The Integrated Science Assessment (ISA) is a concise evaluation and synthesis of the most policy-relevant science for reviewing the national ambient air quality standards (NAAQS). Because the ISA communicates critical science judgments relevant to the NAAQS review, it forms the scientific foundation for the review of the NAAQS for ozone (O3). The existing primary O3 standard includes an 8-hour (h) average (avg) standard set at 75 parts per billion (ppb) and not to be exceeded more than once per year. The secondary standard for O3 is set equal to the primary standard. The ISA accurately reflects the latest scientific knowledge useful in indicating the kind and extent of identifiable effects on public health which may be expected from the presence of (a) pollutant in ambient air (1990, 080701). Key information and judgments formerly contained in the Air Quality Criteria Document (AQCD) for O3 are incorporated in this assessment. Additional details of the pertinent scientific literature published since the last review, as well as selected older studies of particular interest, are included. This ISA thus serves to update and revise the evaluation of the scientific evidence available at the time of the completion of the 2006 O3 AQCD (U.S. EPA, 14 2006, 088089). The draft Integrated Review Plan for the Ozone National Ambient Air Quality Standards (IRP) (U.S. EPA, 2009, 684024) identified key policy-relevant questions that provided a framework for this assessment of the scientific evidence.