much of the observed variability in U.S. surface O₃ concentrations, including the springtime high-O₃ events, with only a minor stratospheric contribution (always <20 ppbv). We conclude that the previous interpretations of a stratospheric source for these events underestimated the role of regional and hemispheric pollution. While stratospheric intrusions might occasionally elevate surface O₃ at high-altitude sites, our results indicate that these events are rare and would not compromise the O₃ air quality standard. We find that the O₃ background is generally 15–35 ppbv, with some incidences of 40–50 ppbv in the west in spring at high-elevation sites (>2 km). It declines from spring to summer and further decreases during O₃ pollution episodes. The 40 ppbv background assumed by EPA thus actually underestimates the risk associated with O₃ during polluted conditions. A better definition would represent background as a function of season, altitude, and total surface O₃ concentration. Natural O₃ levels are typically 10–25 ppbv and never exceed 40 ppbv. International controls to reduce the hemispheric pollution background would facilitate compliance with an AOT40-type standard (cumulative exposure to O₃ above 40 ppbv) in the United States.