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5.0 Environmental Analysis 5.5 Air Quality Avion Project SEIR Page 5.5-2 problem in the San Diego area. Studies show that emissions from the SDAB are sufficient, on their own, to cause ozone violations. 5.5.2.3 Existing Air Quality Existing air quality is measured based upon ambient air quality standards. These standards are the levels of air quality that are considered safe, with an adequate margin of safety, to protect the public health and welfare. The National Ambient Air Quality Standards (NAAQS) and California Ambient Air Quality Standards (CAAQS) are currently in effect, as well health effects of each pollutant regulated under these standards are shown in Table 5.5-1. The determination of whether a region’s air quality is healthful or unhealthful is determined by comparing contaminant levels in ambient air samples to the state and federal standards presented in Table 5.5-1. Air quality is commonly expressed as the number of days per year in which air pollution levels exceed the NAAQS and CAAQS. The air quality in a region is considered to be in attainment by the state if the measured ambient air pollutant levels for ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), respirable particulate matter (PM10), and fine particulate matter (PM2.5) are not equaled or exceeded at any time in any consecutive three-year period; and the federal standards (other than O3, PM10, PM2.5, and those based on annual averages or arithmetic mean) are not exceeded more than once per year. The O3 standard is attained when the fourth highest 8-hour concentration in a year, averaged over three years, is equal to or less than the standard. For PM10, the 24-hour standard is attained when 99 percent of the daily concentrations, averaged over three years, are equal to or less than the standard.