This indicator addresses the highest monitored ozone concentration averaged over every 8 hours for the calendar year. Ozone (O₃), in high concentrations, can cause health effects such as lung inflammation and other respiratory illness. O₃ can also cause damage to trees and plants at concentrations lower than the human health based ambient air quality standards. Standards, with varying time averaging periods, have been adopted to protect human health.

Ozone is a secondary pollutant created by reactions between sunlight and hydrocarbons (HC) and oxides of nitrogen (NOx). The primary sources of HC and NOx include in-basin mobile sources such as cars, trucks, boats, aircraft and off-road vehicles; biomass burning such as wood stoves, wildfires and prescribed burning; and consumer products such as solvents. Ozone is also transported into the basin to a lesser extent from populated areas surrounding the basin, and the ambient concentration of O₃ is highly dependent on meteorological conditions such as sunlight, temperature, wind speed, and mixing conditions.