This indicator addresses the 3-year average of the 4^th-highest 8-hour average ozone concentration, which is the basis of the federal standard. Ozone (O₃), in high concentrations can cause health effects such as lung inflammation and other respiratory illness. Ozone can also cause damage to trees and plants at concentrations lower than the human health based ambient air quality standards. TRPA, and federal and state standards, with varying time averaging periods, have been adopted to protect the public from this harmful pollutant.

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